Meredith Bowles from Mole Architects shares his experience working with timber as a construction material and explains the benefits and drawbacks of the different build approaches.

Interview with Meredith Bowles

Meredith Bowles, Founder and Director of Mole Architects, shares the pros and cons of different timber construction methods. Initially working on additions and conversions of Victorian building stock in London he found the masonry cavity wall approach unsatisfying, messy and difficult. So shifting towards timber construction was transformational. Meredith undertook his own self build project in 2002 and created a sustainable timber home that is still ahead of the government green guidance 20 years later. In this chat, Meredith sheds some light on timber build approaches including stick build, oak frame, CLT (Cross Laminated Timber), SIPs (Structural Insulated Panels), prefabrication and more.

Stackyard in Suffolk

Back to basics – What is timber?

Timber is wood that is prepared for use in building and carpentry, and if you visit a builders’ merchant in Britain this usually refers to white wood which is most likely to be Scandinavian, Lithuanian, or Russian softwood. In timber construction, this is most commonly assembled into stud walls which are sheathed and filled with insulation.

Although the UK has a medieval history of timber construction using large oak framing, the North American approach of using timber stud walls is still not as common as brick and block.

Over the last 20 years, engineered timber has been rising in popularity and involves binding or fixing fibres, strands, veneers, etc. into composite products to increase the strength and efficiency. Plywood is the simplest version of this, but CLT, LVL and glulam are also common.

With the wide range of timber products and construction methods available, timber is a fast growing sector of the construction industry in the UK and an obvious choice as we look to build more sustainably.

Alde Valley Barn ‘stick build’

Qualities of timber

Timber is a sustainable material with low embodied carbon impact

Timber is a renewable resource with low embodied carbon that can be engineered in a wide variety of ways to produce different results. Although timber in itself is sustainable, Meredith stresses that we must try to reduce our consumption of it and use it as efficiently as possible.

“Some advocates of timber make a claim that the growth of timber absorbs CO₂, and therefore, using more timber is a good thing. It’s a perverse kind of argument. If we are going to scale up timber construction throughout the world in order to try and minimise the use of more carbon intensive construction methods, then there’s going to be such an enormous demand on the available construction timbers. Surely it’s better to make it more widespread but to engineer the use of it, so that you are using less of it. I can’t square myself the argument that using more of anything is a good thing.”

Timber frames are better suited to accommodate insulation

Not only is timber a regenerative resource which during its lifetime will help with carbon capture and biodiversity, it also offers a logical way to better insulate our homes. Meredith explains how:

“There are a number aspects of sustainability, the primary one is cutting down our fossil fuel use and carbon emissions, which is what has been fuelling climate change. To do that, we need more insulation and if you’ve got a timber frame building, you’ve got a lot of space in it that you can fill up with insulation. It’s a really easy way of making a really well insulated building, because you’re building it out of stuff with a lot of space. The alternative masonry construction requires you to build two leaves with a really big gap in between to fit the right amount of insulation in, which becomes quite cumbersome and complicated.”

Timber can be utilised as a design feature or entirely hidden

Timber construction allows for more versatility and freedom with your design. Depending on how it is used, timber stud walls can be hidden, as it is sheathed on both sides, or incorporated into the design of the house through exposed beams or entire walls.

Timber is a warm material

Wood offers both an emotional and physical warmth that people respond to. The visual honey tones create a warm and soft interior but there is also a literal warmth in the sense that it has insulative properties. When you feel a masonry surface it absorbs the heat from your skin quickly and feels cold to the touch, whereas, timber doesn’t transfer the heat as well and feels warm.

Securing finance and insurance for a timber build can still be a challenge

Insurance companies and lenders still seem to view timber as non-traditional and therefore as a higher risk. Meredith highlights that these types of entrenched views are rarely as a result of statistical analysis and instead are built on a societal conception. As mentioned before on the podcast, the fairy tale of the little pigs has ingrained a sense of safety in brick-built housing and a sense of insecurity in one made of sticks, despite all of the evidence that suggests otherwise. As a result of this, there are more obstacles to overcome during timber construction which is stopping it being embraced by the mainstream.  

“People say, ‘Well, I can’t get a mortgage for timber frame building’. Of course you can, but it’s not the norm, and therefore it’s slightly harder. Plus, I suppose if you go to any old builder down the road, and say, can you build a house, then they say ‘Oh, yes, we can build out a brick and block’ and they wouldn’t know where they’re going with timber. So it’s a whole industry which needs to change or is changing.”

Fire testing timber products

Fire risks are a common concern, but often exaggerated

The GLC (Greater London Council) has banned the use of timber in any external walls, because of the fallout from the Grenfell tragedy. Grenfell had no timber in the construction, and that was not the cause of the fire, but there’s been increased concern and cautiousness over anything that may be a problem in terms of fire as a consequence.

Timber needs to approached in a similar way to steel in order to resist fire damage

We all know that wood burns, but the reality of timber construction is that the timber frame is covered up, usually with plasterboard. Similar to our understanding of steel in construction, plasterboard is one of the best ways to stop fire reaching the material behind it and by encasing the structure, the heat is not going to reach the timber for some time. Incidentally, Meredith highlights that testing has shown timber to out-perform steel in the case that fire breaks through this barrier. As a result steel actually deforms, bends and can lead to structural failure much quicker than timber does.

Meredith recognises that much more research needs to be carried out in relation to fires, but does highlight that the government could be taking similar steps as Germany and Scandinavia by bringing timber into the same legislative framework that steel currently is in.

Knowing the charring factor of your timber can reduce the chance of structural damage during a fire

The charring factor in timber is another important consideration when it comes to the fire safety of timber framed houses. This refers to the amount of time it takes for the surface of the timber to char and lose its structural properties and how much of the surface is affected. As a result of this testing, timber used in construction has been oversized in order to compensate for the timber that would be lost during a fire so that it remains structurally safe.

The Black House

Meredith’s House

Meredith built his house in 2002 and got inspiration from projects in North America and Scandinavia, which had to deal with the harsher climate. The house consists of a prefabricated timber frame which was insulated using recycled newsprint/cellulose. Meredith’s home is in The Fens and it is common for building foundations to be piled with a concrete ring beam and a beam and block floor. However, to reduce the embodied carbon of his home, Meredith chose to use glulam timbers to create a ring beam above the ground, avoiding contact with water, keeping the structure well ventilated and dry. The finished house was even fitted with an air source heat pump, MVHR and solar canopies to help mitigate overheating.

The glulam timber ring beam

Construction of The Black House

When the Future Home legislation comes into force, the government guidance will have finally caught up with Meredith and his millennium home… just 20 odd years later.

Stick build is the most straightforward method for timber construction.

Stick build is when individual timber elements are cut and combined on site to form the structure. It is often sheathed by plywood or OSB to create a cavity for insulation.

‘Stick building’ at Riverview

Completed home – Riverview

Moisture must be carefully managed in a timber structure

Materials like rockwool or glass fibre insulations are water resistant, which means any build-up of condensation in the wall cavity won’t be absorbed by the insulating material. This means that any moisture will run down the material and begin to pool at the bottom, slowly rotting the plate at the base and causing unseen damage that you won’t discover until it is too late. Meredith highlights that natural insulating materials such as cellulose, recycled newspaper, wood fibre or wool all have the ability to absorb moisture and then pass it out again. This means they take it in during the winter months and release it in the summer, helping to keep the timber structure itself dry and in good condition.

“Building physics has finally been properly understood and we put the layers with a greater resistance to moisture on the inside, and layers with a less resistance on the outside.”

Natural insulations are often not as effective as petrochemical insulations

When comparing the insulative properties of natural and petrochemical insulations, natural materials often don’t perform as well and therefore more is needed to reach the required insulation value. In projects where space and budgets are tight, this can be challenging.

“PIR foam is about twice as effective as any of the natural insulants, so you only need half the space and often when you’re trying to figure out how to fit everything in, that feels like it’s a much better solution. But of course, those insulants don’t absorb any moisture, so the way in which the building needs to be detailed suddenly has to change. We tend to use both forms of insulation in different circumstances because there are benefits sometimes of using a PIR that can get wet without losing its insulation capacity. The building physics of it are quite complicated, and it is necessary to get to grips with that to know how to use one insulation over another.”

Cross Laminated Timber (CLT) is generally used for larger projects

CLT is a form of engineered timber which is created by gluing together at least three layers of solid lumber. The layers are oriented perpendicular to the last, helping to increase the structural rigidity. The configuration of CLT differs from glulam beams in which the laminations are all orientated the same way.

Cavendish Avenue CLT construction

Completed home – Cavendish Avenue

CLT allows you to create a really strong structure whilst retaining the sustainability credentials of standard timber. However, Meredith says that for small scale, self-build projects, this building material is often not needed and would result in using more timber than necessary. There are cases where Meredith has used this material in private residences, but it was often to support the complex geometry of a design, so it is best to assess on a case by case basis. CLT is often best suited for large projects, such as public spaces, schools, etc and is a great alternative for steel beams.

Dune House – CLT

Structural Insulated Panels (SIPs) offer many structural benefits but tend to rely on petrochemical insulation

SIPs are quite broadly defined but Meredith describes them as “a sandwich construction of plywood or OSB on the outside faces, which are glued together with a polyurethane foam insulation, a bit like those ice cream with a wafer on each side.”

SIPs have amazing structural properties and it can be very easy to produce a highly insulated, airtight building using them. SIPs are incredibly lightweight so are easily transported, can be put together quickly, and will reduce your foundation costs.

However, Meredith has concerns about using SIPs in his projects as they require the use of fossil fuels to create and have a potential to release harmful gas from the insulation material and formaldehyde in the glues on either side of the sandwich.

Brettstapel solid timber construction avoids use of glues

Perhaps a less common timber build approach is Brettstapel. This is a simple construction method that allows you to create solid timber walls without the use of fixings or glues. Layers of timber are connected with wooden dowels instead of being glued together, which makes it strong. Meredith believes that, similarly to the use of CLTs in self build construction projects, Brettstapel is not the most efficient use of wood on the market, but the prospect of creating a structurally strong material without the use of fixings or glue is interesting.

Timber cassettes arrive on site

Prefabrication can improve quality control, airtightness and speed up on site construction

MMC (Modern Methods of Construction) refer to the prefabrication of buildings off site before on site assembly. This has the potential to greatly improve quality as the work is carried out in a controlled environment with the right tools on hand and properly trained staff which should lead to a more consistent output. Prefabrication also can include integrated services and windows, and much of the airtightness detailing.

As a result, prefabrication can offer a fast assembly on site, as the majority of work has been completed in a factory. This also minimises the chances of any delays due to adverse weather conditions.

“We’ve all heard the stories, my house was built in five days. Of course, we all know that’s not true, the prefabricated timber frame went up in five days, it’s probably been two months of doing the groundwork before, then it’s another nine months to finish the job. But nevertheless, it’s a very quick way of getting a watertight building and stick built but prefabricated cassettes is the next level up.”

Timber panel lifted into position

The benefits of MMC rely on good quality control practices being in place

Meredith highlights that the outcome of your build is heavily reliant on who is building it, and whilst prefabrication has the ability to increase quality control, it still relies on people to be implementing and performing the necessary checks.

“There’s no reason why a really low tech way of constructing something can’t be as effective and give you as good a quality as someone who’s using a much more automated system. I’ve visited various kinds of factories and our clients have received big consignments of prefabricated timber panels that have been simply incorrect because the quality control wasn’t there, irrespective of the fact that they have all of the machinery so they should be able to make it work. And conversely, we’ve worked with small contractors who are building it in a shed using a jig, two blokes and a tape measure, and it’s gone together perfectly. So, I don’t think there’s one way that definitely works and another way that doesn’t. I think different companies have different reputations and that’s how you should determine it. Talk to people who have used their products before, don’t just look at the fancy machinery and think that that’s got to be better.”

Development with TOWN

Solid oak frames might be beautiful but are not the best use of timber

Meredith is measured in his approach to construction, meaning that every project must be considered on an individual basis and no construction method is entirely off the table. However, he does raise some concerns with solid oak frames in modern construction.

Within the self-build world there is an attractiveness of a contemporary oak framed home, as it is what we see in the traditional vernacular of our buildings. However, with the developments in construction and a focus on sustainability and insulation, large oak framing for small projects is a very inefficient and unnecessary construction method. This is because once you have fitted the stud walls between the oak beams for your insulation, the stud walls could actually hold the structure on their own, making the beams a very large, beautiful but unnecessary feature. In larger buildings, the oak beams may be of more use to the structure.

“We talked earlier about the idea of minimising your use of materials and the idea of engineering something which is efficient is part of sustainable design, not overusing things. So there’s a slight illogic of using oak frame in a contemporary domestic building”

Meredith avoids using tropical hardwoods due to their link to deforestation in the rainforests

When buying timber look for FSC (Forest Stewardship Council) or PEFC (Forest Stewardship Council) accreditation which Meredith believes are governed well.

However, some time ago Mole Architects did stop using tropical hardwoods due to the links to deforestation and logging in rainforests.

“I think even though there are supposedly chains of custody and forms of logging that are deemed to be sustainable, it seems like the destruction of rainforests is of a different order or a different scale of environmental problems to the cutting down of forestry land that is grown for timber from softwood. So, you have to make a decision as to what you choose to do in terms of environmental provenance of the product.”

Every project is different, so try and choose the best method for your build

“My ambition as an architect is to explore ideas and every project is different, every client’s different, and I don’t come with a predetermined idea about what I want to do. So for me, the excitement is each time, it’s an exploration of a new project and what’s going to come out of it. I think that’s the same with the structure and the construction. There are so many different ways you can do things, the idea that one is better over another I don’t think it’s true. You have to figure out what is right for your project.”

Wuduhus home

Find out more

Visit the website of Mole Architects

Wuduhus – Recent project completed in collaboration with Beattie Passive

Collaborators that were mentioned: Håkon Vigsnæs and Kiss House

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