Vapor Barrier
What is a Vapor Barrier?
Vapor barrier is an impermeable membrane that blocks the flow of air through the building envelope.  A vapor barrier is an essential part of the building envelope.
Because the purpose of a vapor barrier is not obvious, this important component is often omitted or installed incorrectly.
The main purpose of a vapor barrier is preventing the passage of the water vapor that is contained in air. Vapor barriers and the insulation affect each other.  They must both be installed so that they interact beneficially rather than harmfully.
Purposes of Vapor Barriers

The specific functions of vapor barriers are:
  • Protecting the envelope structure and insulation from condensation damage.  Many wall materials are permeable to the flow of water vapor from inside to outside, or vice versa.
  • Preventing air leakage through the envelope.  A well-installed vapor barrier prevents or greatly reduces air leakage through the envelope surfaces, although it does not reduce air currents inside the envelope structure itself.  At the same time, the vapor barrier reduces air flow through the insulation, preserving the R-value.
  • Maintaining interior humidification. If humidification is used, a vapor barrier reduces the amount of energy and water required to maintain the desired level of humidity.

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