Crawl space Covering dirt floor of crawl space

Published on October 29th, 2014 | by Henri

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Insulating a stone foundation

Q.    I was going to insulate a stone foundation on an 1830s farmhouse with spray foam. but I seem to remember you answering a similar question and suggesting using fiberglass insulation instead. I was going to insulate from the sill plate to 18” – 24” below grade, 1” thick spray foam. What would you recommend? And if fiberglass, should it be 3 inches or 6 inches thick and faced or unfaced? Thank you for your help.

A.    Spray foam is fine if you can afford the cost. The walls would have to be very clean for the foam to adhere to them, and if there is occasional moisture seeping through them, I wonder if the foam would delaminate from the stones.
Fiberglass would not require that the walls be clean, and moisture would not hurt it, as fiberglass is hydrophobic. Any actual water would run down to the floor.

If your cellar floor is dirt, I would not recommend insulating the band joists with fiberglass, which would prevent any drying in case moisture from the ambient air comes in contact with the sills and is not allowed to dry; they need to be exposed to the air. However, you could insulate them with spray foam, but is it worth the cost to have a foamer come simply to do the band joists? Keep in mind that the canned foam you can buy in hardware and building-supply houses is for sealing cracks and has very little insulating value, but it would be better than doing nothing if you put a lot of it on.

If you decide to use fiberglass, staple the top of the batts to a pressure-treated 2×2, 1×3 or 1×4 screwed or nailed to the bottom of the sill beam and let it hang down to the 18″ or 24″ inches below grade you have in mind.

You should find a way to staple the bottom of the batts to a pressure-treated wood strip fastened to the stone walls to make it more effective. If the stones in the walls are mortared, and the mortar is sound and in good enough shape, it may be possible to use masonry nails, but it might be best to clean the most prominent stones and use a construction adhesive to fasten the wood strips. It does not need to be very regular. You can insert some batting to close the spaces between the wood strips and the stones if the gaps are large.

R-15 (4-inch ±-) batts are easier to handle and should be sufficient. Faced with aluminum would be preferable. Staple the flanges of adjacent batts together.

If the floor is bare soil, it should be thoroughly covered with 6-mil plastic. If there is some leakage through the walls or at their base, try to deal with that from outside to eliminate it, but also dig a small trench around the inside perimeter. Use the dirt to make a small berm and wrap the berm with the plastic, but not the bottom of the trench.

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