Anytime we put a foundation in, we end up with a giant moat around the house as we dig out the earth slightly larger than we need for the foundation to have a little leeway. We usually concentrate right away on framing up the house and leave the crevasse all around the house as an extra security buffer. Now that the framing is well underway, we are trying to make the site a little more workable and safe by filling in that hole.
With the ICF foundation, we have 2.5″ of extruded polystyrene foam insulation on the outside of the concrete. To waterproof, we just apply a peel and stick membrane directly to the foam. You can’t apply a tar based sealer that you would see on a lot of masonry and concrete foundations as that would just react and eat away the foam. We also make sure to lap the membrane down over the top of the footer and onto the side of the footer as an extra waterproofing measure. To protect our waterproofing membrane, we add a layer of half inch foam board and tape all the seams such that the gravel can’t puncture the membrane.
One could argue that this is an extreme overkill, but we have taken to backfilling our entire foundation with gravel. Water scares me and to me the small extra expense of the gravel instead of fill dirt is well worth the piece of mind. The gravel provides easy and fast drainage to the bottom of our footer where we have a french drain and eliminates any hydrostatic pressure on the foundation. The only possible situation where I would have a little reservation about this technique is if we had a basement where we were not able to daylight the foundation drain as that might put too much stress on a pump.
My best test case for this technique has been the Mulberry house. When digging out the foundation we hit an underground spring that constantly delivered water. Plus, there is a creek that runs in the front yard close to the house that carries heavy storm runoff. Suffice to say, it’s a wet site and that crawl space stays dry as as a bone even in the heaviest of storms.
