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.”
Online Learning Resources
Check out these sites for online resources, including webinars and training:
- The Alliance for Sustainable Building Products
- The Green Register
- The National Self Build and Renovation Centre
- Green Building Store
Find out more
Learn more about the work of BEACON
Read Judith's Blog
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