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Another crawl space…

Rob Shuman | Posted in Energy Efficiency and Durability on

I would like to improve upon the poly vapor barrier that exists on the dirt floor of my crawlspace. I have read the ‘crawlspace article’ penned by Martin but have some specific questions.

First, a description of my situation. A full basement under the ell of the house leads to the crawlspace under the main house, which is accessible via a ‘basement window’ (the frame of the window exists, the window does not). Roughly 3 and half walls of the crawlspace lie on the outside of the main house (the remaining wall falls under the junction of the ell and main house). The ‘exterior’ walls of the crawlspace are granite blocks which have been sprayed with closed cell foam, the rim joists have been sprayed as well. The remaining portion of the crawlspace wall consists of a cement ‘hallway’ between the basement and the crawlspace access point and a rubble wall. In general, there is no foam on these interior walls.

Questions:
1.) I would like to use the poly that has been placed to the extent feasible. It is likely 6 mil poly placed over the uneven floor of the crawlspace, including some rocks. Any problems so far?
2.) The existing poly on the floor of the crawlspace was sealed to the exterior walls by the foam in some places but it is not attached to the foundation walls along most of its perimeter. Lacking a convenient way to attach the poly to the foam or the concrete block/rubble wall, is simply placing bricks/rocks on top of the plastic sufficient from an overall moisture management standpoint? If not, do you have any other ideas for effectively anchoring the poly?
3.) The edges of the poly sheets were not taped where they overlap. What is a good tape to use for this purpose? In the crawlspace article I read it indicates using a compatible tape or mastic. What, specifically, would be considered compatible with poly?
4.) The poly has some small holes in it in some of the more traveled portions of the crawlspace. Will the relatively small number of defects seriously undermine the performance of the barrier? If so, could taping them be sufficient and keep me from having to install new poly?
5.) If I were to install new poly, would it be unwise to install the new material over the old?
6.) Given the configuration of the crawlspace relative to the basement, what might be the best way to incorporate an exhaust fan? There are 2 basement windows along the exterior walls of the crawlspace and 3 in the full basement. Should I incorporate a fan into one of the crawlspace windows and replace the window between the basement and crawlspace to (sort of) isolate the crawlspace (the rubble wall is hardly airtight) or something else?
7.) Looking on the internet, most of the exhaust fans I see move air at much higher rates than needed for the crawlspace (estimated at ~20 cfm for the ~900 sq ft crawlspace). Any ideas who might make more suitably sized fans?

Thanks a bunch!!

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    From a moisture vapor control point of view, a few rock holes aren't much of an issue, even though it isn't gas-tight. Detailing the edges to the extent possible is probably more important. Using can-foam or duct mastic to get it to stick to the foundation stones is going to be better than just weighting it down with rocks at the edges.

    Putting a new layer over the old layer does not cause any problems.

    Why do you need an exhaust fan to the outdoors, or even between the crawlspace and basement? With a sufficiently sized window opening in foundation-partion between the basement & crawlspace convection could be sufficient.

  2. GBA Editor
    Martin Holladay | | #2

    Rob,
    Q. "The edges of the poly sheets were not taped where they overlap. What is a good tape to use for this purpose?"

    A. Here is a link to an article that discusses tape performance on polyethylene: Return to the Backyard Tape Test. Of the tapes I tested, the four best tapes for polyethylene were Polyken Shadowlastic, Siga Wigluv, Siga Sicrall, and Pro Clima Tescon No. 1.

    Q. "The poly has some small holes in it in some of the more traveled portions of the crawlspace. Will the relatively small number of defects seriously undermine the performance of the barrier?"

    A. No, but you can tape them if you want.

    Q. "If I were to install new poly, would it be unwise to install the new material over the old?"

    A. No.

    Q. "Given the configuration of the crawlspace relative to the basement, what might be the best way to incorporate an exhaust fan?"

    A. I agree with Dana. It's not clear that you need an exhaust fan.

  3. Rob Shuman | | #3

    Hi Dana, Martin -

    Thanks for the feedback guys. I will research the tapes and work to minimize gaps around the edges.

    The idea that I may not need an exhaust fan for my configuration is consistent with the good condition of the floor joists and supports within the crawlspace.

  4. Rob Shuman | | #4

    Martin -

    A follow-up to our prior discussion. I noticed suitable substrates for Siga Sicrall include hard surfaces (e.g., wood, gypsum fibreboard, plasterboard); I have not yet looked for substrate specs for the other tapes. Do you have any insight into, or opinions about, how tapes such as those you tested would work on an uneven dirt floor in a crawlspace? I suspect the ability to effectively adhere the tape may not be as good as it was when you placed your tape samples.

    Thanks.

  5. GBA Editor
    Martin Holladay | | #5

    Rob,
    Here's my advice: Don't overthink this.

    Siga Sicrall is an excellent tape. If you wipe the polyethylene seam with a rag or a paper towel, so that it isn't dusty, the Siga Sicrall will stick fine. Press it with your hands so that the poly is pushed against the dirt below when you install it, and stop worrying.

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