How to Make Rigid Foam Insulation Waterproof


Rigid foam insulation products are ideal for surfaces on which you intend to install flooring or wall coverings that need a flat, sturdy surface, such as tiles, linoleum or wallpaper. If you install foam insulation in a room in which moisture may be a concern though, like a bathroom, you should waterproof the foam insulation to prevent moisture from seeping into the foam and eventually deteriorating the product.

Things You'll Need

  • Vapor barrier
  • Stud finder
  • Staple gun
  • Staples
  • Tape
  • Waterproof sealant
  • Caulk gun
  • Install the vapor barrier to the wall, ceiling or subfloor before installing the rigid foam insulation on the surface. If you already have the foam insulation in place, take the insulation back down. Don't just install the vapor barrier over the foam insulation, which may still allow moisture to accumulate in the insulation from outside humidity, leaving to the destruction of the insulation.

  • Find the joists in the floor or ceiling or the studs in the wall. Use a stud finder if the joists or studs are not exposed to locate the joists or studs and mark each joist or stud every few inches across the floor or ceiling or down the wall, so that you know where they are when you begin to install the vapor barrier.

  • Roll out the vapor barrier to its full size and hold the corner of the vapor barrier up to the corner of the surface on which you want to waterproof the foam insulation. Staple the vapor barrier into the corner.

  • Smooth the vapor barrier down the one edge of the floor, ceiling or wall. Staple the barrier to the surface at 1-inch intervals. Once you connect the barrier fully down one side, smooth barrier along the surface, securing it to each stud or joist with staples every 1-inch.

  • Use a utility knife to cut holes in the vapor barrier to accommodate wall fixtures. Cut the holes just large enough to allow the fixture to poke through the barrier.

  • Apply a bead of waterproof sealant at the edges of the vapor barrier with a caulk gun. Also apply beads around the fixtures where you cut the vapor barrier.

  • Install or reinstall the rigid foam insulation onto the surface over the vapor barrier per the manufacturer's instructions.

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