BUILDING ENVELOPE: Infiltration

Questions:

• Where are the most critical air leaks in a home?
  
• What is an air barrier, or house wrap?
  
• How effective is covering windows with plastic at sealing a window?

Where are the most critical air leaks in a home?
These are likely to be found in the attic, as holes around plumbing and electrical lines, and other gaps in framing. If this is an existing home, move the insulation out of the way to find many of these. Using a foam sealant, regular caulk and small pieces of foam board, seal all the penetrations possible. Look for other openings in both exterior and interior walls, including plumbing openings behind bath and kitchen cabinets.

Residents should be sure to replace the insulation and avoid leaving gaps between fiberglass batts. In homes with a basement, owners should look for, and seal, the same kind of holes in the ceiling and floor framing that open into the interior cavities of the house.

After sealing is done, consider adding insulation to the attic. An attic should be insulated to an R-38, or about 12 inches fiberglass or cellulose. When adding attic insulation, cellulose can be blown directly on top of either fiberglass or cellulose. Many lumberyards will loan the equipment when purchasing the insulation from them.

For additional help, a do-it-yourself energy audit is available online at the Energy Extension Web site at http://www.oznet.ksu.edu/dp_nrgy/ees/. Some may also choose to hire a certified Kansas home energy rater. Call Energy Extension at 1-800-KSU-8898 for a list of professionals.

What is an air barrier, or house wrap?
These products are primarily designed for use in new construction as a method of reducing air infiltration. They are rolled sheet goods usually installed with staples or tape over the exterior sheathing.

Some brand names are Tyvek, Rufco-wrap, Barricade and Airtight-wrap. Their intent is to minimize the passage of air, while still allowing water vapor through the exterior skin of the building.

Three basic types currently are available. Tyvek is a spun-bonded polyethylene. This is a mat of polyethylene fibers spun-bonded in a patented process. The second type is perforated polyethylene film. The third type is spun-bonded polypropylene, a different type of plastic.
Each of these can be effective air barriers if installed according to its manufacturer's recommendations.

How effective is covering windows with plastic at sealing a window?
Properly applied, a plastic covering can make a window almost airtight. This is one of the most effective ways to seal a leaky window.

A storm window is designed more for convenience and appearance than air tightness. Even the highest quality storm windows allow air to leak around the edges of the sashes. Storm windows typically reduce air leakage through primary windows by about half.

Window plastic can be installed on the inside window surface or on the outside. It will be more difficult to maintain window plastic applied to the outside. Cold temperatures make the plastic brittle, and winds whip the plastic in and out, reducing the seal's effectiveness and sometimes even tearing the plastic.

Newer plastics are very clear when stretched tight, so do not need to worry about window coverings reducing a home's appearance. Special shrink film plastic can be heated with a blow dryer to shrink the window film and eliminate all wrinkles, making the plastic almost invisible.
For maximum leak reduction, it is important to adhere the plastic to the frame surrounding the window rather than to the window sash.