Building with Straw Bale or Adobe: Three Options for Footings and Stem Walls

Reader Contribution by Dave Larson
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In keeping with our goal of creating a pay-as-you-build desert homestead, we used our own labor and, as much as we could, hand tools. This included digging out the trenches for the footings and stem walls of our first two buildings with a pick and shovel.

 By the time we were ready to build our main house, however, my age (the wrong side of 65) and the magnitude of the task at hand dictated acquiring something heavier than pick and shovel for moving dirt.  Buying a used mini-backhoe, dubbed the Tonka Toy, solved our problem. It was powerful enough for our tasks, while small enough to maneuver among our desert plants without too much damage.

Building #1 – The Adobe Bear Cave

When we built our three buildings here on the desert homestead, we had three different sets of conditions for our foundations. Our first building had the heaviest walls by far, being solid adobe. But it was also the thinnest wall, about half of our two-foot plus plaster straw bale walls.

We believed that a poured concrete stem wall over a wider concrete footing, both with steel rebar in the pour, would give us the most economical stem wall while preventing extensive settling and cracking in our adobe wall. Above the footings, we formed in and poured a single pour stem wall about 19 inches deep and 14 inches wide. The extra width on the stem wall gave us a plaster shelf for the adobe. Because we were putting on a moisture barrier of felt paper between the stem wall and the first course of block, we put rebar pegs into the stem wall and mounted the first course of block with holes drilled to match.

Our homestead site is on a bajada, a sloping alluvial fan, so we had to consider the slope of the ground as well as the wall weight and width. The Bear Cave is 320 square feet and the Annex is 192 square feet.  With these smaller buildings, having a slope of about eight inches from high to low points, we simply built with the slope, digging in on the uphill end and building up on the downhill.

The Annex : Our First Straw Bale Building

The Annex, our straw bale utility building, had thicker and lighter walls than the adobe Bear Cave. In the various books we read, owner/builders using straw bale construction have used everything from a trench of rock to railroad ties for a foundation. We had a lot of adobe on hand and not a lot of money, so decided on a treated adobe block foundation for the little Annex.

We used the same “recipe” for adobe block as we did in the building of the Bear Cave except that the liquid used to make the “mud” for the blocks was about 10% asphalt emulsion. The asphalt mixture made the blocks as little darker, as expected. We laid two rows of block and filled the space between the rows with more wet adobe with asphalt. The cost of the foundation, including the 10 gallons of asphalt emulsion, was about $50 in 2006.

When the wall cured, we applied a good coat of pure asphalt emulsion to the outer surface of the stem wall. We let the coating cure about a week and then backfilled. The stem wall was built high enough to provide clearance of about 10″ between the ground and the first course of bales.

A side note on the asphalt emulsion blocks durability: To protect from sheet flooding during monsoons, we had built a 2 foot eyebrow berm around the uphill side of our homestead. We had 8 blocks made with asphalt emulsion left over and put them on the weather side of our berm. They have endured the Arizona sun and rainfalls as heavy as 1 1/2 inches per hour and show modest rounding on the edges of the block. They remain intact and strong.

Our Straw Bale Casa: At Ground Level

The main house was larger and had one more course of bales than the Annex to give us the nine-foot ceilings we wanted, so it posed yet another set of problems. Because the site was larger with more slope, the footprint of the main house dropped about 18 inches from high to low.   

For economic and environmental reasons, we chose not to have a contractor come in to tear up and tamp down the base for the house. So, we did it ourselves. After stubbing in the plumbing and waste lines, we moved some of the dirt from the septic digging and brought the main house building site nearly to grade.

Our nearby sand and gravel people brought in a load of AB and we spread that over the site with the Tonka Toy and then leveled it with a rake and tamped it with a hand tamper. With the nearest equipment rental facility about 75 miles one way, we used hand tools such as the hand tamper for most projects.  After wetting and tamping a few times, we had a good base for our stem wall and footings. Again using the Tonka Toy when we could and pick and shovel where we couldn’t, we dug into our new base for footings and stem wall.

We built a riprap retainer for the leveled AB base, then laid two courses of block 27″ apart to provide a 1 1/2″ plaster shelf inside and out over a reinforced concrete footing. For insulating the floor of a straw bale house using block as a stem wall, we suggest filling the space between the courses of block with vermiculite or some such material. 

It has been more than six years since we began building the Bear Cave and we have been living in our straw bale Casa for two years. All three of our explorations in foundation buildings have worked well. Our adobe Bear Cave is stable and has no cracks. We have no erosion on the Annex foundation and our main house is snug, comfortable, stable, and paid for. Our homestead  has taken a lot of work. The work continues, but every minute of labor is worthwhile. Building one’s own home is truly a labor of love.

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