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Community and Q&A

Vented vs. Unvented Crawlspace

mark_gil | Posted in Green Building Techniques on

We’re undertaking a full renovation of a c.1920 house in the SF Bay Area.  The house has an existing crawl space (varies in height between 6’6″ and down to perhaps 12″ in places), but has a large enough space centrally that we can potentially use as a mechanical room which is an attractive idea.

We’re specifying and investing in having the house above pretty air tight, and mechanically ventilating and so naturally exploring where the edge of our building envelope is, and whether the crawl space is inside, outside or it’s own discretely treated space.

The space is currently vented (perhaps not perfectly) and not finished (there’s a poured slab in the flat area, and ratproofing of to each side as they slope up to grade.  Peer and beam foundation.

Previously the space (in addition to being vented) had a dehumidifier which ran (somewhat) continuously; and during /after heavy rain there is some seepage onto the floor in the main area (which is serviced by a pit and sump pump).

The question is – should we encapsulate the space as we remodel?; and if so how far should we go (e.g. just a membrane to the floor and part way up the walls (taped to the concrete), insulate the foundation walls where they are accessible. Or should we leave it vented, perhaps add mechanical ventilation to increase air flow?  How should we treat the underside of the house in each case?

Thanks for any thoughts!

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  1. Andrew_C | | #1

    I would encapsulate the crawlspace and go all the way up the wall. In addition to reducing possible radon exposure, it should reduce moisture levels. Make sure that you also air seal the rim joist and all penetrations. I'd run the dehumidifier, and suspect that after a couple of weeks it'll run a lot less. Nice to already have a sump pump to drain the dehumidifier.

  2. Expert Member
    PETER G ENGLE PE | | #2

    In most cases, a sealed and insulated crawlspace performs better than a vented one. Just about the only time I don't recommend sealing and conditioning is when the house is in a flood prone area and the crawl space frequently sees water entry. If you have seepage during/after a heavy rain, you should pay attention to diverting your roof and yard water away from the outside of the foundations. There are lots of articles about this here and on other homebuilding sites. Even in your moderate climate, it is probably worth installing R5-R10 of insulation on the walls. If you use foil-faced rigid insulation (like Dow Thermax) and tape the seams, the foil facers give you protection against air, water and vapor migration. I prefer spray foam at the band joists after doing as much sealing with caulk as you can. Depending on accessibility, configuration and price, spray foam for the walls and band joist in one shot can be the best option. You will still probably need a dehumidifier in the crawl space. If you install a Heat Pump Water Heater (HPWH), it may provide adequate dehumidification, or maybe not. Basement humidity should be held at less than 60% all the time.

    1. mark_gil | | #3

      Thanks Peter!

      My bias was to take the vapor barrier (likely Stego or Pango) up the foundation wall, but this raises a question for me as to how i fasten the insulation to the wall (i like the idea of the Dow Thermax, or perhaps Halo Interra with the thermal barrier) as we don't want to build an interior stud wall to hold (or protect) the insulation. I we punch a couple of hundred holes in the barrier to attach the insulation are we creating an issue?

      Thanks again!


      1. Expert Member
        BILL WICHERS | | #5

        Why use something fancy for the crawlspace liner? Convention is to just use polyethylene sheet, usually of 10 mil or thicker thickness. There are a number of companies that manufacturer poly liners like this specifically for this application.

        I like using the press-in plastic insulation fasteners like this:

        They work well, and they are easy to install. If you're worried about the holes, inject a little caulk or canned foam into the hole prior to installing the anchor, which will provide a seal in the hole after the anchor is installed. There are smaller versions of these anchors made to hold the crawlspace liner alone too.


  3. walta100 | | #4

    Consider changing your question in your mind.

    From “to vent or not to vent” to “vent or condition”

    It is all to appealing think “encapsulated” or “seal crawlspace” will take care of itself all too often they end up way to humid making mold and rot a real possibility.

    Are you are willing to spend the money to buy and operate the equipment needed to keep the temp and humidity about the same as the room above? If yes, then sealing it up is a great option but if you are not willing to spend the money the smart move may be to change a little as possible as the space has worked that way for a hundred years.


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