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Community and Q&A

Liquid applied water resistant barrier

Paulson82 | Posted in GBA Pro Help on


I am looking for a solution for a water resistant barrier for the north and west sides of a small 120 sq. foot structure where the aforementioned sides have less air flow, sunlight and are more exposed to moisture from a nearby stream. The structure is already built to the point of being roofed in, and has plywood sheathing. The overhangs are significant, but moisture has already caused surface mold on the back north and west sides (on the plywood).

My instincts tell me that a better solution than housewrap would be a liquid-applied barrier to insure no moisture penetrates into the walls and cause internal mold issues.

My question is: Is there a product that would accomplish this and can it be purchased for only a few hundred square feet?



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  1. GBA Editor
    Martin Holladay | | #1

    In some climates, notably the rainier sections of the Pacific Northwest, it is virtually impossible to prevent the growth of mold on the north side of a building. North facing siding in these climates is often moldy, so one could say that mold in these climates is normal.

    You may find that, no matter what type of water-resistive barrier (WRB) you install, your siding will still be moldy.

    I can't think of any reason why a liquid-applied WRB would do a better job of resisting mold than other common types of WRB. But if you are interested in buying and installing a liquid-applied WRB, there's no reason not to do it. Here is a link to an article with more information: "Housewrap in a Can: Liquid-Applied WRBs."

    1. Paulson82 | | #2

      Hi Martin,

      Thanks for your reply. Ive read a lot of your material on here, particularly on wrb's, so I particularly appreciate getting a response from you.

      My reasoning for thinking a liquid applied wrb may be better is that there is less likely hood for ambient moisture to penetrate into the wall structure, where it seems a paper barrier, even if well installed, could still allow that moisture in as is the nature of ambient moisture in a wet climate.. It will find a way.

      I don't mind if the siding gets moldy I am more worried if the moisture can penetrate the sheating into the wall cavity causing mold there. I live in western North Carolina which is a temperate rain Forest much like the north west. Mold is part of life for sure.

      I've read that article previously in my search (thank you by the way) and it seems all the liquid applied wrb's are available in larger sizes for more industrial and commercial use. I just wondered if there was a product that was more appropriate for a smaller application.

      Thanks again


  2. ohioandy | | #3

    The LAWBs I'm familiar with come in 5-gallon pails. But like Martin alluded to, I agree that you might be fighting a losing battle. Lack of sunlight, a nearby stream, rain and humidity in the air--these things all conspire to grow mold and a layer of fancy paint will not stop it. In fact, LAWBs are vapor-permeable by definition, stopping air and bulk water intrusion but allowing a certain amount of vapor diffusion. How about using a high-quality exterior paint that contains mold inhibitors and is less permeable? Mold is stopped only by having more drying than wetting. I wonder if you need to switch to a non-organic sheathing like cement board or some kind of fiberglass sheet.

  3. Expert Member

    You can stop all moisture moving through the wall in either direction by installing a membrane as your WRB. I don't know enough about humid climates to know whether that's a good idea th0ugh.

  4. GBA Editor
    Martin Holladay | | #5

    I'm not sure what you mean by a "membrane." There are dozens of membranes on the market. Some are vapor-permeable, and some are vapor-impermeable.

    If Shane installs a membrane that "stops all moisture moving through the wall in either direction," that would be a vapor barrier like polyethylene or Grace Ice and Water Shield. Installing a vapor barrier on the exterior side of your wall sheathing is usually dangerous -- it can lead to sheathing rot.

    The exception is a PERSIST house (a house where 100% of the wall insulation -- usually rigid foam -- is installed on the exterior side of the wall sheathing) or a house with lots of exterior rigid foam.

  5. Expert Member

    Yes Martin, I meant a peel-and-stick membrane like Grace I&W. If Shane wants to stop all moisture migrating through from the exterior, that works, while liquid or sheet WRBs don't. Once he has decided to go that way, he needs to design his walls accordingly to dry to the interior, with all the attendant details that come with that approach. Whether walls designed to dry to the interior are a good idea in his climate I don't know.

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