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Community and Q&A

Flash & Batt Condensation Issue

Robert Tonelli | Posted in GBA Pro Help on

I am building a new home in New Jersey. In 2X6 exterior partitions, I am putting 2″ of closed cell spray foam insulation followed by 3 1/2″ of unfaced fiberglass batt insulation.
During my insulation inspection my local building inspector posed the following question:
What is stopping the moisture/humidity from inside the home from traveling into the wall and condensing between the back of the drywall and the sprayfoam?

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  1. Riversong | | #1

    You mean exterior walls - partitions are the interior walls that partition the interior space.

    To answer the inspector's question, there is nothing stopping water vapor diffusing through the drywall and the fiberglass and, if the air barrier is not complete, also the potential for air movement (channel flow) which can also carry moisture into the wall cavity. Channel flow can occur, for instance, through a leaky electrical box and up to a wiring hole in the top plate and out another electrical box or other unsealed penetration or perhaps into the attic.

    But significant condensation will not occur unless the inside surface of the closed-cell foam is at or below the dew point of the interior air (or the air behind the drywall).

    So, what is necessary to avoid problematic condensation is indoor humidity control, which includes spot ventilation from bathrooms and kitchen range, and whole house ventilation, and perhaps summertime dehumidification. Additionally, all framing penetrations, particularly vertical ones, must be sealed. And, if the attic is vented, then any possible penetration into the attic must be completely sealed.

    You should also have the code-required 1 perm vapor retarder on the interior, such as vapor retarder latex primer. Do not use an interior poly vapor barrier, as that will trap moisture between two impermeable layers in the thermal envelope.

  2. David Meiland | | #2

    I give your insulator some credit. When I first started hearing about flash & batt the idea was to install 1/2" - 1" of foam.

  3. GBA Editor
    Martin Holladay | | #3

    Your wall will be fine. Don't worry.

    Condensation (or sorption, or moisture accumulation) only happens when warm, humid air (or water vapor) contacts a cold surface. There is no cold surface in your stud cavities, so you're fine.

    The 2 inches of closed-cell foam provides a thermal barrier and an air barrier -- keeping the interior surface of the foam above the dew point. No cold surfaces = no condensation.

  4. Riversong | | #4

    Yes, the interior surface of the foam should be above the dew point of a moisture-controlled house, but there are thermal bridges at each framing member, and likely many areas with built-up framing around doors and windows and corners and partition take-offs. The wood framing can become a condensation surface if exterior conditions are sufficiently cold and, because the closed-cell foam is non-absorptive, moisture will tend to concentrate in the framing.

    Regardless of the theoretical air and thermal barrier, it's vital to limit interior humidity and make sure there's no opportunity for channel flow air leakage.

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