Step 4: Add the Vapor Retarder

man installing vapor retarder over batt insulation
Photo: Ryan Benyi
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In cold climates, a vapor retarder on the insulation's inside face prevents conden-sation that robs R-value and encourages mold. Batts faced with paper or foil have the retarder built in. Unfaced batts should be covered with 6-mil plastic or MemBrain (shown), a breathable sheet. Apply a bead of sealant to the top plate and to any studs where the sheets overlap, then tack the sheet every 12 to 24 inches to the top plate, studs, and bottom plate.
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    Tools List

    • gloves
      gloves
    • dust mask
      dust mask
    • straightedge guide
      2x4 straightedge
    • utility knife
      utility knife
    • staple gun
      staple gun or hammer tacker
    • caulk gun
      caulk gun

    Shopping List

    1. Batt insulation
    to match the depth of the stud. To estimate how much you need, measure the square footage of each wall and subtract 10 percent.

    2. Vapor retarder
    to estimate how much you need, measure the square footage of the wall and add 5 percent.3. Silicone Sealant