Vapor Barrier Insulation: Definition and What it is
Material layer that prevents moisture vapor from passing through insulation and walls.
What is Vapor Barrier Insulation?
A vapor barrier (or vapor retarder) is a material that resists moisture vapor transmission through walls, ceilings, and floors. It prevents humid indoor air from condensing inside wall cavities and causing mold, rot, and insulation damage.
Vapor barriers work by blocking moisture vapor migration from warm, humid areas to cold areas where condensation can occur. In cold climates, vapor barriers typically go on the warm (interior) side of insulation to prevent indoor humidity from condensing in cold wall cavities. Common vapor barrier materials include polyethylene plastic sheeting, foil-faced insulation, and kraft paper facings on fiberglass batts. Building codes specify when vapor barriers are required based on climate zone—they're critical in cold climates but can cause problems in hot-humid climates where moisture drive is reversed.
Common Vapor Barrier Materials
- Polyethylene Sheeting: 6-mil plastic is standard, 10-20 mil for crawl spaces and basements
- Kraft Paper Facing: Attached to fiberglass batts, provides Class II vapor retarder
- Foil Facing: Attached to batts or foam boards, provides vapor barrier plus radiant barrier
- Vapor Retarder Paint: Special primers that reduce vapor transmission through drywall
Proper Vapor Barrier Installation
In cold climates (zones 5-7), install vapor barriers on the warm side of insulation in walls and ceilings. Use faced insulation with facing toward heated space, or install separate poly sheeting. Seal all penetrations, tears, and seams with acoustical sealant or housewrap tape. In mixed or hot-humid climates, vapor barriers may not be needed or should be on the exterior side—consult local building codes. Never install vapor barriers on both sides of a wall assembly as trapped moisture has no escape route. For crawl spaces, heavy-duty poly ground covers are essential in all climates.
Benefits of Vapor Barrier Insulation
- Prevents moisture condensation in wall and ceiling cavities
- Protects insulation from moisture damage and performance loss
- Prevents mold, mildew, and wood rot in framing
- Essential for long-term building durability
- Helps maintain indoor air quality
- Required by building codes in many climate zones
Vapor Barrier Insulation Cost
Vapor barriers add minimal cost but provide critical moisture protection.
- Material cost (6-mil poly: $0.10-$0.20/sq ft; faced batts include vapor barrier in price)
- Installation labor if using separate poly sheeting
- Sealing and taping requirements
- Crawl space vapor barriers (10-20 mil: $0.50-$1.50/sq ft installed)
- Regional climate requirements
- Building code compliance needs
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