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Community and Q&A

Crawl space insulation options

pbout | Posted in Green Building Techniques on

House is 1-story 1200 square feet, CA climate zone 3, zip code 95062, about 1/2 mile from the ocean. Existing ventilated (passive vents) crawl space has 18″ clearance, 48″ OC 4x floor joists under sub-floor, perimeter foundation and piers along joists. Clay soil is damp, but downspout diversion and french drain in back yard keep the space from having standing water. House is heated by hydronic radiant heat system, on top of subfloor in 1/2″ sleepers.

Looking to for recommendations on how to retrofit the crawl space. Goal is higher IAQ in the house, so minimize infiltration and delta T between house air and crawl space. Air sealing between house and crawl space is a must, but I’m not sure about the other steps. I don’t want to make anything worse (such as block air from the crawl space to house but make it easier for mold to grow).

Option 1 is to leave the crawl space passively vented, and to spray closed cell foam to the underside of my subfloor (2″ thick, as recommended by spray foam contractor).

Option 2 is to add R-5 minimum rigid foam to stem walls and rim joists, seal up vents with same rigid foam. Air seal to outside. This would require the space to be ‘conditioned’, and that’s where I get hung up. I could add an exhaust fan (continuous or on a humidistat or other control) but I don’t want to have a passive vent to the living space, so where would that supply air come from?

Any feedback appreciated, thanks!

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  1. GBA Editor
    Martin Holladay | | #1

    Option 2 is the standard advice for those who care about energy efficiency and home performance. If you install a small exhaust fan in your crawl space, along with a floor grille to allow conditioned air from the house to enter the crawl space as makeup air, you won't get any crawl space odors or moisture in your house.

    All of the details are explained in this article: Building an Unvented Crawl Space.

    All of that said, Option 1 would work in your climate. Note that most green builders try to minimize the use of closed-cell spray foam because of the global warming impact of the material's blowing agents. If you decide on Option 1, a better approach (from an environmental perspective) would be to install a continuous layer of foil-faced polyiso on the underside of the floor joists, with all penetrations and seams carefully sealed to make the layer airtight.

  2. Expert Member
    Dana Dorsett | | #2

    If it's any cheaper (which may be the case since it's easier to install) an inch of closed cell foam on the foundation walls would hit R5 too, and would be inherently air-tight without a lot of detailing. The total square footage of the crawlspace is probably less than the 1200' of floor area you were considering. In many places spraying the foam with an intumescent paint passes muster for fire codes.

    The crawlspace floor needs a vapor barrier such as EPDM (membrane roofing) or heavy polyethylene. Protecting it by covering it with a 2" concrete rat-slab is useful if the crawl space is used to house things like water heaters or air handlers. In radon prone areas one might also pre-install a passive radon system under the vapor barrier (and rat slab) when going this route.

  3. pbout | | #3

    Thank you both for your responses.

    Martin, I am hesitant to install a register between the house and the crawl space for makeup air. We have existing radiant heat installed, so finding a location for this new penetration would be difficult. We also have no air conditioning or forced air at all, so is our indoor air actually considered conditioned enough to work as the makeup air to the crawl space?

    I also appreciate your note about environmental impacts.

    Dana, I will ask about insulating the foundation walls instead. If I go with spray foam under the floor, would I need to paint as noted for fire? Also, we have no appliances in crawl space. Not planning a rat-slab.

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