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Community and Q&A

Spray foam insulation

DRosenfeld | Posted in General Questions on

I live in north Florida. I’m having to remove the batt insulation between the floor joists in the crawl space beneath my house in order to spray for powder post beetles. I’m considering a spray foam insulation (open or closed cell ??) as an replacement alternative. What are the pros and cons to this approach? I’m particularly interested in any associated environmental factors and water vapor transmission effects on the underside of the house.

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  1. Expert Member
    Dana Dorsett | | #1

    It may make more sense (and take less foam) to install a ground vapor barrier sealed to the walls, and spray-foam the crawlspace walls, and ventilate that space with a tiny flow of indoor air (or low cfm exhaust-only. That puts all of the wood in the same temperature & humidity environment as the indoors.

    Per the IRC code-minimum floor insulation for a vented crawlspace in US climate zone 2 (most of FL) would be R13 between the joists, which leaves the joist edges in an air conditioned home colder than the outdoor dew points all summer, making them susceptible to mold/rot/insects., since outdoor air (and ground moisture) pass relatively freely under the house.

    There is no code minimum for crawlspace walls in a zone 2 for unvented crawlspaces, but 1-1.5" of rigid foam board or 1" of closed cell spray polyurethane still has a lifecycle rationale on energy savings. In termite zones insulate the foundation sill and band joist with carefully sculpted 3-3.5" thick rock wool batts, not foam, to allow temporary removal for termite inspections.

    In flood zones there is sometimes a requirement that crawlspaces be able to automatically drain when the tide goes out, but there are reasonably air tight insulated vent replacements that can still work in those areas. eg:

    If closed cell polyurethane, using an HFO-blown version is much nicer to the environment than the more common HFC blown foam. It can be sprayed with an intumescent paint for fire protection.

    For rigid foam going with 1" fire rated Dow Thermax is probably the easiest & cheapest way out, despite being much more expensive than other foam board. Other foam board would need a thermal barrier or ignition barrier such as half-inch wallboard for fire protection, adding another step to the process. 1" foam board can be attached with 1.5-2" cap screws through-screwed into the foundation on a fairly loose grid, with seams taped over with a high quality aluminum tape.

    The ground vapor barrier can be EPDM (membrane roofing) or 6-10 mil polyethylene lapped up a foot or more on the foundation wall, sealed with duct mastic or polyurethane caulk. The foam insulation overlaps the ground vapor barrier.

  2. GBA Editor
    Martin Holladay | | #2

    D. Rosenfeld,
    You may want to read these three articles:

    "Building an Unvented Crawl Space"

    "Crawl Spaces vs. Skirts"

    "How to Insulate a Cold Floor"

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