Tight Construction for a Continuous Air Barrier

Claudette Reichel  |  6/25/2008 10:57:53 PM

Construction gaps and resulting air leakage can increase heating and cooling costs, create comfort and moisture problems, draw in pollutants, reduce fire safety and serve as an entry for rodents and insects. Windows, doors and outside walls can contribute to air leakage, but the greatest losses occur in cracks and holes that are hidden from view and cause air exchange between the interior and the attic, crawl space and outdoors.

Homes should have a continuous air barrier system that surrounds all conditioned space — a combination of materials linked and sealed together to create a tight building envelope with little air leakage. Consistent, stringent quality control during construction is important to create and preserve a continuous air barrier. Newly constructed ENERGY STAR homes must follow a Thermal Bypass Checklist, available at www.energystar.gov. Air sealing is first priority. Once achieved, insulation will have a chance to perform at its rated level.

Exterior air barrier system: Air barrier systems that block outside air from entering wall assemblies are especially beneficial in this climate. They reduce infiltration of hot, humid outside air into walls and also reduce “wind washing” through insulation reducing its effectiveness. Examples include properly installed and sealed housewrap over sheathing, sealed and taped foam sheathing, or spray foam insulation on walls (including any attic knee walls) combined with a sealed ceiling, or airtight building envelope systems such as properly detailed SIPS and shotcrete.

The ceiling, floor, foundation-wall connections and band joists between levels are often weak links in the air sealing system and should be addressed. Spray foam insulation, gaskets or caulk can be used to seal holes and connecting materials in those locations. Other types of insulation do not create an air barrier.

Airtight Drywall Approach (ADA): The ADA is a simple and effective interior air sealing system that connects the interior drywall and other building materials together to form a continuous air barrier around interior space. Drywall is sealed with gaskets or adhesive to top and bottom framing plates, corners and around openings in walls and ceilings. ADA and systems that block inside air from entering wall assemblies are especially beneficial in winter to control the entry of moist interior air into wall cavities where it could condense on cold-framing. It is also an easy, inexpensive way to seal ceilings.

Bypasses: Hidden areas that bypass the building envelope create major pathways for unwanted air exchange with the attic, crawl space or other unconditioned areas. Common neglected bypasses include soffit areas over kitchen cabinets, the return air plenum (path from the air filter to the air handler unit), the area around a fireplace chimney, plumbing stacks, unfinished areas behind tubs and showers, dropped ceilings, chases for ductwork, exhaust vents with no dampers and even insulation.

Penetrations: Holes in the building envelope and air barrier system commonly left unsealed include holes for plumbing, electrical boxes; exhaust fan housings, air registers, attic access doors, and wiring. Airtight electrical boxes with gaskets are available; regular boxes should have wiring holes caulked and be sealed to the drywall..

Recessed light fixtures: Standard recessed lights put a leaky, uninsulated hole in the worst place of all - the attic floor. It’s best to avoid them altogether, and choose surface-mounted fixtures. If recessed lights are used, specify “airtight” models rated for insulation coverage (ICAT-labeled), and make sure the gasket is installed. Existing leaky recessed lights can be retrofitted by replacing the outside trim and inserting a special cone.

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