Strawbale: An Okay Second Choice
Curiously, everything except wood frame and concrete block is now loosely referred to as an "alternative" building material. It is as though pumicecrete, Rastra block, strawbale and, infuriatingly, earth, all have those fringe "green" qualities that are only used by the quasi-rebellious or the cutting edge. Of all the materials listed here, only one has a history ranging back to the very founding of cities, is ubiquitous, is low-energy-embodied and has proven longevity. It seems to me that all other building materials on the planet are alternatives to earth.
Nevertheless, being a proponent of one of the alternative materials leaves me open to invitations to explore the others. It must have been about 12 years ago that I was approached by a fellow contractor in Santa Fe who was possessed by the idea of strawbale; he asked if I would be interested in helping promote it. We talked and within a few hours, I balked. Those who have read this column for a while know how I feel about the hyperbolic promotion of Rastra block -- it's really not the materials or the system I dislike, quite the contrary; it's the sleight-of-hand way it is promoted that I despise. I have (now, now, I reprimand myself: Be fair and say "had") the same problem with strawbale.
Although I think strawbale promotion no longer relies so strongly on the primary environmental harangue that material not used will be pollutingly burned, every strawbale contractor that I have spoken with does eventually come around to explaining the consequences of not using the stuff. Yes, it's true that straw left over after the grain is extracted is burned. But the argument that using the harvest remnants has a favorable impact on the environment is specious.
If you attempt to run the numbers, which is difficult, you will probably reach the conclusion that even if straw were used for 5% of housing starts (very optimistic) the total consumption of the field waste (difficult to calculate) would be something far less than one-tenth of 1%. I imagine the diesel exhaust expelled making the bales and then getting them to the site adds more particulate matter to the atmosphere than burning the same quantity in the field. As with Rastra block, there has been a lack of critical (or even sequential) thinking when it comes to advocacy.
It used to be that strawbale was advanced as affordable. We are definitely past that little delusion, unfortunately, in my view, because it should be inexpensive to build with straw. Strawbale construction is on a par with adobe in terms of cost when, if the code writers based their wisdom on empirical evidence rather than theoretical, it should be much less expensive. But here, as with earth, engineers and compliance offices are suspect of the material. Tradition indicates that they should have faith in it.
Historic strawbale barns in the Midwest (which, by the way, are often semi-subterranean) have walls that have borne their roofs under terrific snow loads for a century with little sign of subsidence. It seems, however, that we cannot trust that evidence and so must build a post-and-beam frame to bear the loads and use the straw as curtain walls. Pity, because then there is no savings in either lumber or dollars.
Where strawbale walls are fantastic is in energy efficiency once built (assuming the other components like roof and windows are built to appropriate standards). This, the secondary level of environmental concern, deals with the conservation of energy, not the questionable issue of pollution.
Here, grudgingly, I must concede a tenth of a point in the earth vs. straw competition; it has a fantastic R-value even when compared to double-wythe cavity walls of adobe; unexcelled values, really. But, straw does not bank heat as earth does, a minor point perhaps, given its insulative value.
So, given access to baled straw (which most of the world does not have) I cannot be chagrined or disapproving when it is used in construction. But earth still reigns for longevity, tradition, and availability.