In exterior wall detailing for a climate with mild winters and hot, humid summers, where should the vapor retarder be applied?

• A. On the warm side of the insulation between the studs
• B. On the exterior side of the sheathing
• C. On the interior finish
• D. On the cold side of the insulation

Answer: (A)

In this climate, moisture moves from the warm, moist interior toward the cooler exterior. Placing a vapor retarder on the warm side of the insulation—between the insulation and the interior finish—helps keep interior moisture from diffusing into the wall cavity where it could condense on cold surfaces. This positioning reduces the risk of condensation and mold inside the wall and allows any remaining moisture to dry toward the interior rather than into the wall assembly.

Putting the vapor retarder on the exterior side of the sheathing would trap moisture inside the wall, increasing condensation risks. Locating it on the interior finish or on the cold side of the insulation would either misalign with the typical vapor pressure gradient or push moisture toward cooler interior spaces, which is undesirable.