I Am Putting A New Metal Roof Over An Existing Mobile Home Metal Roof. I Am Going To Build A Truss System With Purlins To Span The Existing Roof. There Will Be About A 12" Air Space. Do I Need A Vapor Barrier? In Montana We Get Snow But Not Humidity.


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Thomas Rooney Profile
Thomas Rooney answered
Deciding whether or not to install a vapor barrier into your roof is entirely a personal choice. There are no building codes or rules that make it a requirement to have one but the protection it offers insulation may be useful when living in a space that gets snow, such as Montana.

Vapor barriers help prevent as much water as possible from permeating through the walls and reaching your insulation. Installing a vapor barrier before you install your new roof will give you an extra measure of protection against damaging moisture. It is possible to buy vapor barriers from local home improvement stores and they are sold with carrying rates called perms. Perms are the measurement of how well the barrier prevents water vapor permeation. Any barrier that has a perm rating of less than one is sufficient for protection against water vapor in your house.

There are many benefits of having a vapor barrier that may make you decide that it is necessary for you to install one in your new roof system. According the United States Environmental Protection Agency and the Centers for Disease Control, installing vapor barriers is one of the easiest and most effective ways to deter the proliferations of mould and mildew growth. As the water vapor penetrates the walls it forms condensation between them. This moisture will breed bacterial and fungal growth that can reach the inner wall. Once this has happened, the walls will need completely tearing down and replacing. Without a vapor barrier you run the risk of letting your insulation become wet. If this happens its properties dramatically weaker and this can lead to heating and cooling energy loss that in turn will increase your electric bills. If the wet insulation has contact with any framing studs this can lead to wood rotting. Extreme wood rotting can stop the framing studs from being able to support the weight of the walls, a problem that can do a lot of damage.
Arthur Wright Profile
Arthur Wright answered
Best to put one in just for the added safety feature just in case as its better to have it and not need it than need it and not have one

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