The Truth About Vapor Barriers Part 2: Typical 6-mil Poly Vapor Barrier Installation
The following points are illustrated in the diagram at the bottom of this page:
In fiberglass house construction, a 6 mil poly plastic sheet is typically installed as a vapor barrier
During the construction process, the 6 mil poly vapor barrier gets stapled, punctured by drywall screws and torn for electrical receptacles. The integrity of the barrier is compromised, and it now becomes a vapor retarder.
Moisture in air passes through the 6 mil poly, condenses and becomes trapped in the wall cavity.
During winter conditions, as fiberglass insulation’s real-time R-value decreases, the moisture is trapped as water crystals.
In warmer conditions the crystals thaw to water particles and settle on the bottom lumber plate –
the nutrient base for mold growth.
