Where does the vapor barrier go in an attic that has 2 layers of batt insulation?

• A. Above the insulation
• B. Between the layers of insulation
• C. Below the insulation
• D. Stapled to the rafters

Answer: (C)

Moisture control inside a heated space relies on locating the vapor barrier on the warm side of the insulation. In an attic with two batt layers, the warm side is toward the living space, so the vapor barrier should be installed beneath the bottom layer of insulation. This blocks indoor humidity from diffusing into the insulation and toward the cold roof surface, reducing the risk of condensation.

Placing the barrier above the insulation would allow moisture-laden indoor air to enter the attic, where it can condense on cold surfaces. Placing it between the two layers could let vapor bypass the bottom layer and still reach the upper layer. Stapling a barrier to the rafters wouldn’t seal moisture paths effectively. Hence, the barrier belongs on the interior side, below the insulation.