Note 42. Vapor Barriers
Most of the energy conservation measures in the Energy Efficiency Manual that relate to insulation stress the importance of installing vapor barriers properly in combination with insulation. This Note explains the purposes of vapor barriers, where vapor barriers should be installed, vapor barrier materials, and effective venting of moisture.
A vapor barrier is an impermeable membrane that blocks the flow of moisture through the building structure or envelope. The purposes of a vapor barrier are to protect the building structure and insulation from damage by moisture condensation, to block air leakage through the building structure, and to maintain interior humidity.
A vapor barrier is an essential part of the building skin. Vapor barriers and insulation must work together as a system. Because the purpose of a vapor barrier is not obvious, they are often omitted or installed incorrectly. The Note shows how vapor barriers should be installed.
House wraps are often confused with vapor barriers. House wraps are intended primarily to prevent air leakage. If used as a vapor barrier, they may cause structural damage. The Note explains house wraps, where to use them, and where to avoid them.