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Enough straw is currently produced every year in North America to meet all our residential building needs. And the same is true in many other parts of the world, since grain farming is common across most cultures and regions. This fact alone is enough reason to move toward using this abundant renewable resource for construction purposes, even if it held no particular advantage over other building materials. The fact that straw bale buildings can out-perform buildings made from other materials and lighten the load on the planet, as well as on our pocketbooks, makes it a triply effective material with which to build.

What’s in an “R”

Let me offer a slightly different take on what is likely happening with the R-value of a straw bale wall. I question whether the best, most controlled scientific testing would show anything like the R-50 that we have all heard about for [three-string, 24-inch-wide] straw bales. The test used gives a fair first approximation but is widely recognized as being less accurate than ASTM236 Hot Box testing. That said, the difference between R-30 and R-50 is really not that great. It is certainly less than the difference between R-10 and R-30, an apparently equally distant pair of values. This is because R-value, a number derived from U-factor, is the ability of a substance to resist heat flow. To understand how that plays out in actual performance, we need to convert R-values back to U-factors, the measure of how much heat flows through a substance under a predefined set of conditions. U-factor is 1.0 divided by R-value and vice versa.

An R-10 wall will allow 1/10 of one Btu (0.10 Btus) through one square foot of wall in an hour if there is a one-degree Fahrenheit temperature difference between the two sides of the wall. An R-30 wall will allow 1/30 of a Btu (0.033 Btus) through under the same conditions. An R-50 wall will allow 1/50 of a Btu (0.02 Btus) through. Obviously, if your wall is R-10, you are going to make a much bigger dent by increasing the R-value to R-30, than if your wall is R-30 and you move to R-50. It’s the law of diminishing returns. At some point, common sense and the pocketbook say it’s good enough.

However, the tested R-value has little to do with how the wall performs in the real world. This is much truer for straw bale walls than for stud walls. Thermal bridges occur with regularity in stud walls — in fact, at every stud. Straw bale walls have fewer thermal breaks, by far. Moreover, the R-value is measured under what can be called static conditions: you can only take your readings once the wall surface temperatures have stopped changing. This takes about 20 minutes for the average window, an hour or two for a wall, and three to seven days for a straw bale wall. In other words, the conditions at the two wall surfaces must not change for days on end, or the R-value is invalid. Well, how often in the real world does that happen with one’s house? The time it takes heat to travel through a straw bale wall is about 12 to 15 hours. By the time the heat has made that journey, diurnal (daily) temperature swings are driving the heat the other way in the wall. This means that a straw bale wall can give you the real-world impact of an R-50 wall, even though it is really only R-30.

— Nehemiah Stone built his own straw bale house in Penryn, California. The explanation above is adapted from his straw building list serve and presentations on thermal performance of straw bale homes.