An important requirement for a high-performance home is the ability to effectively control the indoor environment. Keeping conditioned air inside and external air outside keeps the indoors comfortable for occupants, and it helps to 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 first understand how air and moisture move across the building envelope and then apply construction Best Practices® to manage this movement.
In general, air moves from warm to cool, carrying moisture with it. For example, 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 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 vapor retarders or barriers.
Air sealing
The goal of air sealing is to help 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. Penetrations by wiring, plumbing, and ductwork create air gaps, however, that must be sealed. For optimum performance, it’s important to seal air gaps throughout the construction process—during framing, before insulating, before installing drywall, after installing drywall and interior finishes, after installing fixtures, and during the final punch-out. To stop air leaks, use low-expanding foams, foam strips, weatherstripping, weatherproof tape, and caulks in these areas throughout the home:
- Sill plates
- Bottom plates of the exterior walls
- Window openings and exterior door openings
- Seams between sheets of drywall
- Attic access hatch cover
- HVAC penetrations in top and bottom wall plates, ceilings, and floors
- Duct boot penetrations
- Bathroom ventilation fan penetrations
- Attic bypasses and chases (open partition walls, dropped ceilings, and duct and flue chases)
- Bathtub drain penetration
- Penetrations for plumbing pipes
- Penetrations for electrical boxes (receptacles, switches, lights, and circuit breaker boxes)
- Wiring and plumbing penetrations in top and bottom wall plates, ceilings, and floors
- Exterior penetrations (porch light fixtures, outside outlets, and phone and electrical service holes)
Vapor retarders and barriers
To control moisture in a home, you have to keep moisture from entering the building assembly, which consists of the exterior wall, floor, and ceiling cavities. You also have to provide a way for the building assembly to dry if moisture does enter it. For effective moisture control, you should install either a vapor retarder or a vapor barrier in walls, floors, and ceilings that separate the conditioned living space from the exterior.
The climate in which you're building determines which product to install and where to install it:
Hot climate. Make it difficult for moisture to enter the building assembly from the exterior, while allowing any moisture to dry toward the interior. To accomplish this, install a vapor retarder on the exterior of the assembly, where the warmest surfaces are located.
Cold climate. Make it difficult for moisture to enter the building assembly from the interior, while letting any moisture dry toward the exterior. To accomplish this, install a vapor retarder on the interior of the building assembly, where the warmest surfaces are located.
Mixed climate. Protect the building assembly from moisture, and allow it to dry to both the interior and the exterior. To accomplish this, 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. Also, be sure to install only permeable materials on both the interior and exterior sides of the assembly, so moisture can flow through without accumulating.
Very cold climate. In very cold climates, such as Minnesota’s and Wisconsin’s, install a vapor barrier if it’s required by building code. However, don't install a vapor barrier when a vapor retarder can successfully do the job, because a vapor barrier prevents moisture from drying in at least one direction. When using a vapor barrier, avoid installing it on both sides of the building assembly.
Air sealing and moisture control have a significant impact on the indoor environment. Air sealing all penetrations and using the right moisture control strategy for the climate can go a long way toward providing your customers with a comfortable home.
More information about air sealing and moisture control can be found in BuildIQ University’s Water Management Suite of courses and the Insulation & Air Sealing online training course.
