Straw Bale Construction/Techniques/Foundations
The first rule of detailing a straw building is keep it dry, and this includes the foundations. It must always be expected that moisture will eventually find its way into even the best wall so foundations must allow any moisture to drain away.
This is typically acheived by building a stone, lime mortar and rubble infill foundation that is built 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.
The golden rule is don't make an impermeable foundation that traps moisture near the straw, as this will cause it to degrade. So it follows that concrete foundations are not an option. Care should also be taken positioning membranes for the same reason.
Also note that straw should not be used as underfloor insulation because if it gets damp it will rot. This is especially important in kitchens and bathrooms where flooding is possible due to plumbing leaks/broken washing machines/over flowing bath tubs etc.
The above is based on the Natural Building Materials lecture series given at the University of Bath (BRE Centre for Innovative Construction Materials)
There are several options for making footings under Straw bale walls.
The most common is standard concrete footing/foundations or thickened-edge-slab-on-grade foundations. Though this is the most common solution it does not fit with environmental considerations as cement uses large amounts of energy in its production. The advantage of nearly all forms of concrete foundations is that it is much easier to attain building approval due to its general acceptance by building officials. This could change when environmental considerations become a integral part of building approval. Concrete foundations constitute by far the least ecologicaly sustainable component in a straw bale house. Depending on the design choices made a concrete foundation contributes up to 70% of the ecological footprint of an average straw bale house.
The easiest most ecologically sound and traditional is directly on grade, on the ground. A staw bale wall due to its thickness impairs light loading to the underground allowing this approach. In practice quite a few succesful straw bale buildings have been built in this way. In general though this is not code permited for permamnent habitation and generally shortens the potential life span of the building. Local circumstances dictate the viability of this approach obviously if the site is prone to regular rainfall or flooding it will not work. Slightly raising the building site above the surrounding area and/or a stem wall of stone blocks raising the footing of the straw bale wall greatly improves the prospects for a long life of the building. Besides directly on-grade there are several other ecological alternatives to concrete foundations.
Pier foundations with joists raised well above ground level are a relatively common option in Australia and Gerrmany. 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. 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.
- 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".