A friend sent me a link on some Straw Bale construction being done in the UK. It’s a quite interesting idea. Building with straw has been around since Paleolithic times, so it’s not exactly a new idea. But now there is increasing research being done, and it’s beginning to look like straw bale construction has a place in both Green and Sustainable building.
But like any construction technique, there are good things, and a few issues to carefully consider.
On the plus side, it’s about as sustainable as you can get. Straw is an agricultural by-product, the dry stalks of cereal grains such as corn, wheat, barley, oats or rice. Besides being used as a building material, it’s used as animal feed and fuel.
A schoolhouse was built of hay bales in Nebraska, around 1896. Without an outer covering of plaster or stucco, it provided years of service before being eaten by cows in 1902. Probably a lesson to be learned there.
How do you resolve the statements that straw is both a wonderful fuel that ignites easily and provides heat, and a fire retardant which will protect your home from wildfires? It’s the same as the difference between a Kleenex tissue and a phone book. In the Kleenex case we have a combustible material with lots of surface area exposed to air. A phone book is also made of paper, but the pages are compressed together so that oxygen cannot flow through it. Without a steady flow or oxygen rich air, a fire cannot be sustained. Loose straw burns easily, compressed straw, not so much. It occurs to me that extremely compressed straw might start to behave like spray foam at some point. Both are basically lightweight, combustible materials that trap millions of tiny air bubbles within a matrix to provide insulation.
Compressed straw also behaves like Shou Sugi Ban, Japanese charred wood siding. When the outer layer of the material chars, it provides an effective barrier against further combustion.
We live in the era of climate change. For most of us, the future is going to be humid. Much more humid than in the past, when most building codes were established.
When building with any organic material, especially straw bales, moisture is the enemy. The current research and practices suggest not using a vapor barrier with inside or outside the walls. The insulating material needs to be able to dry both ways. The traditional outer wall material for straw bale construction has been earthen clay. There are some advantages to be had with clay. First, it’s flexible, so it can expand and contract with the seasons. It is also breathable, being a vapor retarder rather than a vapor barrier. Drywall or clay, or both can be used for interior walls. Both will allow moisture to migrate through their surfaces. Remember that even OSB is a vapor retarder.
When it comes to the foundation and roof, a vapor barrier is the ticket. Keeping water from wicking up from the foundation is critical, as is preventing roof leaks from draining into the straw bales.
One of the potential negatives is that it may be hard to find local people familiar with straw bale building. Local authorities will likely want to apply inappropriate building codes, like vapor barriers, which will create problems. Plus, unless you are using a manufactured product, it’s hard to know if your bales have been properly dried and compressed. Without that, determining the correct R-values is just guesswork. With any building, craftsmanship is paramount, especially around windows, doors, and any other penetration of the building envelope. All of these must me absolutely airtight and leak proof.
I noted in my research that a lot of straw bale walls seem to be around 16 inches (400mm) thick. Not sure where this dimension came from, it could be the result of a standard size for baling machines. In any event, 16 inches definitely qualifies as super-insulation. When you look at the interior photos of straw bale houses, there is something solid and reassuring about those super thick walls. That, along with some excellent indoor air quality a good choice for a home. That it’s one of the most sustainable building products is a major bonus.