Some of you that saw my blog yesterday may wonder why anyone would choose to build a wall out of straw. You may be remembering the first of the three little piggies. There are several reasons. First, straw is a relatively cheap by product of growing wheat or rice. Second, straw has great insulating properties. Third, contrary to what you might think, straw is highly fire resistant. Finally, straw bale construction is relatively uncomplicated and straightforward.
Straw is relatively cheap, but this is a case where the term “relative” is the one you should focus on. While the straw itself isn’t a big expense, it takes more labor to build a straw bale wall than to build traditional framed walls, and in construction labor is a very major percentage of your costs. Part of the reason it takes more labor is just familiarity, people aren’t used to building with straw, but straw bale construction is probably about twenty percent more expensive than typical building costs. The hope is to make back that cost through energy savings.
This piece by Nehemiah Stone is a little technical for the likes of me: http://www.ecobuildnetwork.org/pdfs/Thermal_properties.pdf. But the conclusion is fairly straightforward, straw bale construction is three times better at preventing heat loss than typical wood stud construction. That may translate into an energy saving of as much as 75%.
Here’s another technical piece that examines the flammability of straw bale walls: http://www.casacalida.be/downloads/brandveiligheid/Fire_safety.pdf. The conclusion of this article is that plastered straw bale walls easily can meet the requirements for a one hour fire rating (and maybe meet the requirements for a two hour fire rating). If that doesn’t mean much to you, then in simpler terms it means that straw bale walls meet the requirements for interior fire walls in commercial buildings, and may meet the requirements for exterior fire walls. Typical residential construction only has a thirty minute rating.
In part II, we’ll take a quick look at how straw bale homes are built and link to a few sites that can give you more detail.