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Community and Q&A

Air-Sealing the Subfloor in Vented Crawlspace

TroyElm | Posted in General Questions on

I have a large vented crawlspace in a 1970’s ranch house and I recently embarked on a cleanup and renovation project where I removed the old damaged fibreglass insulation from the ceiling and cleaned out the space. The main goal is to prevent potential moisture issues once and for all, so I’m leaning toward insulating the foundation walls with spray foam, sealing the vents, and installing a dehumidifier. But I’m not still 100% comfortable with the idea of allowing air exchange between the crawl space and the living space above. It’s clear that there’s quite a bit of air exchange and mild odors from below can be smelled upstairs. So my question is this: is there a good reason not to seal the subfloor as well? Maybe a very thin layer of foam or something else just to prevent air exchange through the subfloor? No matter how clean and dry the crawlspace, it seems better not to breath its air. So, are there any drawbacks to completely sealing it away from the rest of the house? Thanks!

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  1. user-2310254 | | #1


    To address your concern, consider installing a small fan to vent the crawlspace to the outside. See this article by Allison Bailes for specific recommendations (

    1. 4khz | | #3

      I know that this post is almost two years old, but I thought it might be a good reference for others. In the linked article, the preferred method appears to be installing a dehumidifier, eliminating the transfer register, and isolating the house from the crawlspace by air sealing the floor.

      I will be finishing encapsulation of my crawlspace. I am planning putting a mini split air handler/ductwork in there and possibly an ERV. Currently, the top of my subfloor is exposed. I'm wondering (if I go with the article recommendation) if a air seal membrane (or air sealing tape) would work before installing hardwood flooring or if all the nails will ruin it.

      The next thing I thought about is the ERV. Would a ERV exhaust duct to the crawl space be beneficial ?

  2. Expert Member
    BILL WICHERS | | #2

    Spray foam is probably the only practical way to air seal a subfloor like you're describing. Anything else would require a crazy amount of labor.

    The usual way to do this is to use a membrane, usually polyethylene sheet, to "encapsulate" the crawlspace. This membrane covers the floor and the walls, and all the seams are taped to form an air and vapor barrier. You then insulate over the membrane. With a system like this, the air in the crawlspace isn't getting contaminated by anything so you're pretty safe allowing air exchange with the living spaces.

    I'd reconsider your use of spray foam on the crawlspace walls. Unless you have something with an irregular surface like a stone foundation, rigid foam insulation is usually a better choice. Consider reclaimed polyiso here.


  3. user-5946022 | | #4

    I can attest from personal experience that other than diligent air sealing, you don't need extreme measures to airseal living space from the crawl space. In my case, the subfloor is Advantek, and the finish floor is either pad + carpet, wood sheet underlayment stapled perpendicular to the Advantek + roll rubber flooring, or sound deadening rolled underlayment + traditional site finished hardwoods. However, all the plumbing, hvac and electrical penetrations between the crawl space and the living space were sealed with spray foam. The above measures are definitely keeping the crawl space pressurized - air is NOT getting through the hardwood, carpet or rubber flooring edges to find an equilibrium between the crawl and living space above.
    I have a dehumidifier that draws air from the living space into the crawl space, so the crawl space is definitely positively pressurized. There are several low voltage conduits from the crawl space to the living space, which I sealed with putty pads to prevent air transfer. You can immediately feel the stream of air come through the conduit from the crawl to the living space if you remove the putty pad. Whenever I remove them for LV work, I replace the putty pad when the work is done to keep the crawl pressurized. The crawl is entirely sealed and conditioned, so I'm not at all concerned about air down there coming into the living space, but I'm trying to keep the separation as it was designed by those who know more than I do about this.

    1. 4khz | | #5

      Thanks for the info and your first hand experience. And a good point on the rolled underlayment which I imagine would help with air sealing as well what you state - to sound deaden the flooring.

  4. frasca | | #6

    +1 on Bill's comment. Install a good, well-taped vapor barrier on the floor and up the stem walls, airseal the sill plate, the band joist, and any old venting or penetrations to the outside, then insulate the perimeter.

    Once that is done, air exchange between the living space and the crawlspace is mostly welcome because you're keeping your framing warm and dry.

    Photo attached of the crawlspace in my 1955 Seattle house I encapsulated with 20mil sheeting - my realtor actually put this in the listing when I sold the place. (I never did get around to insulating the walls before I moved out... sorry, world!)

    As soon as that sheeting went down, all odors from the dirt floor ceased immediately.

    1. 4khz | | #7

      Very nice crawl space. I'm doing what Bill recommends + what Allison Bailes (in the above linked article) recommends adding dehumidifier + isolate the crawl space from the living space. Bill states that the best way is probably to spray foam in between the rafters. I guessing this would be open cell.

      1. Expert Member
        MALCOLM TAYLOR | | #8


        The sub-floor itself is pretty effective at isolating the crawlspace from the house above. You get the most bang to the buck by addressing penetrations between the two, and after that in taping the seams. Spray foam seems like overkill when you don't need its insulating qualities.

        1. 4khz | | #9


          I will take your advice. thanks

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