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Community and Q&A

Insulating around crawl space vent

mjrmason | Posted in General Questions on


I have an older home in Seattle where the temp is only below freezing a handful of days a year. Vented craw spaces are the norm in order to deal with constant ground moisture. Crawl space vents were added to this house by cutting into the rim joist. The joist cavities next to these vents lack any insulation/batting for roughly 4 ft. I am wondering if it is worth it (or against code) to use foam board between these joists to help insulate.


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  1. GBA Editor
    Martin Holladay | | #1

    If your house has a ventilated unconditioned crawl space, it makes sense to insulate the joist bays above the crawl space.

    You can use rigid foam of this purpose if you want. The best way to install the rigid foam would be as a continuous layer under the joists rather than to cut the foam into strips and insert the strips between the joists.

    In either case, the foam should be covered by a protective board (OSB, plywood, or gypsum drywall) for fire safety.

    Q. "I am wondering if it is worth it (or against code) to use foam board between these joists to help insulate."

    A. As long as the rigid foam is protected by OSB, plywood, or drywall, the work won't violate any building code. To determine whether the work is "worth it," we'd need to know more information (what type of fuel you use for space heating, and how expensive the fuel is). You could use an energy modeling program to try to answer the question. It is certainly possible, however, that this type of work would have a very long payback period in your climate -- meaning that the work may not be "worth it."

  2. Dana1 | | #2

    The pay back will be the first winter, where you can then comfortably stand on that section floor in bare feet any day of the year- no financial analysis required. (Been there!)

    But there is a financial pay back too, as well as code minimums- Seattle isn't San Diego. Even if the 99% outside design temps are in the ~20-25F range, it has a 9 month heating season, and deep subsoil temps comparable to most climate zone 5 locations.

    Code min joist insulation for floors over unconditioned crawl spaces in US marine climate zone 4 Seattle is R30 (R19 if the joists are only 2x6). Installing full-depth batts (in contact with the sub-floor) with at least R6 of rigid foam on the bottom (covering the joist edges) is "worth it" from a comfort point of view, even if you're heating the place with high-efficiency ductless heat pumps @ 8 cents/kwh.

    In many cases it will be less material, easier, and cheaper to put down a ground vapor barrier and insulate the foundation walls with continuous R15 (code min for foundation walls in zone 4) with rigid insulation though.

    For IRC 2012 code minimums see:

    Also see Table 2 p.10 of this document for the starting point of the financial rationality limits:

    (Those are "whole assembly" values, with the thermal bridging of the structural elements factored in, not center-cavity values of framed structures. An R13 studwall is roughly equivalent to R10 continuous insulation, an R20 studwall roughly equivalent to R15, etc.)

  3. mjrmason | | #3

    Thanks for the feedback. The crawl space is insulated actually, between the joists. However, when the vents were added by cutting into the rim joist, insulation was pulled out in between the joists that run up to the vents to allow air movement. The rest of the insulation is still there. I am trying to do something about the spot where the insulation was pulled out. I have about 3 inches between the subfloor and the top of the vent to work with.

  4. GBA Editor
    Martin Holladay | | #4

    One other option to consider as you work in this area: you could fill in the vents, which really aren't necessary.

  5. Dana1 | | #5

    Unless there is a ground vapor barrier filling in the vents may be necessary to avoid mold on the joist edges. Unlike most of the eastern US, venting a crawlspace in Seattle lowers rather than raises the average moisture in crawlspace.

    From an overall air infiltration & air quality point of view it's still better to go with the ground vapor barrier, insulated foundation, and unvented crawlspace. It's next to impossible to reliably air seal the crawlspace from the conditioned space, and stack effect forces continuously draw crawlspace air into the conditioned spaces 9 months out of the year (and more). Ground vapor barriers limit the amount of soil gases (including water vapor) entering the crawl to very low levels, to the point where the exterior vents aren't necessary, as long as there is even a very modest amount of ventilation with conditioned-space air. With insulated foundation walls that ventilation with conditioned air does not constitute a significant heat loss (a much lower loss than it currently has with the vents open.)

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