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I need help deciding what kind of rigid foam board to use to insulate a subfloor in a vented crawl space

Gduken00b | Posted in General Questions on

My home was built in 1955 in Southern California. It’s a single story home with a vented crawl space and a dirt floor. The ground gets damp during winter/spring.  Closing up the crawl space is not an option.

I would like to install rigid foam board on the bottom of the floor joists to air seal the floor and keep the moisture out. I keep reading about each type of foam board but  I’m not sure which one is best for my situation. There are many pros and cons with each type of foam board. A secondary concern is insects and rodents.  I would really appreciate any advice.

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  1. Expert Member
    BILL WICHERS | | #1

    If the ground only gets damp, you might still be able to do a full encapsulation project. If you have bulk water in there, then you have other problems and you should look into figuring out why that is happening and fixing it. If you have water coming into your crawlspace, there is the potential for foundation problems over time.

    If you end up just insulating the joists, I would put batts in to code minimum level or better. You then put a layer of polyiso under the joists. I would probably use 3/4" or 1" polyiso here just to get a little more strength so that it wouldn't sag, since it's probably going to end up supporting those insulating batts a little.

    I would go with polyiso here because it's stiffer than EPS, so it will be easier to get flat to the underside of the joists, and it's easiest to tape (tape sticks well to the foil facer). XPS is least green, and offers no advantage here in terms of insulating properties.

    If you need critter protection, a layer of plywood or OSB is probably the best way to go. You only need something thick enough to stay up flat, which probably means 3/8" or so. You could also go with a layer of hardware cloth, but it will likely be harder to install well.


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