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Insulation needs a vapor retarder 

Wall insulation - Fiberglass batts can be applied if walls are opened to the interior. Care must be taken to not compress or leave voids in the insulation in order to maintain thermal performance. Penetrations (such as wires, electrical boxes, pipes) must be carefully sealed with caulking or spray foam to prevent air leakage.

As with a glass of ice water, condensation occurs when warm, moist air  touches a cold surface. When this happens in your home, it can cause  mold, mildew, and damage to building materials and reduce the effectiveness  of the insulation.

A vapor retarder slows the movement of air and water vapor through building materials; in fact, a good vapor retarder will allow very little moisture to pass through. Some vapor retarders also serve as air barriers. A good vapor retarder and good air-sealing can improve the energy-related performance and durability of your home.

Most homes built before 1970 do not have a separate vapor retarder component; however, the predominance of oil-based paints in those homes usually provides an adequate vapor control function.

Whenever you install insulation, always include a vapor retarder. If you’re adding insulation to an area that already has a vapor retarder, you do not need to add another one. Vapor retarders include:

  • Polyethylene sheets are most frequently used as a vapor retarder, but they may not be a good choice if you have central air-conditioning; hot, humid outside air can be driven through the wall assembly and condense in the cooler wall cavity, leading to potential moisture problems.
  • Foil or kraft paper often is part of fiberglass batt or blanket insulation. The vapor retarder should face the interior of the building.
  • Most rigid foam board insulation acts as a vapor retarder when installed under an interior covering material such as drywall. If the foam is also intended to be an air barrier, the seams should be taped—for both interior and exterior use—to improve its performance.
  • Sprayed-in foams (such as polyurethane insulation) may not need a vapor retarder when installed properly. Check your local building codes for vapor retarder requirements with spray foams.
  • “Smart vapor retarders” are a relatively new product that allows moisture to migrate out of a wall cavity under certain conditions. Consisting of a film that is applied over un-faced fiberglass or other insulation (similar to polyethylene sheets), smart vapor retarders have a high resistance to water vapor under low-humidity, winter conditions, thus slowing moisture movement from the inside environment into the wall. During high-humidity, summer conditions, the permeability adapts, thus permitting any trapped moisture in the walls to migrate to the interior.
  • Finally, vapor retarder paint (often a primer sealer) can provide adequate vapor control and can be applied to new or previously painted interior surfaces.