low/no concrete basement? (ship-hull)
GBA regularly sees questions about non-traditional foundation building techniques: “concrete-free slabs” are presently a hot topic, and permanent wood foundations come up occasionally. Along these lines, could a waterproof membrane be used in conjunction with an internal frame on well-drained sites? I’ve attached two sketches, which remind me of a ship’s hull in cross-section.
– a load-bearing internal frame (of steel, reinforced concrete, large timbers, etc) transfers the weight of the building to the footings.
– the site is well-drained, so the membrane never experiences significant hydrostatic pressure.
– the “walls” only need to resist lateral earth pressure (the weight of the superstructure is supported elsewhere, and the site is well-drained). Lightweight fill and an “up-and-outward” sloping wall can be used to reduce this pressure. If frost-heaving and/or expansive soils are of concern, then the wall should slope “up-and-inwards” instead. The wall membrane is then supported from within by light framing (think roof trusses or rafters) and infill (planking, OSB, wattle-and-daub, etc). If the membrane is warm and dry, you can skip the expense of pressure-treated lumber.
– the “floor” area is weighted down with ballast. This could be fieldstone, gravel, cannonballs, etc.
– the membrane is well-protected on all sides. Rigid insulation and/or drainage mat might be good solutions.
– the membrane is continuous and/or seams are welded. The roof membrane and pond liner industries have this down to a science.
– easy access for inspection/repair. Uncover the membrane from the inside, and cut/patch to your heart’s content. Appropriate techniques are well-known for materials such as EPDM and reinforced polyethylene. Cutting through the membrane to access “sub-slab” systems (drainage, radon, septic) shouldn’t be a big deal.
– reduced carbon footprint? My guess is that a membrane won’t last as long as a standard poured concrete basement, but it ought to live an exceptionally long and happy life (stable temperature, no UV, managed water, accessibility for repairs, etc). Estimates for embodied energy still have big uncertainties associated with them, but 45mil EPDM represents ~1/3 the embodied energy (per unit area) of your average 8″ concrete wall, and 15mil poly is <10%.
– end of life management? The holy grail would be ready re-reuse, which probably won’t be the case for membranes. But re-use might be possible with the load-supporting structure (bolted together steel/concrete/timber columns and beams). The membrane could be rolled up for recycling/disposal. Good luck salvaging much from a poured concrete foundation.
All in all, this is probably a solution in search of a problem, and certainly not in the IBC, but an interesting thought experiment. Feel free to poke holes in the
(EDIT: 4/24 title updated to better-reflect content)
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