Ive been doing my research on insulating my crawlspace to allow lower water temps in the above radiant floor(geothermal)

From what I've read, a lot of people are insulating with foam board against the walls, and rim joist. This is where my question comes in.

My house was built in the 1800's and although the foundation has been replaced with newer brick, the rim joist/sill plate is still the old wood beam and there doesn't seam to be any sill sealer (blue foam) between the foundation and sill plate.

If I seal my the gap between the foundation and sill as well as the crawl space preventing any drafts, would this allow any water water to sit between my sill and foundation without being "dried" by the drafts? Is there a way to seal these drafts from outside?

Has anyone encountered this when trying to tighten up a crawl space? What was your solution?

The most important area to insulate, after the attic, in any old house is the rim joist. In fact, we cover this in many of our boiler replacement and radiant floor retrofit work.


There is simply no way to insulate a rim-joist, after-the-fact, save to use 2lb. spray foam. This is especially true of older rubble or field stone foundations.

A blower door test will tell the story and the whole truth.

If you have moisture issues at the foundation, look to the drainage proper drainage of gutters and grade from the house.

Finally, don't forget to cover the floor of every crawl space with 6 mill plastic to stop the migration of water vapor into the space.

It's possible to cut'n'cobble rigid foam into the band joist and seal it in place with can-foam. Cut the blocks with a super-loose fit so that you can get the nozzle fully in the gap to guarantee the air tightness.

Over brick, poured concrete, or CMU foundations you can hang rigid foam with 1x4 furring through screwed to the foundation and tape the seams, seal the top/bottom edges with can-foam. With rubble or field stone 2lb foam is really is the only solution.