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Indoor airPLUS

Technical Guidance to the Indoor airPLUS Construction Specifications

1. Moisture Control

Sections 1.11 - 1.13: Interior Water Management

1.12 When Continuous Water Vapor Barriers Shall Not Be Installed

Do not install continuous vapor barriers on the interior side of exterior walls that have high condensation potential (e.g., below-grade exterior walls in most climates and above-grade exterior walls in warm-humid climates). For the purpose of this specification, vapor barriers are materials that have a perm rating of 0.1 or less (see manufacturer’s product specifications or 2005 ASHRAE Handbook of Fundamentals, Chapter 25, Tables 7A and 7B).


Condensation problems are most likely in walls where:

  • There is a strong source of water vapor outside (e.g., damp earth outside below-grade walls or sun on rain-soaked siding (e.g., masonry) for above-grade walls).
  • There is a combination of building materials that allows water vapor to migrate easily through a good insulator and accumulate on the surface of an intentional or accidental vapor barrier that is a poor insulator.

For example, an interior stud wall erected next to a below-grade basement wall and insulated with mineral wool, fiberglass or cellulose insulation should not have foil-faced paper, polyethylene film or vinyl wallpaper on its interior surface. Water vapor passing from the damp earth through the below-grade concrete or concrete masonry unit (CMU) wall will pass easily through the insulating materials, but accumulate on the backside of a vapor barrier. The entire wall cavity then becomes a cool, damp microclimate. Using materials of 2 perms or more on the interior of the wall allows it to dry into the basement.

An above-grade wall configuration that has high condensation potential is, for instance, one composed of exterior finish material that absorbs rainwater; a vapor-permeable drainage plane, sheathing and insulating materials; and an intentional or accidental vapor barrier on the inside surface of the wall. Consider for example a wall composed of a brick veneer with high-perm building paper or wrap covering an impregnated structural fiberboard sheathing over a fiberglass- or cellulose-insulated stud wall that has polyethylene film or vinyl wallpaper on the interior. During air-conditioning season when the outdoor air is warm and humid, the vapor-drive is from outside to inside and the vapor barrier is in the wrong place, that is, on the cool side of the wall.  This situation is made much worse after a rainstorm soaks the brick. When the sun warms the wall, the vapor-drive towards the cooler wall components becomes many times greater as evaporating water in the pores of the brick increases the water vapor-drive towards the interior. But if all the materials inboard of the brick have a perm greater than 2, (e.g., no vapor barrier, with painted gypsum board as the finish), the water vapor can pass through to the interior, thus avoiding moisture accumulation in the wall.


An appropriate assembly for the inside of a basement wall consists of 1 in. of plastic foam board or closed-cell polyurethane spray foam followed by a stud wall with fiberglass insulation filling the cavities. This assembly is covered with moisture- and mold-resistant gypsum board covered by two coats of latex paint. The foam board serves two purposes: providing a capillary break between the foundation wall and the stud wall and keeping warm, humid basement air away from the earth-chilled foundation. It is good practice to put 1 in. of foam board beneath the sole plate of the stud wall and the basement floor and to leave a ½-in. gap between the basement floor and the bottom of the gypsum board. Doing so provides protection from minor floods and damp concrete floors.

An example of a recommended section for an above-grade wall with an exterior finish that absorbs water consists of brick veneer, a vented air gap behind the brick, building paper or house wrap, plywood or OSB sheathing, stud wall with fiberglass, cellulose or spray foam insulation and painted gypsum board. The vented air gap provides some relief for the water vapor passing through the backside of the brick. Materials on the inside surface that have perm ratings greater than 2 allow the wall to dry to the interior. However, low-perm materials attached to the inside of the wall as a finish (e.g., vinyl wallpaper) or a furnishing (e.g., a mirror affixed to the wall) might accidentally create a vapor barrier in the wrong place. To reduce further the rate of water vapor passage into the wall and to provide additional insulation, 1 in. or more of plastic foam insulation could be installed as sheathing to either replace or cover the OSB.

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