Heating a house pressurizes the inside air and drives it through ceiling penetrations into the attic, which in turn draws in cold replacement air from outside. Simply plugging the air leaks into the attic eliminates problems such as drafts, ice damming and condensation. Effectively insulating ceilings and walls completes the thermal envelope, the boundary between inside and outside air.



Under natural conditions, pressures are always higher at the ceiling than at the windows. Although wind and mechanically induced pressures are sometimes stronger, hot air applies constant pressure upward toward the ceiling and the attic. As a consequence, ceiling bypasses, or holes in the thermal boundary, generate more significant natural infiltration through the heating season than do window leaks.

This doesn't mean that door and window weatherstripping isn't cost-effective, but it does mean that most doors and windows don't need replacement. There are reasons to replace windows, but unless there is glass missing or a large gap between sash and jamb, thermal performance is not a compelling one. There are better places to spend energy-conservation dollars.

Another place to concentrate on is the common wall between the house and its attached garage, if there is one. Air leaks here always have the potential to vacuum car exhaust, solvent and weed-killer fumes, and fuel gases into the living space, so this is a spot that requires a NASA-grade air-seal. Obvious holes are usually easy to find and fix in the open framing. Caulking framing and sheathing joints down to and along the foundation makes a big difference.

Basements and crawlspaces should also get a thorough inspection. Musty odors are a sure sign that moisture and cold air are mixing and that wood is under attack. Crawlspaces are usually built to save money, and difficult access is often a reliable indicator of potentially significant building defects. We often have to saw our way into crawlspaces, where we can find bare soil, open concrete-block cores, no insulation, no sill seal, empty whiskey bottles, mold, decay, and lots of insect and animal debris.

Blocking moisture in the form of water vapor from the soil with 6-mil poly is an important first step to air-sealing here. Cover the ground completely, overlap seams if there are any, and lap the poly right up onto the foundation wall. Then the foundation, sills and band joists can be air-sealed and insulated with sheets of rigid foam and plenty of caulk. It's tough to do perfect work in a tight space. Sometimes it's possible to work from the outside by applying rigid-foam panels or stuccoing the stonework.

With the house depressurized by the blower door, I feel for drafts with the back of my hand and spray expanding-urethane foam into trouble spots. Spiders can also offer clues; they always hang their webs in a draft. If the combustion devices have separate fresh-air supplies, foundation walls should be sealed as tightly as possible all the way to the ground, including foundation vents. Although often required by code, foundation vents allow crawlspaces to load up with moisture in warm months and allow cold air to circulate freely through the thermal envelope in the cold months. If moisture can be prevented from entering this space, then it doesn't need to be vented out.

It's important always to work at the boundary of the thermal envelope. Often during a blower-door test, an air leak to an electrical outlet, radiator pipe or wainscoting will show up in the middle of the house. But it won't do any good to stop the airflow there because the air will just find another exit point. Leave these interior holes alone and track down where the airflow actually enters the envelope. Air can travel long distances through floor bays and interior partitions in conventionally insulated homes. Air-sealing away from the envelope only redirects the airflow to another hole.

Ceiling fixtures and plumbing can be trouble spots. Bath fans are a good candidate for caulking, but the irregular hole around the vent stack is better sealed with foam. Insulated ductwork will keep the warm, moist exhaust air from cooling and condensing before exiting.

Weatherization is tough business -- Insulating a building is physical, dirty work that can take you into tight, uncomfortable spaces. I don't know of anyone who likes working in a confined space at 140°F wearing a respirator.
And what is the payoff for these weatherization efforts? Even in times of relatively stable, low fuel prices, a 20% to 30% return on investment in fuel savings is the norm. Even better are the long-term maintenance issues, such as peeling paint and ice damming, that effective weatherization helps to solve. But best of all is the increased level of comfort for the home's residents: No longer does an old house -- or even a new house -- have to be cold, drafty and difficult to heat.