Spray insulation foam itself acts as a vapor barrier.
Vapor barriers were traditionally applied as an attempt to curb utility costs. The thought was that preventing water movement from outside the house to inside would reduce energy costs. Ultimately vapor barriers often caused more harm than good.
Moisture can come inside a house through air leaks or gaps at the edge of baseboards. If these gaps were fully sealed, we would have no need to worry about vapor. However, existing gaps and the vapor barriers themselves can provoke these issues.
Vapor barriers can be damaging to walls in the case that water gets trapped inside the house. When the temperature difference is great enough, a house can “sweat.” If the vapor barrier prevents this moisture from escaping, it risks damaging the wood and integrity of the walls.
With traditional insulation, the moisture can quickly become a mold problem. For these reasons, a house needs to breathe while preventing the walls from retaining moisture. A house that has movement of air can more easily control the movement of water.
There are a few environmental conditions where they advise installing a vapor barrier. In very hot climates or very cold climates, the vapor barrier serves to prevent moisture from being trapped in the house.
Hot, humid climates should install vapor barriers and may add exterior vapor barriers for extra protection. Cold climates should use polyethylene plastic vapor barriers between the wall and insulation.
Vapor barriers are often installed in basements and crawl spaces to prevent ground moisture from leaking in.
In houses with spray foam insulation, however, vapor barriers are not necessary.
Quality closed cell foam insulation acts as a vapor barrier and does not absorb moisture; at the same time, the insulation prevents leakage by closing air gaps. With proper installation of foam insulation, the house should be well sealed and allow the house to breathe.
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