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Community and Q&A

Condensation within wall cavity in the cooling season

Matt_Salkeld_PEng | Posted in General Questions on

Is it possible for water vapor from outdoors to condense inside the wall cavity (say against the back of the drywall) during a humid summer with the house cooler and dryer than outdoors?

This is a cold climate but also with hot and humid summers and central air conditioning.

The question is for a century home retrofit with the exterior being left intact.  Therefore no Tyvek to prevent outside humidity/vapor travelling from outside to inside. How serious is the driving of humidity indoors in summer compared to winter driving home humidity outdoors?

Would a smart MemBrain vapor barrier help mitigate this to allow humidity in the cavity to travel into the house in summer?  The a/c should dispose of surplus humidity indoors.

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  1. Expert Member
    Dana Dorsett | | #1

    Yes, it's possible for exterior moisture drives to accumulate moisture in air conditioned buildings. It's more likely to occur in buildings with cladding that hold onto dew/rain moisture, such as stucco or brick, than with other types of claddings.

    It's also more likely to happen in homes with true vapor barriers like 4-6 mil polyethylene under the wallboard, or highly vapor retardent interior finishes such as foil or vinyl wallpapers. MemBrain can't force drying toward the interior- the drying rate will be determined by the interior finish materials.

    With standard interior latex on wallboard and with half-inch plywood or OSB exterior sheathing the risks are pretty low even in fairly humid Gulf Coast type climates, as long as the assembly is reasonbly air tight . OSB and plywood are both "smart" vapor retarders, and limit rate that moisture can enter the cavity. In very humid climates it's reasonable to even install true vapor barriers on the exterior side of the sheathing, especially with moisture reservoir claddings such as brick.

    In the US this is largely an issue limited to the southeast (including Puerto Rico and the Virgin Islands), climates zones 3A or lower, though with unvented brick cladding and no roof overhangs to protect from rain-wetting it can sometimes be an issue as far north as zone 7A.

    With a vented 1" air gap between brick and OSB or CDX sheathing and reasonable roof overhangs it's usually not a problem even in steamy zone 1A.

  2. Matt_Salkeld_PEng | | #2

    Thank you Dana

  3. walta100 | | #3

    The walls themselves are unlikely to fall below the dew point but be sure to keep your ductwork inside you thermal envelope and not exposed unconditioned air. As it is could fall below the dew point and get wet.


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