Straw Bale Construction/Techniques/Foundations

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[edit] Foundations

There are several options for making footings under Straw bale walls. Standard concrete footing/foundations or thickened-edge-slab-on-grade foundations have been typical though do not fit with some environmental considerations as cement uses large amounts of energy in its production. Bales can also be stacked over stem walls with joisted floors. With load-bearing straw-bale homes rubble trench foundations or Earthbag construction foundations are increasingly used, as an alternative to conventional footings. Some pioneer designers are even using rock-filled gabions or earth-filled "bastions" in lieu of concrete. Straw bales have been used to insulate the floor from the slab, or to provide subgrade perimeter insulation, but this must be done with care, due to the importance of isolating the bales from undue moisture. (Moisture levels higher than 18% support mold growth in both straw and wood.)

In the same way as a rubble bed, a bed of shells has been used with much success in Denmark. At a thickness of between 119.4 and 124.9mm conductivity is between 0.120 and 0.112 W/mK. Compared to industrial products (such as expanded ceramic or spun glass or rock) shells therefore provide good insulation as a nearly carbon neutral industrial waste product.

While thinking about the design of your foundations, or more specifically the foundation pad, this is the time to think about heating options. One of the options gaining popularity is in-floor radiant heating. You can read more about this in the section on building services under heating and cooling.

This area can still be a major cost as most building codes still require a footing of at least 12 inches or to the frost line, whichever is deeper. They then require that a pad be poured that is at least the width of the bales being used(possibly three inches less if you are going to use rigid insulation on the outside of the foundation) for at least 8 inches above final grade. This is the least restrictive code that has been written to date. If you are not being bound by code (rural area) you might be able to get away with using something much less energy intensive than concrete.

  • Note: Definitely check with the local code compliance or county property appraiser to get their input. Give them a bit of the information here and other places to warm them to the idea. If you are going to be bound by code you need to know that and follow it. Or alternatively, sell that piece of land and move elsewhere.

Further reading

References

  • Thermal insulation of mussell shells, three different densities (2001, Jørgen Munch-Andersen, Birte Møller Andersen and Danish Building and Urban Research.) These tests were to measure the conductivity of Mussel shells carried out in 2001 and can be downloaded in Danish from the Straw_Bale_Construction/Resources/Technical_Studies Technical studies section of this book. The shells were dried in a 60celcius oven before the tests. The tests were carried out following EN 822, 823 (1994) and ISO 8301 (1991). The margin of error is +-2%. There is an article about the tests in The Last Straw Journal (2005 Issue #52) Part of "Straw Bale Houses - design and material properties".
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