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.
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:
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.
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