Is Retrofitting With Exterior Straw Bale Insulation Practical?

When I wrote my post What's The Greenest Insulation? It's Getting Harder To Decide Every Day, a number of commenters wondered why I didn't include straw bale. The main reason was that it is usually considered for new builds, but the real problem facing much of North America and the UK is insulating what we have rather than building new; we don't need a lot of new houses right now. Straw has a low R value, so for it to do a good job, it has to be too thick for a retrofit inside a home.

But not Outside a home. Many houses are retrofitted by wrapping in expanded polystyrene; why not wrap it in straw? Rob Hopkins of Transition Culture points to the work of Keven Le Doujet at the University of Cambridge, who wrote a massive thesis on the subject in 2009.

Keven was inspired by the S house, an Austrian passivhaus built of all natural materials, including a timber panel house insulated externally with straw, plastered in clay and then covered with a timber cladding.

The whole concept and the sustainability performances of the S-House are remarkable and inspiring. Whereas the use of straw bales is a proven solution contributing to very energy efficient new buildings, little attention has been given to it as a retrofit solution until now. Would it be possible to make existing UK buildings more energy efficient using local, renewable and non-toxic materials by insulating them externally with straw bales?

There are also a lot of benefits in insulating on the exterior. It can reduce thermal bridging, there are fewer limits to thickness, and it encloses the thermal mass of the existing house. But there are also problems; Roof overhangs may be insufficient, and windows will be affected as they will now be in deep recesses, reducing natural light. But overall, the author concludes:

SBEI distinguishes itself from many conventional EWI systems thanks to its better hygrometric properties and its significantly lower embodied energy. SBEI also has the potential to be a carbon sink and the materials making it are local, cheap, renewable or abundant, non toxic, biodegradable or easily disposable and require little processing.
These added benefits can realistically result in socio-economic benefits such as adding value to agricultural by-products and promoting local employment. Straw and clay are also adaptable and creative materials, easy to shape, light enough to carry without heavy and loud machinery all of which providing opportunities for construction sites that are socially inclusive and empowering.

There are a lot of things to love about straw bale; it may well be the greenest insulation. It should certainly be considered for renovations as well as new construction.

Found at Transition Culture, where Rob concludes:

What shines through this excellent study, unusually for an academic study, is a real taste of what might be possible, and what it would actually look like if we used local strawbales to retrofit some of our worst housing stock. The benefits, in terms of reskilling, locking up carbon, supporting local farmers, hugely increasing energy efficiency and so on would be huge.