A.Joe Lstiburek, an engineer and principal with Building Science Corp. in Westford, Mass., responds: Installing a vapor barrier on both sides of a wall is never a good idea in any climate. In the Houston climate, a vapor barrier should be located on the exterior, so your choice of exterior foam sheathing was a good one. The concern is the foam sheathing you installed on the interior.
The good news is that because of the thermal resistance of the interior foam, the wall cavity will rarely be below the dewpoint temperature of the exterior air (see “Psychrometric Chart,” below). The bad news is that if moisture ever gets into the wall — say, due to a window leak or negative pressure caused by leaky attic ductwork — it won’t be...
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