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Do you need a vapor retarder in your wall assembly?
Do you need a vapor retarder? Learn why modern building science often suggests avoiding them, especially when you have continuous exterior insulation.
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Groundwork ResearcherIn most modern wall assemblies with continuous exterior rigid insulation, an interior vapor retarder is not only unnecessary but can be counterproductive by trapping moisture. Focus on creating a continuous air barrier first. If you are concerned about moisture, prioritize high-quality air sealing and consider a 'smart' vapor retarder that allows for seasonal drying.
Do you need a vapor retarder? Learn why modern building science often suggests avoiding them, especially when you have continuous exterior insulation.
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Yes, the kraft paper facing on fiberglass batts acts as a Class II vapor retarder. In many cases, this is sufficient for code, but in high-performance homes with exterior insulation, it may still be considered too restrictive for optimal drying.
No, using 6-mil polyethylene plastic as an interior vapor barrier is generally discouraged in modern building science. It prevents the wall assembly from drying toward the interior, which can lead to trapped moisture and long-term structural damage.
A smart vapor retarder is a membrane that changes its permeability based on the relative humidity within the wall cavity. It acts as a vapor barrier in winter to prevent moisture from entering the wall and becomes more permeable in summer to allow the wall to dry out.
A wall dries correctly when it is designed to allow moisture to escape in either direction—inward or outward—depending on the season. This is achieved by avoiding low-perm materials like plastic on the interior and ensuring the exterior cladding is properly ventilated with a rainscreen.
No, edge vents generally do not work when buried in snow. Snow acts as an airtight seal, blocking the intake path and stopping the airflow required for attic ventilation. This can lead to moisture buildup and ice dams during the winter months.