Controlling Hidden Moisture

Indoor air in cold climates may be more humid than outdoor air, and vapors will then flow from the warmer interior to the colder exterior surfaces of the walls, ceilings, and floors. This can leave the building envelope permeated with moisture. Wet insulation becomes less effective, and dry rot, a fungus disease of timber that can cause it to become brittle and crumble into powder, can afflict wood structural members.

In hot humid climates, you need to keep the moisture from getting into the interior of the building. A drainage plane inside the exterior surfacing material, such as tarpaper, will let moisture that gets through wick away to the inside. This is safer than using a vapor barrier that may keep the moisture trapped in the wall.

When the moisture content of the air rises inside a building, it creates vapor pressure, which drives water vapor to expand into areas of lower vapor pressure like the exterior walls, seeking equilibrium. When there is moist air on one side of a wall and drier air on the other, water vapor migrates through the wall from the moist side to the drier side. Water vapor will also travel along any air leaks in the wall. Most building materials have relatively low resistance to water vapor. When the temperature at a given point within a wall drops below the dew point at that location, water vapor condenses and wets the interior construction of the wall. This condensation causes an additional drop in vapor pressure, which then draws more water vapor into the area. The result can be very wet wall interiors, with insulation materials saturated and sagging with water, or frozen into ice within the wall. The insulation becomes useless, and the heating energy use of the building increases. The wall framing materials may decay or corrode, and hidden problems may affect the building's structure. The

Wood siding

Plywood sheathing

Insulation

Surface air

Wood siding

Plywood sheathing

Insulation

Surface air

Figure 17-1 Vapor barrier.

Gypsum wallboard

Vapor retarder

Surface air also provides some insulation.

Figure 17-1 Vapor barrier.

amount of vapor pressure within a building depends on the amount of vapor produced, its inability to escape, and the air temperature.

A solid coat of exterior paint that keeps the water vapor from traveling out through the building's wall will trap vapor inside. Vapor pressure can raise blisters on a wall surface that will bubble the paint right off the wall. This is sometimes seen outside kitchens and bathrooms, where vapor pressure is likely to be highest.

By using a vapor barrier (Fig. 17-1) as close to the warm side of the building envelope as possible, this creeping water vapor can be prevented from traveling through the wall. The vapor barrier must be between the main insulating layer and the warm side of the wall. In a cold climate, the vapor barrier should be just under the plaster or paneling inside the building. In an artificially cooled building in a warm climate, the warm side is the outside.

Vinyl wallcoverings or vapor barrier paints on interior surfaces offer some protection, but don't replace the need for a vapor barrier. Aluminum-foil faced insulation is effective for thermal insulation, but does not make an adequate vapor barrier. Plastic films are tight against both moisture and infiltration. Where adding a vapor barrier to an older building is not practical, plugging air leaks in walls and applying paint to warm-side surfaces, and providing ventilation openings on cool-side surfaces clears moisture from the construction interior. Special vapor retardant interior paints are available for this purpose.

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