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Community and Q&A

Sealing a Leaky Crawlspace

noew | Posted in Energy Efficiency and Durability on

I am trying to bring down the heating load for our 100 year-old house in CZ5. One of the main problem areas seems to be the crawl spaces under the 80 sq ft addition. Attached is a floor plan for the basement and first floor levels. The addition is in the NW corner.

The kitchen addition sits on a vented crawl space with an unsealed dirt floor. According to the energy efficiency consultant, the kitchen floor is sufficiently insulated in that area.

The little half bathroom sits on an unvented crawl space (except, see below!) with an unsealed dirt floor and a window, but I can’t make out much more — it’s tiny and basically inaccessible. The bathroom is heated with an electric baseboard and gets pretty cold otherwise.

Our main basement is unconditioned and largely uninsulated (except for air-sealed rim joists stuffed with fiberglass, which I will be replacing with rigid foam held in place by spray foam). There is a concrete slab.

There are at least two connections between these spaces that concern me. First, the huge holes around the plumbing penetrations from the unvented crawl space to the basement. Second, a sort of “jump duct” leading from the kitchen floor through the vented crawl space into the unvented crawl space underneath the bathroom. I’m including pictures of the inside of that duct and where it dumps into the crawl space.

My questions:

1. Why was this “jump duct” built, and does it actually serve that purpose? I can only assume that the builder somehow thought that the unvented part of the crawl space needed to be conditioned with air from the living space — perhaps because of the plumbing? As far as I can tell there are only drain pipes running through the crawl space. The supply pipes run above floor level.

2. Can I seal off these different spaces from each other? I would seal the hole in the rim joists with rigid foam and canned spray foam. I would leave the “jump duct” in place for now, but try to wedge some rigid foam in there close to the wall of the crawl space, as well as near the kitchen floor, and hold the pieces in place with spray foam.

3. If I can get to the little window in the unvented crawl space from the outside (hidden underneath the deck), can I board it up with some plywood/rigid foam (in what order)? I don’t think I can brick it up due to lack of skill and room to work.

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  1. walta100 | | #1

    The way I see it you have 3 choices in attic and crawlspaces vented conditioned and moldy.

    It seems like vented has been working for a long time but it come with an energy penalty.

    If you seal up the vents are you willing to take the bitter pill and heat and cool the space enough to keep the humidity under control and avoid getting moldy?


  2. Expert Member
    BILL WICHERS | | #2

    You have to provide air flow to sealed crawlspaces per code. That is probably the original purpose of the "jump duct" you mentioned.

    If you have enough room to get into that crawlspace, you can put down a proper membrane (10 mil or thicker poly sheet), and insulate the space. That will likely solve your problems with the cold spaces above it. You'll want to be sure to completely air seal ALL of the exterior walls (rim joist areas mostly), so that the newly encapsulated crawlspace is connected for airflow only to your interior spaces.

    If you don't have enough room to be able to get into that small crawlspace, then I'd try to do as much air sealing as possible to seal the crawl space off from the outdoors, leaving it connected to the indoors. You don't want stagnant air in there. You could potentially have seasonal moisture issues without the poly liner though.


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