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Do people report mildew problems in their homes as a result of using spray foam insulation?

[email protected] | Posted in General Questions on

I am considering using spray foam insulation in my metal building and then building out an apartment inside. The a/c contractor is discouraging me using the spray foam because he claims that he has encountered homes that have a “wet rag” odor because of problems regulating the moisture in homes with spray foam insulation. Is this likely?

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  1. GBA Editor
    Martin Holladay | | #1

    Your A.C. contractor is wrong.

    Installing spray foam insulation often results in a building with a low rate of air leakage. That's a good thing.

    Spray foam is an air barrier. It seals leaks through cracks. That makes the house tighter and therefore easier to heat and cool.

    Every builder should be striving to reduce air leakage through the floors, walls, and ceilings of the homes they build. Tight houses are well-built houses.

    Every tight house needs a mechanical ventilation system. For more information on ventilation systems, see Designing a Good Ventilation System.

    If a building has a "wet rag" odor or mildew (mold) problems, it is probable that (a) the indoor relative humidity is too high, and/or (b) the house lacks a well designed ventilation system. The problem is not caused by spray polyurethane foam insulation. (That said, a very small percentage of spray foam jobs have odor problems. This rare phenomenon occurs when spray foam installers make an error during installation.)

    High indoor humidity can have many causes, including (a) a wet basement or crawl space, (b) crowded conditions (for example, a large family living in a small space), or (c) unusual hobbies like raising bananas indoors or raising tropical fish. If a building has high indoor humidity, it's important to identify the sources of humidity and mitigate the effects of these moisture sources. It is often useful as well to improve the ventilation rates.

    Finally, it's usually a bad idea to install spray foam insulation directly against the steel siding panels of a steel building, for many reasons -- including the fact that the spray foam makes future repairs difficult.

  2. [email protected] | | #2

    Martin, You have previously mentioned the idea of spraying the foam directly onto the metal as a bad idea. Other than the fact that is makes future repairs difficult, what may be other problems this scenario could create? What would be my options if I still want to use the foam? Also, obviously, I am trying to create a space that is well insulated to reduce my energy costs. I suppose another option would be to install batt insulation with one side vinyl (as a moisture barrier) against the steel, leave an air space and then install traditional batt insulation in my wood frame buildout. Is this over-kill? What are your thoughts?

  3. GBA Editor
    Martin Holladay | | #3

    As long as you recognize the downsides, you can install spray foam against your metal siding if you want. It's your building. If you look around at other buildings with metal panel siding, however, you will often see that these panels begin rusting at the bottom due to splashback. It won't be easy to replace a rusting panel in the future if everything is glued together with spray foam.

    Ideally, you need to install an intervening material between the steel siding and the spray foam. One possibility is to install a three-dimensonal plastic drainage mat bonded to a sheet membrane -- perhaps something like Stuc-O-Flex WaterWay Rainscreen drainage mat or MTI Perforated Control Cavity -- with the drainage mat facing toward the steel siding, and the membrane facing toward the spray foam.

    If you decide to use a different type of insulation, you need to create a framed wall on the interior side of your steel building. This framed wall needs an air barrier. It's a challenge, but people do it.

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