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REScheck 101 Training: Envelope General Requirements |
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Moisture Control
Non-vented framed cavities (walls, floors, and ceilings) need an approved vapor retarder with a maximum perm rating of 1.0. Non-vented areas are framed cavities without vents or other openings allowing the free movement of air. The ability of a material to retard the diffusion of water vapor is measured by units knows as "perms" or permeability. The lower the perm rating, the better the vapor retarder. Vapor retarders help control the amount of moisture that gets through the insulation and are typically installed on the "warm-in-winter" side of the insulation. Vapor retarders if required by the code, in warm, moist climates are sometimes installed on the warm-in-summer side of the insulation.
Exceptions
- Where moisture or its freezing will not damage the materials
- Hot and humid climates
- Where other approved means to avoid condensation are provided (must be approved by the jurisdiction)
Types of products that may meet the perm requirement:
- Polyethylene sheeting over unfinished batts
- Foil and kraft-backed insulation
- Some blown-in insulations (check with the manufacturer)
- Certain paints (check with the manufacturer)
Example of vapor retarder installation.
The Importance of Vapor Retarders in Certain Climates
Examples of Installed Vapor Retarders
Additional Resources
The Importance of Vapor Retarders
DOE Consumer Guide: "Vapor Barriers or Vapor Diffusion Retarders"
EEBA Article on Moisture Control - pages 4-5
"Understanding Vapor Barriers"
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