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

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

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

Answer: (C)

Moisture diffusion needs to be blocked on the warm side of the insulation to keep the attic space from getting damp. In a two-layer batt setup, the best spot for the vapor barrier is between the two layers. The bottom layer sits against the interior space, so placing the barrier there stops interior moisture from moving into the upper layer and toward the roof deck. This keeps the upper insulation and roof structure drier and helps prevent condensation, mold, or rot. If the barrier were placed above the insulation (against the roof deck), moisture could be trapped in the insulation and on the underside of the roof, increasing the risk of condensation. If it were placed below the insulation (on the interior side of the bottom layer), it would be less effective at protecting the upper layer from moisture diffusion toward the roof. Sticking the barrier to the rafters isn’t correct because it wouldn’t reliably create a continuous barrier on the warm side of the insulation.

Moisture diffusion needs to be blocked on the warm side of the insulation to keep the attic space from getting damp. In a two-layer batt setup, the best spot for the vapor barrier is between the two layers. The bottom layer sits against the interior space, so placing the barrier there stops interior moisture from moving into the upper layer and toward the roof deck. This keeps the upper insulation and roof structure drier and helps prevent condensation, mold, or rot.

If the barrier were placed above the insulation (against the roof deck), moisture could be trapped in the insulation and on the underside of the roof, increasing the risk of condensation. If it were placed below the insulation (on the interior side of the bottom layer), it would be less effective at protecting the upper layer from moisture diffusion toward the roof. Sticking the barrier to the rafters isn’t correct because it wouldn’t reliably create a continuous barrier on the warm side of the insulation.