Saturday, April 09, 2005


Underneath both the bathhouse and the kitchen we'll need some kind of foundation. I've investigated a few different kinds and here's what I've found.


This is a common rural type of foundation made up of concrete piers set on footings about six feet below grade. Little earth work is required, little concrete and it can even be done by hand if there's enough labor available. The underside of the building floats above grade creating a space under the building. This has the disadvantage that air moves under the building making floors cold. Animals and other critters can make that space home. If building with straw bale this technique can be expensive because straw bale weighs alot and the beams you have to use to support it might cost more than the difference in the cost of a slab foundation.


This is the most common type of foundation in New England because it provides excellent frost protection and a place to put all your junk. Deep footing walls create a protected air space under the building that stays about ground temperature if not heated or cooled. The space provides a place for mechanicals and easy underfloor access. This kind of foundation uses a lot of concrete and is usually installed by a professional with formwork and is often one of the more expensive parts of a building.

Frost Protected Shallow Foundation

This type of slab foundation, pictured right, was championed by Frank Lloyd Wright and has been approved for use all over the United States and Canada. It's been proven to perform as well as a basement foundation in frost protection but uses a fraction of the concrete and labor. It does usually require the use of foam insulation which I'm not sure is nasty to fabricate, but seems like it probably is. A large skirt of this insulation spreads out from the building, more in the corners, to keep frost from getting under the building and heaving it. Temperature records of FPSFs show that even in the harshest of climates the temperature under the slab remains above freezing. A disadvantage of a large mass foundation like this is that it takes a long time to change its temperature, which could be an issue for Destiny since in October, for instance, the slab would not warm up much at all during the course of the gathering. Passive solar design might be able to help with this and keep the slab warmed enough even in winter months.

Straw Bale

One of the more radical foundations I uncovered is the straw bale foundation. A two inch layer of concrete is poured over a stone bed, straw bales are laid with spaces between them, then another layer of concrete is poured over the bails, filling the spaces between which support the floor above. The straw provides insulation and the small concrete infil columns provide structural support. A firm in Quebec, ArchiBio, pioneered these and several are in good condition today, although in some the straw has rotted, which doesn't structurally affect the slab which is entirely supported by the concrete between the bails. This kind of foundation seems a little iffy to me, but I'm going to look into it more.


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