foundation

Part 3 of our 15-part Q&A series on all aspects of green building from the publishers of HealthyHouseInstitute.com. Click here for the introductory post and furthur details.

Question: What is the healthiest type of foundation?

Answer: There are four basic foundation types: concrete slab, crawl space, basement, and pier. In general, pier foundations are the easiest to build in a healthy manner because they lift a house up off the ground. When a house is well separated from the soil, it’s more difficult for radon, subterranean termites, and ground moisture to get into the living space. Pier foundations are often used on hillside lots or in hot/humid climates.

Actually, any foundation can be healthy—if constructed and maintained carefully. You need to be concerned with moisture (both liquid water and relative humidity) which can lead to mold or rot, radon (a cause of lung cancer), subterranean-termite control (toxic chemicals should be avoided whenever possible), energy savings (to minimize heating/cooling expenses, and reduce the potential for condensation), and selecting materials that are inherently healthy.

Question: Don’t concrete slab foundations tend to be damp and moldy?

The very thick walls of straw-bale houses (also rammed earth, double-thickness adobe, etc.) need an equally wide foundation (18-24 inches or more) to support them. In mild climate with sandy soil this is not a big problem, as the concrete foundation can be modest in depth (1-2 feet). But where deep frost-heaving or expansile soils are present (much of the country) you may require foundation depth of 4 feet or more. Making a 2 ft wide and 4 ft deep foundation takes a hell of a lot of concrete and rebar.

I have 2 ideas to reduce the materials required in such conditions. One is to engineer your foundation in the shape of an I-beam instead of a solid rectangular block. The I-beam principle is widely used in steel, engineered wood, and reinforced concrete to maximize strength while minimizing weight and material (hence cost as well).

--see cross section drawing below -- At the bottom of the foundation you'd have a 1 ft high by 2 ft wide base, then a web 8-10" thick by 3 ft high, then another 1 ft by 2 ft section sitting on top of the web -- total height of your reinforced concrete I-beam ~5 ft.. You would undoubtedly need an engineer to certify this design, but it would be easy to build and save a lot of cement and iron.

The second scheme I've thought of could be substantially cheaper and even "greener" -- an old idea but seldom used, why I don't know -- the grade-beam foundation. I associate this idea with FL Wright, but he may not have invented it. You excavate a trench to required frost-stable depth, say 5 ft deep by 2 ft wide, fill the trench with rocks to approximately grade level. Then build your foundation form, say 14" deep by 24" wide, directly on top of the rock base. On the rock base, Wright even used heavy wood timbers fastened together, instead of concrete, for some of his foundations; and if you had a bunch of used railroad ties or creosoted bridge timbers you could easily concoct some mixed-media foundation, sandwiching the timbers and reinforced concrete, using the timbers on the outside as "form," reinforced concrete in the middle, all tied together with rebar and reinforcing mesh.