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Musings of an Energy Nerd

Worries About Trapping Moisture

While some types of wall assemblies and roof assemblies are risky, many worries about ‘trapping moisture’ are baseless

I'm not sure what this product is good for — but if you are the type of homeowner who worries that building materials might “trap moisture,” this product might give you nightmares.
Image Credit: Kontrol

A significant number of questions posted by readers on the GBA site are variations of, “Will this wall detail (or roof detail) trap moisture?”

When I entered “trap moisture” into the GBA search box, I got 182 results. The search terms “trapping moisture” yielded another 104 results. Clearly, there is a high level of concern around the issue.

Many of these worries are baseless. Most worries arise from the presence of a vapor-impermeable layer — for example, polyethylene, rigid foam, or a foil-faced product — somewhere in the building assembly. But the presence of one of these products isn’t necessarily risky. For a product to contribute to a moisture problem — or, to use the popular phrase, to “trap moisture” — you need two factors: (1) a mechanism or driving force that allows moisture to enter the building assembly, and (2) no easy way for the moisture to leave the assembly. In other words, you need some type of ratcheting mechanism — a one-way valve or a “lobster trap” — for a problem to develop.

Here’s how a building scientins would express this concept: “You won’t have a problem unless the rate of wetting exceeds the rate of drying.”

Misunderstandings about “trapping moisture” occur when homeowners fail to think about basic moisture drive mechanisms and drying mechanisms. If you’re interested in figuring out these puzzles for yourself, ask yourself these questions:

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  1. EDUB6 | | #1

    Tile Shower on Exterior Wall
    We are building an EIFS wall in Zone 5 with 4" ext. foam and interior 5.5" Roxul batts.
    The shower stall is on two exterior walls and will have a vapor barrier.
    This creates a wall cavity with two vapor barriers.

    Drying by diffusion seems to only be able to work through studs, top and bottom plates.
    Ventilation is also only available via any gaps between studs, top and bottom plates.

    Is this an issue?

  2. Expert Member
    Dana Dorsett | | #2

    Not all foams are vapor barriers.
    At 4" unfaced type-II EPS is still over 0.5 perms, which is more vapor permeable than 2x timber, and more than half as permeable as dry half-inch OSB. The vapor permeance of the EIFS finish varies, and that (more so than the foam) would be what makes the difference.

    But even if air leaks into the vapor-trap stud bays, with 4" of foam on the exterior the sheathing stays warm enough to not collect moisture. The only real risk is from bulk water incursions.

    To prevent convective transfer of moisture into the stud bays you really need to caulk "...any gaps between studs, top and bottom plates..." with a high quality polyurethane caulk, since that presents a higher risk (and bigger heat loss), than the interior side vapor barrier.

  3. GBA Editor
    Martin Holladay | | #3

    Response to Eric Burhop
    Lots of houses with exterior rigid foam on the walls also have vapor-impermeable finishes on the walls of showers. As long as the interior wall finish materials are installed with attention to airtightness, and as long as the exterior is properly flashed to prevent bulk water entry, experience shows that these walls are fine.

    If I were you, I wouldn't worry.

  4. Jon_R | | #4

    other directions?
    My guess is that your interior side sheathing won't form a perfect air seal against the studs and when there is a humidity difference between the shower stud cavities and adjacent (presumably permeable drywall covered) ones, air will flow horizontally, causing drying (it doesn't take much). You could do something to encourage such flow (a thin mesh layer between the sheathing and the studs?).

    This is worth reading:

  5. guilfordvtbuilder | | #5

    Hello, I’m in the process of renovating a 1980s house with 2x4 fiberglass walls with interior plastic sheeting. I’d like to put recycled polyiso insulation on the outside and not mess with the inside. Is this wall assembly risky?

    1. GBA Editor
      Martin Holladay | | #6

      No, it won't be risky. Ideally, you will choose to install rigid foam that meets the minimum thickness rules described in this article: "Calculating the Minimum Thickness of Rigid Foam Sheathing."

      Assuming that you understand those rules, you're probably worrying about the interior polyethylene. You probably don't need to worry, however, as explained in these two articles:

      "The Exterior Rigid Foam is Too Thin!"

      "Rethinking the Rules on Minimum Foam Thickness"

      1. guilfordvtbuilder | | #7

        Very helpful, thanks!

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