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Community and Q&A

Last-minute change to an unvented crawl space

Joe Norm | Posted in General Questions on

Climate 4C, Pacific Northwest.

I will be pouring a crawlspace foundation in the coming weeks and have been planning to go vented as its the norm around here.

If I want to change it to a sealed space down the line can I just fill the vents, insulate the walls, and figure out how to add a mechanical vent to the outdoors?


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  1. Expert Member
    Zephyr7 | | #1

    If you convert it to a sealed crawl space down the road, you would seal the vents, and insulate the walls, but you would NOT add a vent to the outdoors. The whole point of a “sealed” crawlspace is that it’s sealed off from the outdoors.

    If you think you might seal the crawl space in the future, I’d do a few things now during the build:
    1- install a floor slab, even if only a thin “rat slab”, and install polyethylene under the slab. This will help keep the crawlspace dry if you seal it in the future, and will help to limit dampness even in a vented crawlspace.
    2- consider installing rigid foam insulation under the slab. This will help insulate the crawlspace if you seal it in the future.

    If you can (water table isn’t too high), I’d seriously consider digging out a full basement. You have a lot more flexibility with a basement than you do with a crawlspace, and the cost isn’t too much more during the initial build. Costs to dig a crawlspace out later are usually way to high to be justifiable, so you really only have one chance to do it and that’s during the initial build.


  2. Expert Member
    Malcolm Taylor | | #2


    If you are considering at some point converting to a conditioned crawlspace, it's important to integrate your crawlspace into your mechanical ventilation strategy. The three options shown in our BC Building Code are:

    - Provide enough air grills so that the crawlspace and the floor above function as one space. The downside of this approach is that the two spaces will share air, and also can not be kept at separate temperatures.
    - Provide mechanical supply air from the floor above, and a return air grill (same problems as above).
    - Provide a mechanical exhaust fan in the crawlspace and a grill in the floor above for supply air.

  3. GBA Editor
    Martin Holladay | | #3

    Here is a link to an article that describes all of the steps you'll have to take to convert a vented crawlspace into an unvented crawlspace: "Building an Unvented Crawl Space."

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