I have an older home with a cement floor crawl space. It is a cold air space and I would like to insulate and heat the area going forward. The cement floor is in good condition and is mostly covered with repurposed underlay and carpet (mostly to make the crawling bearable). The are no signs of moisture on the floor on on the walls neither. Can I seal off the vent to the outside and insulate the walls with rigid foam board without a vapor barrier? Should I also look at insulating the floor and are there other considerations I am missing? Regards, Brian

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    Some may argue that the cold space vented crawl space is one thing that helps to keep it dry in there. Sealing off may invite dampness.
    – Michael Karas
    Commented Nov 6, 2017 at 11:03
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    Is this a pier-and-beam foundation? At present, the space is dry; the floor beams, floor joists, subfloor, etc., all dry. This is a condition that should not be risked without a certain and major benefit. Are water and drain pipes exposed in this crawl space? For how long has the concrete been covered with underlay and carpet? Is this just laid in or is it adhered to the concrete? Commented Nov 6, 2017 at 11:16
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    What's the end goal?
    – isherwood
    Commented Nov 6, 2017 at 12:35
  • The crawl space walls are formed concrete with 2 by 10 floor joists. There are water and drain pipes in the crawl space attached to the floor joists so the intention is to heat the crawl space. The carpet and underlay are lose on the floor and have been there for 10 plus years with no sign of moisture anywhere.
    – Brian D.
    Commented Nov 10, 2017 at 1:32

1 Answer 1


You can seal off the exterior vent if you are adding a fan that pushes air in and out of the crawlspace (air circulation, mimicking what that outside vent does). However, those vents are passive and require no power, so why not leave it open and seal it off when temperatures drop below 15-20 degrees Fahrenheit?

You would not gain much from insulating the walls with a rigid foam board. Your best bet would be to insulate under your floor with an R-30 or higher fiberglass batt insulation. That will seal your home tight with insulation from the bottom, while still allowing your crawl space to function properly with ventilation.

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