Plants as building materials – five key questions

Earlier in the summer I gave a talk on this subject at the Royal Welsh Agricultural Show. This post is a referenced/hyperlinked reflection based on what I spoke about.

Space and water heating in our homes is responsible for around a third of the UKs CO2 emissions. Under the Climate Change Act we need to reduce our emissions by 80% from 1990 levels by 2050, so we need to think about how to heat homes more efficiently. We cannot simply build new homes; at current replacement rates at least 70% of the buildings that we will be living in by 2050 have already been built. The UK has the oldest housing stock in Europe, with 38% of our housing dating from before 1946. So we need to talk about retrofitting these homes.

As well as thinking about housing in terms of its relationship to CO2 emissions, it’s worth thinking about the social and financial costs as well. The cost of poor housing to the NHS is estimated to be around £2.5 billion/year across the UK. In terms of manageable health risks, this puts it in the top 5; it’s less expensive than obesity (£5 billion) and not far behind smoking (£3 billion). Meanwhile here in Wales, 23% of households are in fuel poverty, meaning that they spend at least 10% of their income on energy costs.

So we need to retrofit existing houses to make them more energy efficient. It then becomes a question of what materials we use to do this, and how we might choose between materials. In the post-carbon economy, where fossil fuel based materials are either unavailable or costly, we will need to do more with plants and other biological materials. Consequently, I am interested in developing more plant based building materials, and to my mind there are five key considerations about how we might do this in the future.

(1) Which plant should we use for which end purpose?

This sounds like a completely obvious question, and one that we should maybe have answered already, but the history of building is opportunistic; very often plant based building materials have been based on waste products or co-products such as straw or hemp shiv, or decisions have been made because of what is locally available. This isn’t necessarily a bad thing and can result in highly imaginative and creative solutions. However it does suggest an opportunity for design – how much better would a material be if it was bred and designed for a specific end purpose? All designers need a design target, and plant breeders are no different. So we need to have conversations about what the building industry actually needs a product to do, and then translate this into design criteria for plant breeders. It may well be that a suitable plant already exists, or that a particular variety of a plant already in use would serve our purposes better. More on this in another blog post…

Left: Hemp shiv, originally a byproduct of rope and sail making industries. Right: straw, historically a byproduct of grain production.

(2) How many ‘eco’ building products do we actually need for any given application?

There are lots of plant based building materials available already. If we take the example of loft insulation, we can get it made of hemp, sheeps wool, seaweed, cork, cereal straw, mushrooms and wood. Not to mention recycled products like plastic bottles, textiles and shredded newspaper. (Yes, I know these are not all plant based products, but you get my point about there being enough ‘eco’ choices available already). Loft insulation is an extreme example and there is certainly less choice for many other building products, but those of us interested in developing products need to be brutally honest with ourselves; does the world really need my new product? If we were to take pharmaceutical development as a model, any new product would need to have a clearly defined sphere of application and to have been demonstrated to be at least as good if not better than other products that were already available.

‘eco’ insulation materials. How much choice is too much?

(3) Natural materials are safer?

There is a pervasive idea within the eco-building industry that natural materials are safer than man-made ones. This is in no way evidence based; it stems from a world view. Many of us have an intrinsic attraction to natural materials, but we shouldn’t forget that nature is full of toxins, poisons and allergens. We have probably all heard scare stories about volatile organic compounds (VOCs) from synthetic carpets and paints, and yet the most serious indoor air quality risks in UK housing stock relate to cold and damp and the mould that results. Cigarette smoke and inadequately ventilated combustion appliances are also highly significant, as are outdoor sources such as the particulates from diesel emissions; these are not natural, but they are far more worrying than any choice we might make about building materials. The tragedy is that we have normalised these failings. If we are going to discuss the relationship between building materials and air quality, we must be willing to be value-neutral about it and base the discussion on proper epidemiology; eco-blindspots are dangerous and self-indulgent, and the risks posed by natural materials should be taken seriously.

mouldy wall
Mould is a significant cause of poor health in UK houses, but receives much less attention than the volatile compounds emitted from man-made building materials.

(4) How well does the product perform, and what is the performance gap?


Most of the plant based building materials being developed are insulation materials, and it is vital to retain focus on the goal; reducing CO2 emissions. If an insulation product does not provide the necessary performance, we shouldn’t use it. Unfortunately, we cannot simply rely on manufacturer’s data in this regard because of the difference between test procedures and real life. Many products (eco and otherwise) are fiddly to install and result in a performance gap; a much lower performance in practice than that predicted by a numerical model or calculation. Real life monitoring is not something the building industry is particularly good at, and there is a lack of data on what types of failures occur and over what time frame. What we do have is data from thermal imaging studies on how poorly insulation is often installed in the first place.

Thermal imaging revealing badly installed sheet internal wall insulation.

Architects need to design out building details that are particularly difficult to get right. There is also a real need to regard building as a skilled profession, improve training opportunities and attention to detail. Builders used to be artisans, and in some countries they still are. In the meantime, whenever we’re creating a new product we need to be mindful of how easy it is to do a good job of its installation. There isn’t a clear divide here between material types that are good and bad, but it is critical to understand where common mistakes are made and to design these out when developing new products.


(5) Where does the water go?

Understanding how buildings are designed to function in terms of water vapour is critical to prevent degradation of the building fabric. There is a significant problem with damp and mould in much of the existing housing stock. We need to increase living temperatures, and that means insulation, but we also need to ensure that we have a plan for how we are going to manage internal air and ventilation. A key risk with the use of synthetic insulation materials for solid walls is that they rely on the idea that it is possible to create a perfect vapour barrier to prevent any moisture from getting into the walls. This may work in theory, but if not properly installed there is a significant risk of creating condensation within walls that then rots timbers or harbours mould that will negatively affect indoor air quality. Materials that are vapour permeable may reduce this risk, but since air holds very different amounts of water at different temperatures, the underlying building physics is not simple. Managing moisture and developing a ventilation strategy is a critical part of any insulation retrofit project.

Final thoughts

We need to retrofit the existing housing stock to be warm and comfortable and properly ventilated. This requires products and systems. An individual product or building component may be relatively simple, but systems are not, and we must think about houses as systems of interacting components, with an added layer of complexity when we consider the role of the building occupant in both producing their living environment and modulating it. Plant based building materials may well have a role to play in retrofit of our housing stock but it is vital not to lose sight of this bigger picture.


This post is based on work undertaken as part of the BEACON project, which is funded by the European Regional Development Fund via the Welsh Government. The event at the Royal Welsh was made possible by a grant from the Welsh Government via their NRN Low Carbon Energy and Environment programme.

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