Tag: straw bale

HPH285 : Developing plant-based building materials – with Judith Thornton
Dr Judith Thornton of Aberystwyth University explores how plant-based materials are being used in the building industry, and why they are so important for our future.

Interview with Judith Thornton

Judith works within the Department of Biological, Environmental and Rural Sciences at Aberystwyth University. She specialises in projects at the interface of plant science and material function, with her main interest being the use of plants as building materials.

It’s about more than just building with straw or hempcrete

Judith says that, historically, because of the finely honed skills required for each approach, there has been a tendency for the natural building industry to divide off into those that build with straw, and those that build with hempcrete. There are of course many other plant types that can be used, depending on the design criteria.

She explains that we shouldn’t assume that we could build the whole project out of plants either, and nor should we assume that only one plant type will be suitable for each of the different building elements.

Judith uses walls as an example, where because they provide the dual functions of structure and insulation, it might make sense to separate those elements out and use different materials. She says that otherwise, wherever you’re trying to get a particular product to perform multiple functions, you may well end up with significant compromises.

Do your research

Even within strawbale building there are different types of straw that can be used, but Judith says people rarely demand a particular variety, and generally just accept what they’re given.

“I have yet to meet a strawbale builder who knew what wheat variety was in their building!”

Historically the tendency is to use wheat straw, within which there are also a number of varieties, but sometimes barley straw is used. Barley straw however is far more prone to rotting than wheat straw, which Judith says should be a bit of a red flag when thinking about building with it.

In Wales they have tested out building with miscanthus straw, which is a grass that can grow to three metres high, so is much tougher than wheat or barley, and presumably should be more resistant to breaking down.

Judith says that there tends to be an assumption that strawbale buildings are inherently low energy, but that’s not necessarily the case. If they’re not built properly they can be leaky and rot, not very airtight and have high energy bills in the same way that any other building can.

There is a lack of aspirational standards

“We’ve got a regulatory system where we’ve got a fairly ineffectual stick and absolutely no carrot. So, ‘this will comply with building regs’, and people will say that as if that’s a good thing. Well, complying with building regs is as bad as you’re allowed to be before you get prosecuted. That isn’t an aspiration, is it?”

Judith believes that there is a lack of incentive to build to the Passivhaus standard using low impact materials. She sees it is a combination of lacking aspirational standards and not valuing building enough as a trade, when it’s actually a very skilled profession and a challenging undertaking to build to Passivhaus standards.

One of the key challenges with Passivhaus is getting the airtightness, and particularly concentrating on the junctions between different materials. It can be as problematic for man-made materials as it is with plant-based materials though. Particular care needs to be taken when designing details at junctions between timber and straw for example, where you might get some shrinkage in the plaster when it’s butted up against a timber frame.

There would be no sense in trying to build an entirely plant-based building

Judith thinks it would be ludicrous to even try. Using airtightness tapes as an example, she explains it as the law of diminishing returns: where a small amount of a very high tech materials provides a massive benefit. She says there is no point in obsessing about whether your strawbales are tied with polypropylene string or whether your airtightness membrane has got formaldehyde glue in it.

“Partly because it makes almost no difference to the total carbon in the building and partly because it’s based on this completely false premise that natural things are safe and manmade things are dangerous. That simply isn’t true. But it’s a very commonly held environmental world view.

“We do this weird moral balancing in the UK where we say, ‘I’ll fly abroad on holiday, but I did do my plastic recycling’. The whole plastic debate for me is seeking to shift blame to manufacturers as an alternative to examining our own behaviours.

“It’s not rocket science. As individuals, we need to eat virtually no meat, we need to stop flying on holiday, and we need to insulate our houses.”

Establish what the building industry needs

Judith thinks that academics can have a tendency to come up with solutions for problems that don’t necessarily exist. So part of her role is having conversations with the building industry and the plant scientists to work out the areas that need new materials, and how the plant science community can contribute.

One example is of insulating solid wall houses in Wales and doing that with vapour permeable materials. She says that hemp lime or another material mixed with lime is an excellent way of achieving internal wall insulation whilst minimising risks of interstitial condensation. It then becomes a question of analysing what it is that the plant material is doing in that mixture, seeing whether other plant materials are available, and then carrying out testing.

Prevent biodegradable building materials from biodegrading!

There are significant differences between various plant materials in how easily they biodegrade, and this will impact on the longevity of a building. The key thing is to stop moisture from getting in.

Plants contain variable amounts of silicon and lignin in their cell walls which are protective against being broken down by microorganisms. Even different wheat straw varieties will contain different levels of lignin, which would impact on how quickly an individual piece of straw would rot if it were exposed to moisture.

So one of the initial decisions to make when planning your build is as to how you plan to deal with moisture, and whether for example you have a vapour-permeable construction where moisture can diffuse into and out of the wall, or whether it will be a completely sealed envelope that relies on mechanical or window opening systems for moisture management.

We will need to move to a plant-based economy

Before fossil fuels we had a plant-based economy, and going forward this is something that we will need to revert to once again.

Judith is involved in the university’s BEACON project, which looks at plants as products and how biorefining can help us move to a natural feedstock-based economy.

“Everything we use on a day-to-day basis, be it building materials or plastics or everyday items, is going to need to be made out of plants and it’s a question of how we best do that.”

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July 29, 2020
HPH252 : How to reduce the embodied carbon of your new home – with Chris Magwood

Chris Magwood, a director at the Endeavour Centre in Canada, explains the importance of embodied carbon and how building with plant-based materials could transform our new homes into carbon sinks.

Interview with Chris Magwood

Chris Magwood has been working in sustainable building for the last 25 years. He’s also written several books such as Essential Sustainable Home Design, Essential Prefab Straw Bale Construction and Essential Hempcrete Construction.

For the last 8 years he’s been teaching at the Endeavour Centre which is a sustainable building school based in Peterborough, Ontario.

What is embodied carbon?

Embodied carbon is the total carbon emissions associated with materials through initial harvesting of raw materials and through the entire transportation to manufacturing facility and everything that happens in that manufacturing facility. And then transportation to a job site and the associated emissions with actually doing the installation work on the job site.

Carbon dioxide is an expression of all greenhouse gases

As carbon dioxide is not the only greenhouse gas, other emissions are translated into a carbon dioxide equivalent.

Chris says: “The expression that we use in the calculations is CO₂-E – so, carbon dioxide equivalent. So, if it’s carbon dioxide, one molecule equals one. But if it’s methane, one molecule would equal twenty-five or whatever the number happens to be.”

Energy isn’t necessarily a proxy for carbon

For a long time the term that was being used was embodied energy, but it’s important to realise that energy isn’t necessarily a proxy for carbon.

So, two factories could use the same amount of energy, but if one is all solar and one is all coal powered, then the emissions profile of the two would be very different.

Even though in embodied energy terms, that number might look the same. So, this is specifically trying to make emissions the focus and not just energy use.

The carbon footprint of building materials vary drastically

While we probably understand that different building materials have different carbon footprints, when Chris crunched the numbers he was surprised by the extremes.

Chris comments: “I was pretty stunned at the results in that some things just had such a huge carbon footprint and I had never really considered that impact being so big before. And then some things were miniscule.”

Plant based materials give us the opportunity to turn our buildings into carbon sinks

As plants grow they pull CO₂ out of the atmosphere and release oxygen. Effectively they put the carbon into their physical structure.

So if you’re building with a material that has atmosphere carbon stored in it – basically any plant material – in terms of the calculations, it’s a negative emission. In other words there’s a drawdown of carbon rather than an emission.

“Building with a lot more plant material, we could not only reduce that carbon footprint to zero, we could actually take it into the negative numbers and make drawdown buildings that are in their way drawing down climate change rather than contributing to it.”

Straw is one of the best materials

Chris compares two plant-based materials as an example of how even a little processing can increase the embodied carbon.

Although cork requires quite a bit of heat in its manufacture, it still ends up being a net store of carbon.

Straw, however, has almost no emissions in terms of the harvesting and manufacturing. It’s also the waste product of an annually renewable crop. And in the case of grain straws, the carbon in the stalks would have just returned back to the atmosphere in fairly short order.

Chris adds: “Even though all of these plants more or less store a similar amount of carbon, the amount of processing energy that’s gone into them does change the net profile.”

Endeavour class at a straw bale raising

Six carbon storing insulation materials

Timber has a more complex picture

It’s much harder to attribute carbon storage to timber because the carbon that’s in that tree, depending on when the tree was cut, that tree could have gone on drawing down more carbon as a living tree than it does as a stored tree.

And if you only end up using about half the volume of the tree to make a timber product and the rest of the carbon from that tree ends up back in the atmosphere, you haven’t net stored any carbon. And if you’ve released a whole bunch of carbon from the soil of the forest floor, then it may be worse than a draw down. It might actually cause emissions.

We could become more intentional about what our buildings are doing

Chris is not advocating that all buildings need to be entirely plant based, but he does see a massive opportunity to be more intentional.

“If you can weigh up the carbon storing materials with the carbon emitting materials, you can actually plan a building to land on a target that you’re aiming for, whether that’s carbon neutrality, storage or a slight number of emissions.”

Environmental product declarations report the environmental profile of a material

In carrying out this research, Chris used environmental product declarations, an ISO standardised way of reporting on the environmental profile of a material.

This is third party document where the third party goes in and looks at a manufacturer and traces the entire chain of that product and essentially does a calculation to look at its global warming potential from harvesting of the raw materials through the entire manufacturing process.

Chris explains how he used this information: “I’m basically using those numbers to then multiple it out on my own buildings. So, if a particular environmental product declaration says there are two kilograms of CO₂-E per kilogram of this material, then I figure out how many kilograms of that material are in my building and then I can multiply that out and do that for all of the materials and then total that up and get a net, whether it’s a net emissions or a net storage.”

Concrete, glass, foam and steel all have high carbon footprints

Perhaps as one might expect concrete, glass, foam, and steel all have significant carbon footprints associated with them.

Building a Passivhaus will not necessarily have any impact on embodied carbon

If you’re building a Passivhaus, then your choice of materials is really important.

“The way a lot of people will achieve Passivhaus standard here in Ontario, Canada, is they’ll throw a whole bunch of foam insulation at that building. And when I modelled that, the embodied carbon footprint, the upfront emissions of making that building, don’t ever get paid back by the energy reductions. So, if somebody was using that strategy because they thought they were doing something for climate change, they’ve actually done exactly the opposite.

The carbon profile of the grid varies according to where you live

This goes back to energy not necessarily being a proxy for carbon.

Energy efficiency – in terms of carbon emissions – is only useful if you use dirty energy (with a high carbon content).

“Here in Ontario, we shut down our coal plants a few years ago, and so our electrical grid is relatively carbon free. And so, savings in energy efficiency, especially moderate ones, our code is now pretty good in terms of baseline energy efficiency. Going from that to Passivhaus, while it may have good implications for the owner in terms of lower bills and comfort and all that kind of stuff, from a carbon emissions profile, it’s almost meaningless with the difference between a code built house here in Ontario and a Passivhaus in terms of the emissions profile of operating that building, it’s negligible because the carbon profile is so clean on the grid.”

Canada’s Greenest Home

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June 1, 2019
HPH225 : How to create a load-bearing straw bale Passivhaus home – with Andrew Goodman
Architect Andrew Goodman explains what it took to create the UK’s first load-bearing straw bale Passivhaus home.

Interview with Andrew Goodman

Andrew Goodman is an architect with over 20 years experience running his own practice, currently as a sole practitioner based in Hertford. One of his recent projects has been for Dave and Mabel Howorth, who were looking to build a straw bale home for their retirement, with the added challenge of wanting to achieve the Passivhaus standard.

Dave Howorth already had an interest in straw bale buildings and had previously participated on one of Straw Works‘ projects. He was convinced that it was a good way to build and was also attracted by the affordability it provided.

The simple form suited both the straw bale construction and Passivhaus elements

It took several years for Dave to find his plot of land. When he bought it, it already had planning permission for a chalet style bungalow, which wasn’t ever going to lend itself to either being built with straw bales or to the Passivhaus standard.

The plot was in the curtilage of an old, listed dwelling with thatched roof and lime plastered walls, and the local vernacular for Dutch barn style curved roofs also gave the extra ceiling height they were looking for, so these both provided a starting point for evolving the design.

Being a detached dwelling in an established orchard meant the trees would have some impact on shading the building in a positive way.

Photos copyright of Agnese Sanvito

The house needed to be suitable for their future requirements

It was important to Dave and Mabel that the building should meet the Lifetime Home standard, which was about at that time (but since been partially absorbed into Building Standards). The standard had a dozen or so categories covering how someone with restricted mobility might approach and access the building, move through it with ease and move between levels if applicable.

The brief was to provide three main bedrooms, living accommodation, separate kitchen, dining and living room. The Lifetime element was incorporated by providing an extra reception room on the ground floor with access to a wet room, which could be a bedroom with en-suite in the future if needed.

A pre-application with the planners was largely positive

The proposed building had roughly the same floor area as the original planning permission which was a help, and although the finishes were very different, they were judged to be appropriate for being in close proximity of the listed building, and planning permission was granted.

The natural movement of straw may have an impact on airtightness over time

The foundation is an insulated slab system. Then there is a taped connection between the slab and the internal wall finish, which crosses over the base plate detail that you need on any straw bale construction. Instead of building first floor joists into the walls, they used a ledger beam approach attached to the box beam, with a membrane between the ledger beam and the box beam that connects the two plaster layers, the ground floor and first floor.

The curved roof uses a standard Intello membrane on the underside of the timber structure, which again is then connected to the render at first floor level.

A previous attempt by Fran Bradshaw to build the first load-bearing Passivhaus straw bale house just missed out on achieving the standard. Hers used a structural timber frame, so the bales may not have been as compressed and loaded as those at Haven Cottage. Andrew’s assistant was convinced that load-bearing the straw bales would be the best way to go, and by using a wet lime plaster on the inside as the air barrier, it shouldn’t be any different than using it on a brick block cavity wall. Haven Cottage also has an external lime render.

In terms of the construction, part of a ring beam is placed that can be used to compress the bales using wires that run from the base plate up to the top plate, which are then tightened. There is a typical compression of around 50mm for each storey height. Once it’s compressed it is loaded with the next level of construction up to the underside of the roof.

Together with the structural engineer, they worked out a system of curved laminated beams that are actually within the overall depth of the roof. The roof has 40mm of Warmcel insulation. The laminated beams are only 300mm deep.

“So, there’s a double joist system that minimises thermal bridges, loses the curved laminated beams – perhaps unfortunately, but it keeps the underside of the ceiling simpler and therefore the Intello membrane just runs in a very, very simple way.”

Airtightness tapes were used to connect the windows to the wet plaster reveals, and time will tell whether the added movement that straw bales naturally have will cause cracks in the render and impact on the airtightness. Andrew recommends taking care to look at the building over time, checking the plaster and renders and making sure they are kept in good condition to ensure there is no moisture ingress.

The straw bale construction process provided an education for others

Dave was able to take a 12 month sabbatical from work and project manage the construction. He used a specialist substructure and groundworker who did the slab and drains and anything else below the ground. There were a couple of carpenters who worked collaboratively throughout on all of the timber elements. Straw Works provided teams of people to build the walls in their usual method, providing training to people who were interested in learning about the skills of straw bale construction. There was also a specialist roofing contractor.

Dave did all the airtightness taping and installation of the MVHR system himself.

Solar PVs provide the majority of hot water

The original design and PHPP model for the building was using a gas-fired boiler for heating, but Dave decided, partly for cost-saving, not to install the gas main.

“There’s a duct into the building which a gas main could be fitted into in the future, should someone want to do that, but fortunately with the introduction of PHPP 9, the primary energy renewables option enabled it to be certified. Without that, it wouldn’t have been certifiable because it’s electric-only heating.”

There are three heaters in the house in the hallway and bathrooms, and a post-heater in the MVHR which raises the temperature a little bit above whatever it has come through the heat exchanger with.

Haven Cottage has solar PVs which provide all the required hot water in the summer, with a bit of Grid electricity needed in the winter to top it up. Passivhaus Plus Magazine has calculated that the building is actually making a profit on the electricity side.

Lack of straw bale experience was no obstacle

Andrew didn’t have any experience of straw bale buildings before coming into this project, but wasn’t daunted as he found sufficient information around to understand the normal procedure, and from that work out how to do things slightly differently to meet the needs of achieving the Passivhaus standard.

“I would love to build more straw bale buildings. I’m not convinced it’s a solution to our mass housing needs but there is definitely a place for it, especially within the self-build world, because of that collaborative nature of the process itself.”

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September 8, 2018
HPH141 : What is a structural engineer? – with Andrew Collinson
Andrew Collinson explains the role of a structural engineer.

Interview with Andrew Collinson

Andrew Collinson is a structural engineer based in Herefordshire. His degree course in the subject was mostly based on theory, so his real training came from learning on the job for a big contractor. From working on site he moved to the design office and this background gave him a good understanding of the practicality of a design.

“The basic job of a structural engineer is to make things stand up”

The engineer’s involvement will usually begin with receiving architects drawings, and their job is to make sure the foundations are suitable, and any beams, floor timbers, roof timbers etc are the correct size.

Quite often they will be appointed through the architect who already has the building design but just needs engineering help in certain areas, like the foundation details or beam sizes. In these cases Andrew will be dealing with the architect directly and may not need to meet the client.

Most people don’t go to the expense of having a site investigation

For most jobs the ground conditions aren’t fully known before work commences. The structural engineer will have to make a best guess that will cover most circumstances, by writing down the assumptions that the ground is likely to be something suitable and that particular foundations will work with that. If, once the contractor starts digging, they find anything outside those parameters then they will come back to the engineer for a revision. Andrew says that in most cases standard foundation solutions will work, but there will always be a few that don’t, based on whatever surprises are found once digging starts.

“Until you dig in the ground you don’t know what’s under there so you either pay up front to get someone to come and have a look at it, or you take the risk when you dig that there might be a slight delay.”

And it’s important to remember, that everything can be resolved – it just may come with a heavier price attached!

Most cases are covered by Part A of the Building Regulations

Part A of the Building Regulations has a series of descriptions, and depending on the loading of the wall coming onto the foundation, it stipulates the minimum foundation width and how deep they need to be. Andrew says that most people could read off these tables if they knew what the loading coming down the wall was. There are variations for example if there are trees nearby or it is clay soil. Doing something smaller than is covered by Part A of the Building Regulations might need more careful engineering, and for that you would need to know soil conditions.

Natural materials are less well covered by the Building Regulations

Masonry builds are very well covered by the Building Regs. Timber frame for example is less well covered by the documents but often that is designed by the supplier who will have their own engineers.

It is more challenging to convince building control officers that materials like rammed earth, cob and straw bale have been designed right and won’t fall down, because they are not so well defined structurally and it is difficult to put a figure on their strength.

Andrew recommends that if you’re wanting to use one of the less common building materials it’s good to get an engineer on board early so they have time to research, and ensure there aren’t any difficulties that are likely to arise.

Steel or timber frame?

Steel is very stiff and an efficient material from an engineering point of view. Steel beams can be made to be near identical, and if you have limited head room it can be hidden effectively. It is readily available, fairly inexpensive and builders know what to do with it. With steel though you would normally need to get someone else to cut it for you and it has to be right first time.

One advantage of timber is that it can be cut on-site to get the exact size required. Every timber beam will be different so can have more of an aesthetic quality. And being a natural material it tends to have more movement, particularly from drying and shrinkage with green oak. Timber also benefits from less embodied energy than steel.

As a client, it’s important to manage changes in the right way

Andrew uses the example of a current self build project he’s working on, where the scheme is virtually complete but the client is very hands-on, has done a lot of research and is open to making changes. Andrew says that it is refreshing to work with a client like that who is able to understand the more technical details and can have more informed decisions about whether to make changes to improve the scheme. He does add however that there’s a fine line between that and an interfering client who keeps changing things!

“Because when you’re trying to move things along quickly, change needs to be managed as a process. Nothing wracks up bills more quickly than constant changes by the client because it can often involve going right back to square one and re-doing a whole load of stuff, which isn’t readily apparent to someone who doesn’t understand.

“So it is important to manage changes in the right way as a client. Which doesn’t mean to say that you can’t change things but you need to understand there may well be consequences.”

Use personal recommendation to find a good structural engineer

Recommendation is the best way to find a good structural engineer. Ask a previous client about whether they produced things to a reasonable time scale, were the costs reasonable, and even just getting a gut feeling about them from speaking on the phone.

“Do they feel a bit stand-offish, are they someone you could have a discussion with? Are they the sort of person that you would want working for you or not? Because sometimes if people don’t speak to you nicely on the phone the first time, how are they going to talk to you later on?”

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August 10, 2016
HPH089 : Can You Build to Passivhaus Standard Using Only Natural Materials? – with Bjørn Kierulf from Createrra
Bjørn Kierulf from Createrra explains how you can have the best of both worlds, using almost entirely natural materials to create a house that reaches the Passivhaus Standard.

Bjorn-Kierulf from Createrra

Interview with Bjørn Kierulf

Bjørn is an industrial designer and his wife is an architect, and together they run Createrra.

They have always been into building with natural materials, but now have adapted their approach to achieve Passivhaus Standard as well.

Natural Materials are Often Good Insulators

Most natural materials are fibrous and as such tend to be good insulators. They also have protection against summer overheating, though do need to be protected from humidity and moisture.

In order to reach high levels of insulation, however, a greater thickness of the material will be required (compared to artificial insulations such as aerogel).

Straw has the Ability to Take Structural Loads

When compressed, straw has very good insulation values and unlike most other materials also has the ability to take structural loads. While there are a number of load-bearing straw houses the structural calculations can be quite difficult which is why timber is usually used as well.

A further benefit is that it can be added to the compost heap, rather than landfill, when the building is demolished!

A Straw Bale House is Not a Flammable One!

Fire needs air, but straw bales are so densely pressed that there is no air to burn. When combined with a typical clay plaster it can have a 2-hour fire protection, meaning that even with a 1000 degree fire burning for 2 hours on the inside, there is no noticeable temperature difference on the outside.

Copyright Strawbuild

A Danger of Lime Render is High Water Absorption

Lime render is typically used by self-builders who are building with straw. Bjørn references the recent International Passivhaus Conference where the dangers of high water absorption in lime render were discussed. At Createrra they are looking into other options, such as industrial certified render plastered on wood fibre board which creates a thinner layer.

There is Often a Trade-Off When Using Natural Materials

Bjørn explains the difficulty of trying to get the right balance between using natural materials and materials that will last well under humid conditions. He sees this situation persisting for the foreseeable future, though predicting huge development and innovation in this area.

Natural Materials are the Basis of Innovative Products

Bjørn uses the example of a Swiss company who have experimented with using enzymes to treat the straw, thus giving it very different properties once it has dried out.

He also explains how a Danish company are using crushed mussel shells to make a foundation. The shells don’t absorb water and with air gaps they do have some insulating properties.

Much More Experimentation is Required for Natural Foundations

Bjørn feels they are still some way off an ideal natural foundation that he would recommend for clients. Hempcrete is another example he gives which, while appropriate for some areas, doesn’t have particularly good insulation values so wouldn’t create a thermally efficient building.

Airtightness is Crucial and Has Often Been Overlooked by Self-Builders

Problems are caused by humid air going through gaps in the construction and condensing in the structure.

One solution is to use a clay plaster, which is airtight, however it is difficult to achieve full airtightness because the plaster will always need to connect somewhere, such as where there are electrical sockets or beams going into walls.

Createrra are helping to develop a system using Ecococon straw panels where the airtight layer is on the outside of the straw insulation. This is in contrast to the typical way of doing things, where the airtight layer should always be on the inside in cold climates. These panels are wrapped in a recyclable airtight membrane and covered with wood fibre panels / ventilated façade. Early blower door tests have found that they meet the rigorous Passivhaus Standard.

The Ecococon panels are a combination of straw with wooden construction which are used to create the structure.

It is Difficult to Achieve an Entirely Natural Building

There are areas where construction using natural materials can be achieved easily, but areas like the foundations are more challenging. When also considering the whole of the interior, things like electrical cables being wrapped in PVC shows where there is still so much more that can be done.

Bjørn says that only around 500 materials were used in the construction of buildings at the beginning of the 20^th century, but now that figure is closer to 50,000. He sees the goal of having a building that at the end of its life can be put on the compost heap or recycled as being a greater challenge than solving the world’s energy crisis.

Createrra Have Completed 5 Passivhauses Almost Entirely From Natural Materials

The exceptions were the foundations and some of the roof membranes.

Feedback from their customers has been extremely positive, with perfect comfort levels maintained throughout the seasons. Bjørn thinks when people see the flat straw panels and the efficiency of the construction they see it as an attractive proposition, when perhaps they might not previously have considered straw.

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May 9, 2015
HPH078 : Externally Insulating a Bungalow with Straw Bales – with John Butler
John Butler tells the story of how he retrofitted a bungalow to be more energy efficient. Having carried out a lot of the work himself, he also shares what he’s learnt about airtightness and overheating.

Interview with John Butler

Although it had been John and Anna’s dream to build their own house, they found themselves muscled out of the local market for plots by builders.

John had attended a straw bale building course and was struck by some of the slides which showed a bungalow in the Czech Republic which had been wrapped in straw bales on the outside. With plenty of bungalows in the local area with poor energy efficiency their focus shifted away from the idea of building from scratch and instead towards searching for a property to retrofit and extend.

Straw was Always Part of the Plan, Whether for a New Build or a Retrofit

Having been on straw bale building courses and volunteering on builds, John felt that straw was a fun material and enjoyed the atmosphere on site from the types of people it also attracted. The more involved he got the more he realised its sustainability potential; being well insulating, breathable, accessible and a waste product from agriculture.

Gaps Need to be Eliminated Between the Bales and the Original Walls When Wrapping the Building Externally

Firstly the walls were levelled with clay to ensure no rodents or insects were able to crawl through the channels. Gaps would also potentially allow air flows which would bypass the insulation. Once the walls were level they were able to create a firm connection for clamping the bales onto.

Foundations Were Needed for the Bale Walls

As John was trying to avoid using concrete foundations they dug down and used gravel, compacting a layer, before adding further layers by the same method and finally finishing up with a strip of limecrete. The bales themselves were elevated from the ground by an insulated plinth brick wall to ensure that any moisture or leaks would run through the bales and drain out through the foundations.

Airtightness Should be Planned From the Beginning

Unfortunately airtightness was something John only became aware of later in his project! He discovered the importance of airtightness for energy efficiency through following others on social media, and then tried to incorporate it into the build. It was easier to do this in the extension, but in the bungalow they also managed to use tapes around the windows and are hoping that the plaster skim will keep it airtight. An air test they carried out before completing the finishes showed the weak areas and where they needed to go back and make adjustments.

John admits that with hindsight he would have planned better and carried out more research before starting, to ensure he had a really good airtightness system. He notes though that he would be interested in finding out about the sustainability credentials of the tapes and membranes themselves.

Initial Plans Were Designed and Drawn in Sketchup

Once John and Anna had thought about the spaces they were looking for in their new home, John drew their ideas in Sketchup before sending them to their consultant in the Czech Republic for suggestions.

A Vaulted Ceiling in the Living Area Helped Compensate for the Loss of Height From Insulating the Original Floor Slab

One option would have been to excavate the original concrete slab, however with the waste materials being sent to landfill and the risks of excavating within an existing building, it probably would have been wise to have knocked the bungalow down.

Instead, the chosen option was to use wood fibre insulation boards on top. This does mean however that they’ve not managed to get enough insulation in the floor, without digging the concrete slab out, for it to reach Passivhaus standard.

Solar Tubes and Roof Lights Provide Welcome Additional Light, But Have Their Drawbacks

The solar tubes have been effective in providing additional light without structurally taking up much roof space. In summer however, they have found that the direct sunlight beaming through their various roof lights has resulted in overheating of the spaces. This is something they had prepared for on the living room’s two large roof lights and installed external shading, however this does mean that using the shutters blocks the light from coming in.

Again, with hindsight John would have planned for this and used the PHPP (Passive House Planning Package). It is an annoyance for John that they had planned for saving energy and not being cold in the winter, but now have the opposite problem of overheating in the summer.

Self Building Has Been Satisfying But Draining

While John finds it very satisfying to point at the work he has done himself, he readily admits that it has also been all encompassing and draining. He recommends having people to help out at least with elements of the build to provide stimulation and motivation.

He sees self building as a balancing act, of paying people to do the labour meaning you could get back to work earlier, against the potential financial savings and satisfaction of doing the work himself.

The result is a house where he and Anna are enjoying their new light, warm and comfortable spaces.

Find Out More

Visit John Butler’s blog on his self build

Follow John Butler on Twitter

Transcript

Download a transcript of the interview with John Butler.

The Future of Housing – Premiere!

The date is set for the first screening of our documentary The Future of Housing – And How Airtightness Can Help.

This project was crowdfunded on kickstarter earlier in the year and on Thursday 4th December 2014 you can see what we’ve been up to!

The venue will be the Mildmay Community Centre, which was the UK’s first non-domestic retrofit to the Passivhaus standard. So there’s a building to check out too!

We hope you are able to join us for what will be a special evening. Capacity is limited, so get your ticket now.

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November 12, 2014
HPH057 : Low Impact Living Affordable Community (LILAC) – with Joe Atkinson

Joe Atkinson explains how the LILAC cohousing community came together and why their project is affordable for everyone.

Interview with Joe Atkinson

Around 2005, Joe got quite interested in fuel depletion and climate change. At the time, Joe had a nice job in IT and was getting paid well but felt that his life wasn’t very fulfilling. He was looking to get involved in something that would provide this fulfillment, and be in line with his ethics.

Eco-building Can be Really Simple

Joe did a Masters in Architecture at the Centre for Alternative Technology in Wales, at Machynlleth which he found absolutely fascinating. Being a bit of a techie, he really liked the technical side of the course and the physics behind how buildings work. This got him really interested in eco-building and he feels that a lot of it is actually really simple and backed by common sense. Instead of constructing thermally leaky buildings, we should be building sensibly.

How Joe Got Involved in LILAC

Joe went to an event where some of the members of LILAC (it was previously known as Leeds Eco Village) had a stall and were trying to recruit new members. Joe was at a point in his life where he was looking for somewhere to live. LILAC appealed to him as it was community of people who were building eco-houses that he would be able to afford. It was a no-brainer for him from that perspective and he got involved.

Being on the Board

When Joe joined, the group was really keen to recruit people on to the board. As Joe is the kind of person who commits totally to something new that he gets excited about, he decided to join the board.

Being on the board meant taking legal and financial responsibility for the project collectively with the other board members. Once Joe joined, he found his own little niche as the person who is a good team-builder. The board at the time was populated by lots of people who were really dynamic and making stuff happen, and his supportive role on the board really helped things along and played to his strengths.

Making Community Decisions

The board needed to think about how the whole community functioned and to avoid a two tier decision making system between the board and non-board members of the community. To address this concern they had an away weekend where an external facilitator came in and they did a whole community visioning and strategic planning exercise. They discussed the whole community’s goals and looked at different options for how to get there. Smaller task teams were set up to look at particular aspects of the project such as their relationship with the existing community, recruiting members, publicity and so on.

Joe joined the group that was to liaise with the professional team- the quantity surveyor, the project manager and the main contractor. The group went through a tender process to appoint a contractor.

Developing a Relationship with Leeds City Council and the Existing Community

The group had been in discussions with the Asset Management Team at Leeds City Council about various sites around the city and looked at two that were very suitable. It took the group a while to develop a relationship with the Council because when they first approached them, they probably came across as a bit of a rag-tag bunch of optimistic, well-meaning do-gooders who wanted to build some eco-topia. It wasn’t until they came back to them with a project manager and a serious business plan that the Council started to really take them seriously as a credible group who they could sell the land to.

They ended up choosing the site in Bramley, where they did a community consultation with the local residents. The community really liked the idea of what they were trying to do which gave the group a really good feeling about choosing that site.

LILAC- (L)ow (I)mpact (L)iving, (A)ffordable, (C)ommunity

LILAC is an acronym and stands for three main elements- Low Impact Living, Affordable and Community. The Low Impact Living element comprises of buildings built using the fabric first approach, an airtight construction with mechanical ventilation with heat recovery and the use of renewable materials such as timber, straw bale, lime render and lime plaster and triple-glazed windows. There is solar PV on the roof and solar thermal for the houses but not for the flats. They have used the Passivhaus approach, and are not far off from it, but haven’t spent the extra money to get the accreditation. They also have fewer car parking spaces- there is half a car parking space per home and two secure bike parking spaces. They have also set up community agreements for lots of other different aspects like sub-letting agreements, the car policy and the installation of electric car charging points.

The LILAC Mutual Home Ownership Society

Joe explains how the affordability model of LILAC is achieved through their legal and financial structure which is called The Mutual Home Ownership Society. It is essentially a housing co-operative and it uses a new financing model unique to Britain, as each member is effectively buying shares in LILAC Mutual Home Ownership Society Limited.

The total project cost is divided proportionally between each of the twenty homes, with each home representing the notional build cost for each unit, e.g. where the four bedroom units are more expensive to build than the three, two and one bedroom units. The amount that people pay to live there is related to their earnings. So all the members pay thirty five percent of their net income and the amount that relates to determines the exact size of their home size.

Once the debt associated with your flat or house has been paid off, you drop down to just paying ten percent of your net income. At that point you’re just paying a small amount as a service charge or ground rent. If you had a normal leasehold, you’d always be paying a small amount for things like insurance and management fees, etc.

The group is decoupling the value of the homes from the housing market, and that’s intended to keep these homes affordable in perpetuity. So they will always have the same ratio to earnings. Living at LILAC isn’t about investing in property; it’s about creating affordable housing as a legacy of the project and about buying the service of a home. It’s about homes as shelter, not as a nest egg or an investment.

LILAC is about Creating Good Quality Affordable Housing

According to Joe the process has worked so far and they are keen to support other groups who are interested in this model. At LILAC, they are not creating social housing, but affordable good quality housing. Joe can’t imagine anywhere else where you can get to live in an eco-house with all this amazing community and it’s affordable and you get to own it. This is potentially a model for anyone who is excluded from home ownership for whatever reason.

They have an active waiting list and they are receiving new applications all the time. This gives the current members an added level of security. If a member were to lose their job and couldn’t afford to live there anymore, there’s a whole process that takes place before they have to move out. And if at the end of that process they still have to move out, there is a waiting list of people that already want to buy your shares and move in.

Membership by Consensus

Everyone who lives at LILAC- apart from a couple of the very early founders – has had to apply to join. Because the group operates a consensus decision making model, everyone in LILAC has the right to veto someone applying. When Joe’s partner moved in, she had to write an application too and there was a period when somebody could’ve voted against her moving in. She sailed through with flying colours, recounts Joe happily.

Living in Cohousing Comes with its Own Challenges

Joe feels that it has been quite interesting going through a process where you have to prove that you can afford to live at LILAC because the affordability model relies on everyone declaring their financial situation. Although they now have a finance task team who look at all the finances, previously applicants had to submit their application with their financial information to their future neighbours which could be quite uncomfortable.

Living in an intensive cohousing community requires you to adapt and change the way you think. It can be really nice and sociable, but at times overwhelming. One of the real strengths of the cohousing design is that you have got your own place and you can retreat to it if you need to. But when you’re ready to come out, you’ve got a whole community there waiting.

Everyone at LILAC has been very supportive and open minded and have a real willingness to learn about living in a community and to learn about themselves as well. Living in cohousing challenges you in ways that you might not expect- it is a bit different to living in a traditional street in Britain. Joe admits that there have been a lot of challenges so far and there are likely to be more in the future, but they look forward to them.

Find Out More

LILAC’s website

Transcript

Download a transcript of the interview with Joe Atkinson.

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June 11, 2014
HPH056 : Which Build System is Best to Create a Low Energy Home? – with Piers Taylor from Invisible Studio Architects
Piers Taylor from Invisible Studio Architects explains what a build system is, why you would use one over another and whether any are preferable when it comes to creating low energy houses.

Interview with Piers Taylor

It was after seeing an inspirational talk by Glenn Murcutt in Sydney that Piers Taylor decided he wanted to become an architect.

He now runs Invisible Studio Architects as well as helps self builders on the BBC TV show The House That £100k Built.

A Building System is the Method Chosen to Construct the Building

Piers talks of these in terms of lightweight and heavyweight, and on-site and off-site construction, as most one-off individual homes will fit into these categories.

It is Easier to Achieve a High Quality Finish With a “Lightweight” Timber Building

When answering the questions of what is most efficient, most economically sound and best environmentally, timber is the material which Piers instinctively selects.

Timber is a renewable and readily available material, comparatively cheap, very workable on-site and can be used by people with varying degrees of skill and experience.

With Passivhaus requiring airtightness and quality of workmanship, Piers believes that a stud framed building is ideally suited.

It is Generally More Efficient to Build On-site Rather Than Pre-fabricated

Piers had laboured under the misapprehension that to build in a factory was the better way to do things, however when the expenses of factory rent, labour and heating are taken into account, when it comes to single buildings he believes that making things by hand on-site is more efficient.

There are Far Fewer Passivhauses Made of Masonry Than of Timber

While masonry buildings do have the advantage of thermal mass, Piers explains that it is harder to achieve a good quality finish with blockwork and wet trades. He puts this partly down to tradesmen putting them together crudely, with the assumption that someone else will be doing the finishes.

There is a Place for Concrete as an Environmentally Sound Material in Big Span Construction

Concrete has a smaller carbon footprint than if it were replaced in big span construction by steel for example. The problem is often the amount used, rather than the material itself, with laziness often causing its overuse. Piers doesn’t see a place for it in domestic houses, unless there is a particular desire for a thermal mass in the floor.

SIPs (Structural Insulated Panels) Don’t Have Any Real Advantages in Individual Houses

The advantages of SIPs panels lie in larger projects where economies of scale are a factor. For a domestic property they are expensive as the factory overheads are included in the cost, they contain all sorts of formaldehydes and glues, and they can’t just be made on-site.

While SIPs panels appear to be constructed quickly, it could be argued that making a timber frame in a factory would be just as quick.

Piers summarises that the ideal use for SIPs panels are for buildings with big roof spans, multiple buildings where an application is being repeated and where the walls are required to have a structural capability without the need for beams.

Big Volume House-Builders are Efficient in Terms of Their Construction

Piers accepts that, whilst we may not like what they build, big volume house builders have honed their methods of construction to be hugely efficient, as they’re making all the component parts on-site, and are therefore not paying factory overheads.

The Passivhaus Standard is Achievable With Any Construction Method But is Largely Dependent on the Quality of Craftsmanship

As Passivhaus is about achieving a good standard of fit to create building performance, rather than being interested in thermal mass, it is something that is not as easy to achieve with masonry.

Straw Bales May Not be That Efficient But Their Advantage Lies in Being so Locally Produced

Straw bales are relatively inefficient because they are bulky and not great insulators compared with some other materials. However, they are loved by some as a construction method as they are low tech, available, cheap and usually haven’t had to travel far – something that is sometimes overlooked with timber which, unless it’s come from a local sawmill, can often be shipped from Scotland, Scandinavia or Canada.

A Build System is Invariably Better When it Influences the Look of the House

Piers’ preference is to have the constructional system visible and legible to show what the different parts are actually doing, rather than the sanitised approach to current construction where everything is hidden behind plasterboard.

Piers sees a beautiful precision in timber, unlike the less aesthetically pleasing concrete block and steel which invariably have to be covered up because the fit is not good.

As with SIPs panels, there is often no call for steel frames in a domestic building as the spans don’t necessitate it and it is an expensive option.

Projects are Immeasurably Better if How They’re Made Forms an Early Part of the Building Decision

An early decision should be made on what is going to be the most appropriate material to use as this will give the building its character. By exploring all sorts of options you can gradually hone in with greater certainty on one thing.

Part of this early conversation should be focussing on efficiency, in terms of using materials intelligently and sensibly, without being wasteful, and these are things that need to be balanced with what the building is going to look and feel like.

Timber is the Best Material to Use For Keeping it Simple

Many self-builders will be doing a project for the first time so Piers’ advice is to keep it simple. Timber is quick, efficient, available everywhere, anyone can use it and it doesn’t harm the environment if it is chosen wisely, and with its only real drawback being lack of thermal mass, Piers considers it to be the best system to use.

Find Out More

Follow Piers Taylor on Twitter.

Transcript

Download a transcript of the interview with Piers Taylor.

Today in the Comments Section

Which build system did you use? And what were the pros and cons?

UK Passivhaus Awards 2014

The shortlist for this year’s UK Passivhaus Awards has been been unveiled. Good luck to all those involved!

Check out some of last year’s projects.

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June 4, 2014
Achieving Passivhaus Standard with Natural Materials – Bjørn Kierulf from Createrra

Is a straw bale construction really a good way to reach the Passivhaus standard?

And would a straw bale Passivhaus be robust over time?

In this article I speak to Bjørn Kierulf from Createrra about achieving Passivhaus standard with Ecococon straw panels.

Bjørn Kierulf was originally an industrial designer and won a number of national awards for his work between 1993-2001. He lives in Slovakia with his wife, who is an architect. Since 2007 their firm Createrra has focussed on Passivhaus and they have now completed 50 buildings that meet the standard.

They also have developed a reputation for achieving the Passivhaus standard using natural materials and have recently completed their fourth Passivhaus using Ecococon load-bearing straw panels.

Interview with Bjørn Kierulf

1. What is the difference between building with straw bales and building with straw panels?

The main advantages are quality and ease of execution. Anybody who has been on a traditional straw bale building site knows that the same problems arise every time: different sizes and densities of straw bales, protection of the straw from moisture during storage or building and lots of details that are not always solved properly. The work is very labour intensive and is often done in workshops. Professional builders are aware of the risks involved and have problems pre-calculating the amount of work needed. Usually the only suitable period for building is after collecting the straw from the fields and before the winter sets in.

Straw panels Ecococon can be pre-produced at any time of the year and assembled in just a few days on the building site. The panels are immediately covered with a membrane providing airtightness and protection against rain. The precision of the panels are 1-2mm and the straw fills even the smallest gap at 110kg/m3. Each panel is pressed with a force of 5 tonnes. The surface of the straw is completely flat and can be covered with clay plaster.

Going from straw bales to straw panels makes it possible for any builder with experience in wood to assemble a quality house built from straw in record time at predefined costs.

Lintel assembly

2. How does the process work?

Usually any house plan can easily be turned into a 3d panel model. Because of modularity, there are nearly no constraints and the architect is free to produce what he thinks is best. After a panel plan is produced, also the exact costs can be determined. Production of the panel is started after the buyer and the architect have signed the panel plan. Usually a few weeks later the panels are delivered on site ready to be assembled. Assembly of 100 m2 or more of wall is possible in one day with a skilled 4-man team.

3. What determines the size of these panels?

All panels are 400 mm thick. There is a standard panel of 120 cm width x 300 cm height, but smaller panels are custom made to suit the design. The minimum width or height of the panel is 400 mm. To complete the modularity, there are panels with an inclined top (0-45°), braced panels, sills and lintels to be installed above the windows. Special elements are in development for integrating shadow devices in the openings.

4. Are there any limitations as a designer by using these panels?

You need to take into account structural concerns (above 2 floors the structural design should be carefully thought through in advance). It helps if the designer has an understanding of the modularity and types of panels, especially for cost optimising the design. The designer should also understand the concept of airtightness we apply and design accordingly connections to other building elements.

5. What is needed on site to put these panels together?

The panels can be heavy (50kg/m2) but can be moved on rollers or by tilting over the corner to the necessary place. For unloading from the lorry and building the second floor a crane is usually needed. Other than that a good screw driver and screws are all you need. Special clamps can be obtained by the producer to pull the panels together before adding the screw.

Screwing the panels together

6. How are these panels rendered? And is the render the airtight layer?

The cut straw surface is an ideal surface for clay render. Over the wooden construction 5 mm thin wood-fibre boards are tacked on for the clay plaster to stick better. There is no special preparation necessary before rendering. We did our first two houses with an airtight clay render inside, but this was cumbersome and resulted in a lot of work. The result was very good in the end, but we used a much simpler method on the next houses we built. We now add a membrane on the outside of the straw, between the straw and the wood-fibre board. It is very important that the membrane has an Sd<0,2m, in effect the construction is open for vapour to cross all layers, but any air-draught is minimised. It is usually air movement that transports most moisture into the construction, and then causes damages from condensation. We have achieved airtightness as low as n50=0,14 1/h and have calculated this construction with WUFI to be sure that there is no moisture build up over time.

Clay plaster and rounded edges at the window connection

7. How does building with panels affect the speed of construction?

As one builder said: “Everything took longer than we thought, except the assembly of the panels.” Of course the time saved on the building site is spent in the factory producing the panels, but overall the advantages are huge. With the airtightness concept and the wood-fibre board on the outside we have developed a system that is in general very effective to build.

8. How cost effective is this construction method?

The higher the cost of labour, the more cost effective the solution is. Production is already partly mechanised and can be made even more effective in future. The truth is, that building with natural materials does cost more than with widely available materials. A major part of the price difference comes from lower turnovers and necessary higher margins. Even if today a straw panel house with clay plaster probably costs a bit more than a traditional house, it does not have to be so if the system gets adopted widely.

9. What’s the biggest challenge of building this way?

Tradition. In the building industry it takes sometimes generations to change perspectives. Traditional approaches, ridgid regulations and ignorance need to be overcome. Technically there are no obstacles, only a huge potential to develop the system further.

Passivhaus across the border in Austria built with straw panels

10. Do you think that Passivhauses built with natural materials can ever be a large part of the market?

If we want to take the last IPCC report seriously, I think aiming at anything less would just be irresponsible. Passivhaus has proven its place effectively over the last twenty years. But after managing global warming, humankind will still be faced with an even larger challenge: limited resources and the necessity of proper materials management. The only solution I can see longterm is the implementation of Cradle to Cradle concept across the building industry. With the massive amounts of materials involved, major use of natural materials will be inevitable. This does not exclude the development of new materials based on renewable sources.

11. Have you achieved Passivhaus standard with buildings constructed of other natural materials?

Our first Passivhaus was built as a light wood frame construction with blown in cellulose and 30 t of clay bricks and plaster. We have since then moved to massive wood construction with elements from the German producer Lignotrend, creating exclusive interiors with wooden acoustic ceilings and clay plaster. The inclusion of straw is a natural step in the right direction — straw, wood and clay, all renewable materials available in abundance can be turned into beautiful Passivhaus buildings and I sincerely hope many other architects will follow in our footsteps.

Passivhaus in northern Slovakia built with straw panels

April 17, 2014
HPH042 : Can a Straw Bale House Reach Passivhaus Standard? – with Fran Bradshaw from Anne Thorne Architects
Fran Bradshaw from Anne Thorne Architects shares her experience of building an ultra low energy home using natural materials such as straw bales, lime render and clay plaster. The house is also aiming for the Passivhaus standard.

Interview with Fran Bradshaw

Fran Bradshaw was a builder before she became an architect but she’s still very much interested in the practical side of things. A lot of the work she does at Anne Thorne Architects has a focus on sustainability and community-based work.

This episode is about a straw bale house that she’s building for herself in Norfolk, UK.

Straw Bale Building Allows People to Get Involved

Fran got inspired to build her own house with straw after working on a project for Haringey Council.

Local people helped both design and construct the eco-hub, which Fran describes a wonderful experience: “It was really exciting being on site and just seeing how enthusiastic people were and what a great community benefit it was.”

Volunteers help construct an eco-hub in Lordship Park

Natural Buildings Feel Different

Upon completing the eco-hub, Fran noticed that lots of people commented on the quality of the environment.

The acoustics, humidity and the colours are all very different in a building constructed of natural materials – people liked this.

The finished eco-hub (credit: Rae Parkinson, Straw Works)

Getting the Details Right is the Biggest Challenge

While there have been straw bale houses in mainland Europe that have reached Passivhaus standard, there is yet to be one in the UK.

The challenge is in turning what is a rather chaotic building material into something that is actually very precise, because to achieve the Passivhaus standard all the details on all the joints have to be very neat and carefully airtight.

The timber frame goes up on Fran’s house

The Team Must be Led by Someone Experienced

Although one of the reasons Fran chose to build her house with straw was to involve her friends, she leveraged the experience of Straw Works.

So guided by one or two experts, the rest of the team members just need to be enthusiastic and prepared to do a little bit of physical work.

Barbara Jones from Straw Works examines the quality of a bale

A Reed Thatch was Chosen for the Roof

The site is very close to the Norfolk Broads and Fran discovered that reed thatch was being cut a mile away.

She describes this as too tempting: “Yes, it’s just a very good roofing material. It’s very durable. It’s got a natural insulation value so there was no reason not to use it. I mean it’s more expensive going with thatch rather than clay tiles or something and that was my extravagance really.”

Reed Consistency is Important

In order to thatch well, the reeds need to be the same strength, the same length and have the same diameter.

This is partly to do with how well and methodically the reeds are cut, which is why reed cutters are given long contracts to tend a certain patch.

The Airtight Layer in the Roof is Below the Thatch

The thatch is on fireboard which is moisture permeable but airtight. It’s taped at the joints so that the airtight layer is just underneath the thatch.

Warmcel – recycled newspaper insulation – then goes in between the plasterboard and the underside of the fireboard.

The Floor is a Reinforced Raft

Fran says the connections between the walls and the floor are always tricky to get right and, as this project is aiming for the Passivhaus standard, she opted for a reinforced raft.

While there was a level of compromise here it is quite economical on concrete and it gave Fran the opportunity to employ a local contractor (whereas many parts of this build have needed specialists).

Due to Timber Penetrations the Airtightness Layer is on the Outside of the Wall

Normally the airtightness layer would be on the inside of the house to avoid warm, moist air from the room going into the wall fabric.

As there’s timber inside and the straw is outside the timber frame, in this case it would have been very difficult to make the airtight barrier work well (because there are timber penetrations and quite big bits of timber).

After discussing the best approach with their airtightness expert Paul Jennings, they decided to use the outside layer of render as the airtight layer.

So the concrete slab is the airtight layer in the ground which connects to the lime render on the outside of the straw walls and then to the fireboard on the roof.

A Theoretical Risk has been Managed

There is a theoretical risk to having the external lime render as the airtightness layer as Fran explains: “Inside you’ve got clay plaster and that effectively is airtight as well but for instance at the windows where it’s all taped it’s on the outside, so theoretically you could get moist air going at the junction between the internal plaster and the window and condensing within the straw.”

They are pretty confident this won’t be a problem because all the materials are moisture permeable and hygroscopic as well, and the MVHR ventilation will be operating at the coldest times.

Moisture monitors will also provide on-going data, so should any issues arise they can be addressed.

The Clay Plaster and Lime Render Will Need to be Maintained Every So Often

As the lime render is the airtight layer, any cracks that appear could affect the airtightness and thus the energy efficiency of the house.

However, this is just part of using these materials – these cracks will need to be repaired. Fran is expecting this at the end of the drying out process but is not sure how often the maintenance will be after that.

Nearing completion!

There’s a Role for an Airtightness Geek!

Working with these kind of materials and trying to achieve high levels of airtightness, Fran can definitely see the need for a ‘very precise airtightness geek person’ who has a close eye on all the details as the build progresses.

This could develop within building firms or be a role that is taken on by a broader group of people.

Prepare for the Key Moments

There are certain key moments where you’ve got to be absolutely spot on, so find out where these are.

Planning for those moments and making sure that there are the right people around at the time will stand you in good stead.

You Can Always Do Things for Less BUT . . .

While Fran knows that there are always ways to cut costs, she derived great pleasure on this project from being able to work with people who she knew were going to do a really lovely job.

Although she may have paid a little bit more it’s been worth it and she still believes it’s been reasonably economical given the materials used.

Invest in People

Look carefully at what people have done in the past and trust your judgement about them.

For work like this Fran is not in favour of endless competitive tendering. On the whole she’s selected people who she enjoys working with and who charge a reasonable rate for what they do.

Passivhaus is a Language We All Understand

The first air test of Fran’s house recorded a figure of 0.9 air changes per hour.

She still hopes to meet the Passivhaus standard: “I’d really like to certify it because it’s a language that we all understand. Either you’ve achieved it or you haven’t. If we don’t achieve it, that’s okay, we’ll have got very near it and the difference in the building performance between not quite achieving it and achieving it is really quite small. It’s just very exciting for me to have this house and this opportunity to live in it.”

Find Out More
- Anne Thorne Architects
- Read their blog all about Fran’s straw bale house
Transcript

Download a transcript of the interview with Fran Bradshaw.

Our Question for the Comments Section

Have you managed to incorporate natural materials in your project? And if not, why?

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February 18, 2014