As a insulation contractor, I meet a lot of people who are sick and tired of cold, draughty houses. Their problem—and maybe yours, too— is that they live in homes that don't work very well. These houses, new and old, cost too much to heat and to cool. Their paint peels, their roofs dam with ice, and they sometimes make their owners sick. Simply put, these homes lack a good thermal envelope, or an insulated, air resistant boundary between conditioned inside air and outside air.
Incomplete thermal envelopes are common in old houses, but new ones can have the same or similar problems. Open-web joist systems, cantilevers, balloon-frame walls and mechanical penetrations allow outside air to penetrate buildings. Sometimes the problem is poorly installed insulation with too many voids, but more often, holes in the building are the real culprits.
Caulks and weather stripping can help plug small holes, but this is like using Elasto-plast to treat major wounds. The total area of the holes I'm talking about is measured in square feet, not in square inches. Even so, these problems can now be fixed simply and economically, and buildings a century old can be routinely upgraded to higher performance levels than typical new homes. And the principles and methods are applicable to new construction.

Air movement in floors, walls and ceilings is bad

Air infiltration is the predominant heat loss mechanism for most buildings (drawing above), so the primary goal of any insulation effort should be to control air infiltration. Not all infiltration is bad; humans, pets, and furnaces and other combustion devices need a continuous supply of fresh outside air, and the air in most homes should be replaced (either naturally or mechanically) about six to eight times per day.
But relying on a home's air leaks is not a good way to provide fresh air. I've worked on buildings that have suffered as many as 30 air changes per day. At that rate, the conditioned air doesn't hang around long enough for the house's insulation to have much of an effect on keeping it in.
Air infiltration forces warm, moisture-laden air into cold, dry places. The buoyant nature of hot air drives it into every ceiling penetration, and if there are large holes, the house acts like a giant chimney, pulling

Effective insulation and air-sealing take
place at the thermal boundary

A thermally efficient building must have a well-defined boundary between indoors and outdoors. The holes and voids that allow outside-air intrusion are obvious breaks in the thermal boundary, but sometimes it takes a little head-scratching to figure out just where the boundary is. It's a waste of time and money to insulate an area that is actually outside the thermal boundary, so it's important to attack the right combination of floors, walls and ceilings to yield a complete thermal envelope. For example, although attics and basements are usually thought of as being transitional areas between inside and outside, they really aren't. There is no "in between" in a properly insulated house. I generally consider basements and crawlspaces to be inside the thermal envelope because it is difficult to isolate these areas from the living spaces above. Besides", combustion appliances always belong inside, where they operate more efficiently and can contribute to the heated space. On the other hand, vented attic spaces should always be outside. If the attic is used often, treat the access as an exterior door, and insulate the stairwell walls and under the stairs. When an attic is rarely used, a well-sealed foam hatch over the well is sufficient. I like to use surplus sections of stress-skin panels here. They are heavy enough to compress the gasket we place around the well, they are well-insulated, and the drywall is ready for paint.
Areas behind a kneewall can fall either inside or outside, depending on the use of the space. Because air infiltration can be a real problem here, special care should be taken to seal off the floor, kneewall and sloped ceiling from the outdoors. Air sealing and insulating the rafters down to the bottom of the band joist brings this triangular storage space inside so that it can be used for easily accessible storage. If this space is inaccessible or unusable for storage, my favourite technique is to solidify the entire volume by packing it densely with cellulose, which air-seals it and insulates it at the same time. (For more on dense packing cellulose, read on.)
A blower door and careful investigation help to find the holes—I use pressure diagnostics to help direct my air-sealing efforts. Depressurizing the inside of a house with a blower door quickly reveals the most significant penetrations of the thermal boundary. But it doesn't take a blower door to find a lot of the major holes in the thermal envelope.
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 weather stripping isn't cost-effective, but it does-

deck. The 12 in. of insulation in the ceiling is yielding an R-value of close to 0. This isn't the time to add more useless insulation or ugly vents or an ice-dam membrane under the shingles. The best way to correct the problem is to dig through existing insulation and find and seal air leaks. Partition walls without top plates in older homes are often a major source of air leaks. Plumbing, wiring and chimney penetrations should be checked, and light beaming up into the attic from a ceiling fixture below is a sure sign of trouble. I also look for blackened insulation . As warm air finds a hole and jets through the insulation and into the attic, the dirt gets filtered out. Batt insulation wasn't designed to stop the loss of warm air from a building, but it does a good job of cleaning it. Framing around chimneys should be sealed to the masonry with sheet metal and high temperature silicone caulk. Mechanical penetrations are usually filled with nonexpanding polyurethane foam from a gun, while larger holes are best stuffed with fiberglass insulation wrapped in a poly bag (photo top right). We generally recycle our empty cellulose bags this way. For bigger holes, fasten down appropriately sized sheets of rigid foam, metal or -in. oriented strand board and caulk the edges. There are sometimes areas where it is difficult to rebuild a solid, continuous thermal envelope. For example, suspended ceilings are usually real trouble. When batts are laid over the grid, the assembly behaves like an open skylight, and air flows up through all the openings. In cases such as this, dig into the building until something solid and patchable can be found. Dense-packed cellulose fills the gaps and stops the leaks—Once the flow of warm, moist, indoor air is cut off from the attic, the space can be prepared for adequate insulation. Because of the difficulty of installing fibreglass batts properly in an attic, I like to use blown cellulose. Potential ignition sources such as unrated recessed lights and chimneys should be dammed with sheet metal in order to keep them from contact with insulation. Hatches and soffit vents can be dammed with scrap lumber or plywood. This typically involves cutting and fitting a 10-in. or 12-in. deep box, or well, around framing so that it surrounds whatever needs to be dammed and keeps loose insulation out. Flagging electrical junction boxes is a nice touch that you or your electrician will appreciate when it comes time to find them again. Finally, cellulose can be blown in at low pressure and low speed on top of the existing material to yield an honest R40. Treating open cavities is the easy part of insulation