Tips to Winterize Your Crawl Space

Yesterday I got a call from one of our DIY customers down in Chattanooga. His name is Wade and he had an issue with something going on in his crawl space and he was concerned about how to winterize the crawl space. So today I’m going to answer his question.

Okay, so Wade from Chattanooga called me up and he had bought some foam board and some vapor barrier and dehumidifier from us because he’s doing his own crawl space. It’s a new construction house. Basically what was going on is, he put the foam board on the walls, on the block wall and then he put in the dehumidifier to get it going. But it’s been in the 7-10 degree range here in East Tennessee, so what he noticed was, is that his rim joists started sweating. And the reason why that was happening is because it was seven degrees outside and it was probably 60-65 degrees inside. So that rim joist, that piece of wood, or the ban board, where ever you’re from, was starting to sweat and condensate.

He started up the dehumidifier and that was the best thing he could do to dry out that rim joist. He began to dry out the rim joist and his question was, should he go ahead and put the rim joist insulation on the wet wood? I told him to not to, I told him to wait until the wood was dried out, let the moisture content get low, because foam board can trap moisture, or even insulation, fiberglass, if you throw fiberglass up in the rim joist to insulate it, it can trap moisture as well.

How to Prepare Your Crawl Space for Winter

Be careful if you’re winterizing your crawl space. Some people, they’ll just close the vents, for example, in the crawl space, in the winter, but they’ll leave the heat on. Obviously, you’ve got heat going if your ducts are down there, so you could be warming the crawl space up with the vents closed, but the dew point is still high. You still have a lot of humidity down there because the heat is drying out that summer moisture that had been accumulating in the wood and in the fiberglass and it’s making it go airborne. That can cause condensation in the crawl space, but, also, if you have a warm crawl space and a cold outside and you don’t insulate the rim joist, you could get condensation on those rim joists.

When you’re winterizing your crawl space, make sure that you do everything. You control humidity, you put foam board on the walls, or you have insulation between the floor joists if that’s your preference. And, you might want to make sure that you insulate those rim joists.

The other thing you want to think about is if you’re going to seal up the vents, that’s gonna help also with frozen pipes, if you use a good foamboard and spray foam to seal up those vents. If you just put a piece of cardboard over the vents or something that doesn’t have any R-value, it’s not really gonna do a whole lot to block the cold and the wind. If you’re gonna take the time to seal up those vents, make sure you use something good like an R10 foam board or something like that.


This is a lightly modified transcript of our YouTube video, Cold Crawl Space | Winterize Your Crawl Space | Crawl Space Insulation | Keep Crawl Space Warmer.

2 thoughts on “Tips to Winterize Your Crawl Space”

  1. My crawl space stays at around 50-55 deg in the winter, even when it is 20 deg outside. Is this because the heating ducks in the space radiate heat, the floors transfer heat to the space, or the dirt ground (half the house is 6 feet deep on a slope) radiates heat? Or a combination? I am not complaining, this prevents the pipes from freezing. Just curious. Thanks.

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