I have a crawl space under the house that is a bit damp, what is the best insulation application. I am wanting a R value of 38 or 39


I have a crawl space under the house that is a bit damp, what is the best insulation application. I am wanting a R value of 38 or 39

Asked by Larry Heihn

I have used fiberglass in the past and am not a big fan. I have read up on the polyethylene foam and am wondering if this is one of newest innovations that are top of the line. I also want to be able to install it my self. I am wanting insulation that ranks at R-39 and works better than fiberglass. Thank for your time!

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Monica Rokicki's picture

If your crawlspace is damp, the first thing is to solve the moisture problem. Be sure that you have a properly lapped and sealed vapor barrier on the crawlspace floor and extending up the sides of walls and piers, proper grading away from the foundation on all sides of the home, and make sure that gutters and downspouts are taking rainwater away from the edges of the foundation, as well. Once this is done your crawlspace should be drier, but it will only be as dry as the ambient humidity. An open crawlspace will never be lower humidity than the outdoors: this is why it is often recommended to close up the crawlspace and bring it into the building envelope. But assuming that you do not do that, the second thing to do is to seal up any penetrations in the subfloor (plumbing, electrical, and any other 'holes) and also to seal the band joists. For this, open cell spray foam is a great product. While you are doing it you could just spray foam the underside of the floor but to get to R-value of 38 or 39 you'll need 10 inches of open cell foam. You could use closed cell but that may be more expensive and also may become a condensation surface with an open moist crawlspace. Assuming that you've solved the crawlspace moisture issues and air sealed the underside and bands, probably the best and most cost-effective solution will still be fiberglass insulation, as long as it is properly installed (in contact with the underside of the floor, not compressed, paper side up, continuous and supported per manufacturer specs). Note that in your region, R-values at the attic should be R38 - 49 - more than that will not deliver much more efficiency. At the underside of the floor R19 batts meet code requirements, are inexpensive and will be sufficient. Going to higher R-values at the underside of the home will not deliver much more energy efficiency. The primary driver of efficiency at that location is air sealing. Fun fact: Way back when, open crawlspaces were meant to bring fresh air into the home, drawn in through the "Stack Effect" by openings at the top of the home. The function of an open crawlspace was misnamed a 'vented' crawlspace. With the current configuration of most homes and the advent of mechanical heating air air systems combining with higher costs of energy, the open crawlspace lost its original function and performs more like a rainforest machine in humid weather. I hope this helps!