Controlling Moisture Movement With Air and Vapor Barriers

An important requirement for a high-performance home is the ability to control the indoor environment and, in turn, control the homeowner's energy bills. Keeping the interior dry is important, too, because when moisture gets trapped inside a building assembly, it creates problems such as wood rot, mold and drywall swelling. To effectively control the indoor environment, a builder must understand how air and moisture move across the building envelope and apply construction practices to manage this movement.

In general, air moves from warm to cool, carrying moisture with it. In a hot climate, moist air moves from the hot exterior of the home to the cool, dry interior. In a cold climate, moist air moves from the warm interior to the cold exterior. As air and moisture move between the exterior and the interior, moisture condenses onto the cold sheathing or cool drywall, degrading the R-value of insulation and promoting mold growth. Two key strategies that builders can use to control air and moisture movement are air sealing and the use of vapor retarders or barriers.

Air Sealing

The goal of air sealing is to create a continuous air barrier between the conditioned living space and the outdoors. Drywall, interior sheathing and floor decking create this air barrier in large part, especially if they're glued to the framing. But penetrations by wiring, plumbing and ductwork create air gaps that must be sealed. For optimum performance, it's important to seal air gaps throughout the construction process—during framing, before insulating, before and after installing drywall, after applying interior finishes, after installing fixtures and during the final punch-out. Air sealing should not simply be relegated to late in the construction process, because at that point many areas are inaccessible. To stop air leaks, use low-expanding foams, foam strips, weatherstripping, weatherproof tape and caulks in the following areas:

• Sill plate
• Bottom plates of exterior walls
• Window openings and exterior door openings
• Bathtub drain penetration
• Wiring, plumbing and HVAC penetrations in top and bottom wall plates, ceilings and floors
• Penetrations for plumbing pipes and electrical boxes (receptacles, switches, lights and circuit-breaker boxes)
• Duct boot penetrations
• Bathroom ventilation-fan penetrations
• Exterior penetrations (porch-light fixtures, outside outlets, and phone and electrical service holes)
• Attic bypasses and chases (open partition walls, dropped ceilings, and duct and flue chases)
• Seams between sheets of drywall
• Attic access-hatch cover

Vapor Retarders and Barriers

First, some terminology. A vapor retarder controls the entry of moisture into a building assembly. It has a permeability of greater than 0.1 perm but less than 1.0 perm, making it a semi-impermeable material. A vapor barrier, on the other hand, stops the entry of moisture. It has a permeability of 0.1 perm or less, making it an impermeable material.

The goal of using vapor retarders and vapor barriers is twofold: to prevent moisture from entering the building assembly from the exterior or interior, and to enable the building assembly to dry to the exterior or interior or both if the assembly gets wet. To achieve this goal, install either a vapor retarder or a vapor barrier in the building assemblies. Here are tips on which product to install and where to install it, based on the climate in which you're building.

• In a hot climate, install a vapor retarder and an air barrier on the exterior of the assembly, where the warmest surfaces are located. This makes it difficult for moisture to enter a building assembly from the exterior while allowing any moisture to dry toward the interior.
• In a cold climate, install a vapor retarder and an air barrier on the interior of the building assembly, where the warmest surfaces are located. This makes it difficult for moisture to enter the assembly from the interior while letting any moisture dry toward the exterior.
• In a mixed climate, install permeable materials on both the exterior and interior sides of the assembly so moisture flows through without accumulating. Use a hot-climate or cold-climate strategy in addition to a dehumidification system, or install a vapor retarder in the middle of the building assembly. This protects the building assembly from moisture and allows it to dry to both the interior and the exterior.
• In very cold climates such as Minnesota's and Wisconsin's, install a vapor barrier only when building code requires it. Don’t install a vapor barrier when a vapor retarder can successfully do the job, because the impermeability of vapor barriers prevents moisture from drying in at least one direction. When using a vapor barrier, avoid installing it on both sides of the building assembly.