It Starts with a Slab

Concrete slab for a straw bale house
Construction starts with the concrete slab. The slab is an important part of the passive-solar heating – it soaks up solar energy on winter days – so it is 5 inches thick rather than the usual 4 inches.

Concrete has a high carbon footprint. Our slab has a somewhat smaller footprint because roughly 50% of the cement in the concrete has been replaced with fly ash, the primary by-product of burning coal in power plants. Fly ash actually makes the concrete stronger. Not to mention that it provides a use for a waste product that is otherwise hard to dispose of without making an environmental mess

The concrete – later stained – is our finished floor downstairs, so the surface was carefully smoothed when poured and expansion joints were cut after it hardened.
Post-and-Beam Framing
Framing for a straw bale house
The first straw bale houses, in Nebraska in the 1890s, simply stacked up bales that became load-bearing walls. That can still be done in a simple structure where the ground is very stable, but not in earthquake-prone California. Most straw bale houses today use some sort of post-and-beam framing where 4 x 4 posts spanned by header beams provide the load-bearing frame of the house with the bales stacked between the posts to provide the substance of the wall. Post-and-beam also allows you to frame in windows and doors – pretty useful things that are hard to achieve when the bales themselves bear the load.

Actually, seismic building codes in California require you to go beyond a simple post-and-beam method. As the photo shows, there are steel reinforcing pieces inside the walls that were later covered by the bales.

The headers – one seen in the photo – are engineered wood beams. These have the same strength as a solid wood 2 x 8, but instead of cutting down a tree they are made from wood scraps and epoxy.

For practical reasons, only the downstairs has bale walls. The smaller upstairs is conventional framing but with 2 x 6 studs rather than the usual 2 x 4 studs to allow for thicker wall insulation.
The Bales

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.

Wrapping bales in a straw bale house
People wonder whether the straw will decompose. Or attract pests. Or be a fire hazard. The answers are no, no, and no. Straw will rot if wet or in contact with the ground, but dry straw will last essentially forever. Straw in excellent condition has been found in Egyptian tombs thousands of years old. The straw has no nutritional value – the nutrition was all in the rice grain – and does not support pests. Although mice do find that it makes a nice bedroom if they can find an entry point. Termites much prefer wood to straw, and there’s a reported case in which termites ate a house’s wood windows but left the straw.

As for fire, stuccoed straw bales are exceptionally fire resistant. The thick, hard coating is a formidable barrier, and the interior is pretty much an anaerobic environment because residual bacteria in the straw use up all the oxygen. In one fire nearby a few years ago, all that was left of one residence was a straw bale wall. Most of the straw-bale houses that were in the path of recent large fires in Northern California survived.

To finish the walls, the exterior is covered with waterproof barrier, then the exterior and interior are wrapped tightly with a heavy-gauge wire mesh seen in the photo. No vapor barrier is used on the interior because the bales need to “breathe” to avoid moisture buildup. The mesh is then stuccoed inside and out. The exterior stucco has added color so that the outside never has to be painted.
Insulation and R-Values

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.

Blown in cellulose insulation
The upstairs has blown-in cellulose insulation, held in place with a thin vinyl sheet, rather than the more common batting. As the photo shows, blown-in insulation fills every little nook and cranny, leaving no air gaps. The 6 inches of insulation itself is R-20. There’s some heat loss through the framing – lumber has poor insulation value – but additional insulation with drywall on the inside and stucco on the outside. Overall, the whole-wall insulation is about R-15.

The inaccessible “attic” above the second floor has 12 inches of blown in insulation, giving R-36.

By far the weakest link, the one that limits the limits the thermal performance of the house, is the windows. Even really good, double-pane windows have poor insulating value. Despite ads that would try to convince you otherwise, window coverings don't really help. The window issue is described in more detail on the Passive Solar Design page.

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