The bales are made of rice straw – the husks that remain after the rice grain is harvested. This is a sustainable building material that uses a waste product. Rice is grown in the Sacramento River Valley in Northern California, and disposing of the straw after harvest is a serious issue. Farmers used to burn the fields, but doing so creates huge pollution issues and burning is no longer allowed. The rice farmers are very happy if someone wants to take the straw off their hands. In addition, the embodied energy – the energy needed to create the building products – is almost zero because any energy inputs were for the edible rice, not for the straw.
Our bale walls are 20 inches thick: 18 inches of straw, then an inch on each side of wire mesh and stucco. Some early measurements of single bales reported R-vales (the insulation value) at nearly R-50. For comparison, a standard insulated wall with 2 x 4 studs is typically R-10.
The problem with measuring a single bale is that a wall consists of many bales with gaps between them and also gaps between the bales and the framing. Gaps allow air to penetrate, reducing the insulation value. It’s not easy to measure the R-value of an entire wall, but a few brave laboratories have tried. It seems that the insulation value of an entire wall is roughly R-30, three times that of a standard 2 x 4 wall.
To make sure we achieve that, I spent several weeks after the straw bale party, while there was still lots of loose straw around, wedging handfuls of straw into every gap, especially around window and door frames where the bales didn’t always press tightly against the frame. It must have worked. The blower door test – described on the linked page – found that the house is really tight.
We live in a high-risk area for fire. We’ve designed the house to survive a fire. The exterior stucco walls are non-flammable. The roof uses concrete tiles that have a Spanish-tile look but aren’t as fragile.
In wildfires, hot embers blow well in front of the actual fire line. These embers can be sucked into the attic of a house through the eave vents, and, indeed, this one of the most common ways that houses catch fire in a wildfire. We don’t have attic vents in the eaves but, instead, baffled vents that slip under the roof tiles. In addition, the eaves themselves are not wood, which would be typical, but cement board, a non-flammable building material made from cement and cellulose fiber.
The bottom line is that there’s almost nothing flammable on or close to the house. Any landscaping that might easily burn is kept away from the house. We do have oak trees next to the house, but the landscape situation around us makes it very unlikely that a crown fire could get started in the trees.
We hope to never test it, but we think we would survive a wildfire that sweeps through our area.
© 2021 Randy Knight