The first rule of building with straw is to keep it dry ("good hat and goot boots"). This includes the foundations. Moisture will eventually find its way into even the best wall so foundations must allow any moisture to drain away. An impermeable foundation will trap moisture near the straw and cause it to degrade quickly. Care should also be taken positioning membranes for the same reason.
One possible solution is building a stone, lime mortar and rubble infill foundation. Build it up above ground level to protect against rising damp and rain splash. Other alternatives include rubble piers, rammed earth depending on the site and the desired aesthetic.
Car tires have recently gained in popularity as a cheap but labour-intensive foundation material.
Concrete footing/foundations or thickened-edge-slab-on-grade foundations are another option. A building with concrete foundations is generally much easier to attain building approval for. While the most common solution, the cement used in concrete does require large amounts of energy for cement production. Concrete foundations contribute up to 70% of the ecological footprint of a straw bale house.
To further prevent water damage, on top of the foundations one will generally find a toe up, a basis for the straw bale wall. It is made out of two parallel 5x10 (2x2") or 10x10cm (4x4") beams. These are spaced evenly slightly less than the width of one bale apart. This is important to avoid water getting into the bales. The space between the toeup beams is filled with somewhat water resistant insulation like perlite, sheep wool, crushed shells or alternatives less friendly to the environment.
Pier foundations with joists raised well above ground level are a relatively common option in Australia and Germany. Even if the piers are poured or pre-fab concrete a vast savings on concrete is made. This technique also has the added bonus of allowing the use of straw bales as underfloor insulation as they are raised well above grade.
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.
The use of straw as underfloor insulation is usually discouraged because straw will rot and grow mold if it gets damp (>18% moisture content). Avoiding moisture is especially important in kitchens and bathrooms where flooding is possible due to plumbing leaks/broken washing machines/over flowing bath tubs etc.
A commonly used option for insulating joisted floors is sheep's wool.
A bed of shells has been used with much success in Denmark as a combined rubble bed and insulation. 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.
Another practical alternative for underfloor insulation is hempcrete, consisting of hemp fibers mixed with natural hydraulic lime. As this is a fairly lightweight mix, it can also be applied as roof or ceiling insulation.
Recycled foam glass is yet another option. It's cheap, insulating and hydrophobic: no capillary effect, and no loss of insulation value if placed in a high humidity environment.
One experimental building in Belgium tried to avoid moisture problems in a straw insulated floor with a clever foundation floor structure. From bottom to top: big stones, sand, strong plastic tarp, sand, recycled building bricks (spaced so air can flow between them), straw blocks, and rammed earth for the floor.
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.
Foundations 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.
- Jay H. Crandell, Design Guidelines for Frost Protected Shallow Foundations (2Mb PDF), 1994, U.S. Department of Housing and Urban Development
- 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".