How to Vapor-Proof Rigid Foam Insulation

Vapors and moisture travel through the exterior walls on a house, following the air flow as it travels from the hot area toward the cold where the moisture condenses. If this moisture becomes trapped between the walls in a house, wood rot or mold may occur. Rigid foam insulation is considered a vapor diffusion retarder, which slows the movement of water through the walls, giving it a chance to evaporate properly. Install the foil side pointing outward, before vapor-proofing the insulation.

Closed-cell expanding spray foam
Foil tape

Instructions

- 1
  Attach the nozzle extension to the closed-cell expanding spray foam. Place the nozzle between any rigid foam insulation boards with a gap larger than 1/4 inch. Fill the gap with the foam. Continue filling the gaps between the boards.
- 2
  Let the expanding spray foam dry completely. Cover all the seams between the boards with foil tape on foil faced insulation. If the insulation is not faced with foil, use contractor’s tape.
- 3
  Add more tape, covering the corners of the structure and any other openings in the insulation. Tape up the seam between the insulation and windows and doors. Run a strip of foil tape around the bottom of the insulation to prevent insulation degradation.

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