Straw Bale-Building: I'm a Believer

Brothers and sisters, I’ve seen the light streaming through the wheat fields. I’ve made my confessions, renounced the old ways and become reborn as an acolyte. Witness: Just after my straw-bale building class last weekend, I had a bit of an evangelical discussion about straw with a friend who’s had her head in green building for some time now. Her concern was: “What if the straw gets wet?” (Actually, my friend phrased it like this: “I’ve heard the straw always rots.” So many unbelievers, wanting for the truth!)

So I told her a funny little story about a building I used to have that, because it lacked gutters and was built right on the ground with nothing to lift it up and away from moisture, attracted a happy little colony of famished ants who ate at it until it basically fell apart. (Or would have fallen apart, if I hadn’t have given it a little push.) Point is, no building material is impervious to nature’s many whims — read this book yet? — but straw, even on its own, beats wood pretty squarely.

Let me break it down for the unconverted. Straw is what farmer folk call the stalks leftover from harvesting grains like rice, wheat, oats, barley and hemp. Hay, on the other hand, is a grass, eaten by horses and moo-cows and the like. And, while bales as bales are new – baling machines were first patented in the 1870s — people have been building huts and houses out of straw for centuries.

A few reasons why:

  • It’s a waste product — one that until quite recently was burned by farmers looking to clear their land. In the United States, farmers annually clear enough straw to build 4 million 2,000-square-foot homes each year — nearly four times the number of homes that are currently built every year. Four million homes, out of what would otherwise be burned or thrown into the compost. Compare that to stick-framed houses, built of hundreds of 2x6s hewn from wood grown specifically to make 2x6s. (As one of our instructors pointed out this weekend, “There are no old-growth straw fields.”)
  • Straw’s insulating properties far outweigh whatever you might throw up between those 2x6’s. A code-built residential wall — 2x6s in-filled with fiberglass batts — has an R-value of 19 or 22. A nice tight straw wall ranges from R-30 to R-50. And fiberglass has something like 125 times the amount of embodied energy as straw — 125 times the energy to produce an inferior product.
  • It’s user-friendly: Straw bales can be used for load-bearing walls — where the walls support the roof — or as insulating infill on a post-and-beam structure.
  • As long as you give it a good roof, protection from ground moisture and breathable protection from the elements, it can endure almost any climate: Witness, friends, the straw-bale Burritt Mansion in Alabama, where the average humidity ranges in the shower-like percentiles. The place was built in the 1930s — but when some straw-scientist types recently drilled its walls for straw samples, they found it to be just as dry and stable as the day it was stacked.

One secret to dry bales is letting them breathe. Trapped moisture creates problems: mold and decay. Most straw-bale buildings are covered in lime-based plaster, which is a bit like Gore-Tex for walls – it sheds water, but it’s also breathable. Meaning any humidity gradients — more ambient moisture inside than outside, say — are allowed to balance out. Vapor is allowed to move through your walls, not stay inside and mold everything up. So while you don’t want to wrap your walls in waterproof (and, ahem, nonbiodegradable) plastic, like you might a standard wall, you do need to protect it from direct moisture, just as you would with wood framing. That means lifting the main walls off the ground with a good foundation and footing, and using gutters and whatnot to divert rain from the roof and base of your building.

So, ye unbelievers, straw can rot, but it takes a lot of bad planning and poor design to get it there. 

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