Almost every house has a large number of hidden air leaks that rob it of heating and cooling energy. Although sealing these leaks is not the easiest job, it is one of the most cost-effective things you can do to increase your home’s energy efficiency. You’ll want to do this before adding any insulation, as fixing the leaks is much more difficult once extra insulation is installed. In addition to saving energy and making your home more comfortable, sealing air leaks also reduces ice dams and freezing pipes, and helps control moisture movement that can contribute to mold and structural damage. People often think that windows and doors are the source of most air leakage in a house, but the largest leaks are almost always hidden in attics, basements, crawlspaces, and other unexpected places.



Identify the most important leaks first; sealing them makes the biggest difference in comfort and energy. The priorities are big leaks, and leaks at higher pressures. Air movement and leakage areas are not always intuitively clear, but remember that air does not move through most solid objects (concrete block and masonry in general are notable exceptions). Air leaks mostly through connections and spaces between building materials and other unexpected places. Of course, if you’re not inclined to “do it yourself” you can hire a professional to do the air sealing work, but the following sections will show you what the issues are.

Image courtesy of the Taunton Press

Seal the high and low leaks

The greatest pressure differences are those at the highest and lowest points in your house, and your top priority should always be the attic. Doing a complete job of sealing leaks between the house and the attic will help stop cold winter air from seeping in at the bottom of the house.

Sealing the attic also helps keep out moisture, reduce ice dams, and prevent hot air from leaking in from the attic or roof in the summer.

Once you have sealed the attic, the next priority is the basement, crawlspace, or slab—the home’s foundation. Sealing leaks there will help prevent cold air from coming in during the winter and cool air from escaping during the summer. Then you can look for significant leaks in other parts of the house.

Attic Air-Sealing


When walking around your house, you’re likely to see only the plaster or drywall. What you don’t see are all the connections where building materials meet (or don’t meet). The biggest leaks are hidden behind the walls, where you can’t see them. The key to air-sealing an attic is to block off the large openings and seal the smaller ones as completely as possible—from one end of the attic to the other. (Note: You might skip the air-sealing if there are lots of complicated leaks as well as leaky ductwork in the attic, or if the attic is a space you plan to convert to finished space in the future. If that’s the case, consider sealing and insulating the roof instead with sprayed urethane foam.)

Image courtesy of the Taunton Press

Attic Bypasses

Although there are many variations, here are some common attic bypass situations:

  1. Dropped, or soffited, ceilings are common in kitchens above cabinets, in bathrooms over showers or vanities, and sometimes over stairways (A in illustration).
  2. Plumbing chases (D in illustration) can be much larger than the pipes that run through them.
  3. Chimney chases often run all the way to the basement of a house (C in illustration).
  4. Tri-level homes often have open cavities that extend from the lower level into the attic, along the wall between the levels. These stud spaces are bypasses, as are the large openings that may occur near the stairs (A in illustration).
  5. Top plates and exterior walls may have significant openings into the attic from below (G, H, and I in illustration).


Cape-style Kneewalls

If you have a cape-style house, there is typically a huge leak hidden behind the “kneewall” in the second floor. The floor framing is often open into the attic space, leading to sometimes massive leakage and often contributing to ice dams. To properly stop this problem, these spaces need to be sealed off, or the roof sealed and insulated to bring the entire kneewall attic inside the heated space.

Image courtesy of the Taunton Press

Recessed lights

Recessed “can” light fixtures installed in an insulated ceiling are often big air leaks. Most common styles are full of holes to vent the fixture so it doesn’t get too hot. Also, an older unit can’t have insulation above, or within 3 in. of any part of the fixture, so each one represents a big “hole” for both air and heat flow. If you have a lot of them, the result can be an energy disaster. The best approach is to replace them with airtight fixtures that are rated for insulation contact, or “IC.” The IC rating allows insulation to surround and cover the light fixture. Not all IC-rated fixtures are airtight (AT), but building codes in most states require airtight IC fixtures, so they are easy to find.


Other special situations

Attic air-sealing is complicated by the fact that there are so many types of houses, but there are some fairly common situations. First, don’t miss hidden attic areas. Sometimes an addition, dormer, or ell may have a separate attic that you can’t see, because the new roof was built right over the first roof. Suspended ceilings can hide tremendous leakage areas, particularly if they have attic spaces above. In older houses, such ceilings are often installed to hide deteriorating plaster. There may be large holes in the ceiling or high on the walls that are hidden by the dropped ceiling. Sometimes a suspended ceiling hides open framing that is insulated with only faced fiberglass batts. This is a huge air leak: neither the insulation nor the suspended ceiling is an air barrier. It’s best to remove the suspended ceiling and install new drywall. Another potential air-leakage disaster is a wooden tongue-and-groove ceiling. Whether the ceiling is flat or cathedral, if there is no air barrier behind the planks (which is typical), the hundreds of lineal feet of cracks in even a small room can add up to a big leak. Occasionally, air leaks occur in places that are virtually impossible to reach. If your house has a lot of cathedral-ceiling areas, you may have many plumbing vents and wiring holes that lead from partition-wall cavities up into the roof. These provide a path for warm humid air to escape in winter and for hot attic air to leak into interior walls in summer. Unless you are planning to re-roof or to gut those partitions in a remodel, your most likely options involve dense-packed cellulose insulation or two-part foam. These are the types of situations where you should definitely get some help from a home-performance professional or energy auditor to help you make the right choices.

Basement/Crawlspace Air-Sealing

After the attic, the basement or crawlspace is the next most important area to seal for several reasons. Like the top of a home, the bottom typically has the largest air pressure pushing air in or out of the house, so sealing those leaks results in larger energy savings. Second, though often smaller than attic leaks, basement and crawlspace leaks tend to be larger than those in exterior walls. More important, in most climates, air leaks in from the bottom in cool or cold weather. This leaking air can carry radon gas, mold spores, gases from pesticide treatments, water vapor, and any other subsoil nasties that may become airborne. Sealing leaks at the lowest level of your home helps prevent these undesirable elements from entering your home. Finally, most pipe-freezing problems result from cold air leaking in around the sills, where the pipes are often located.
Basement walls
Generally, consider it most effective to create a good air barrier between the basement and the outdoors by sealing the walls. This is typically true even if you intend to insulate the floor over the basement. There are four common leakage areas in basement walls: service penetrations, stepped-wall transitions, bulkhead doors, and the sill/band joist area.
Service penetrations Every home has basic service penetrations, such as those for electricity, telephone, plumbing, fuel lines, and cable TV. Often, these penetrations are made in the band-joist area, also called the box sill or ribbon joist in conventionally framed houses (in older homes, this may be one large sill beam). Sometimes penetrations come through the foundation (especially plumbing drains, which are always below grade).
Stepped foundation walls are often built on hillsides in cold climates. The vertical parts of the steps are often particularly leaky.
Bulkhead doors Another common source of basement leakage is the bulkhead door. Bulkheads are designed to keep out rain but not to stop air. If your basement is unfinished, the best way to deal with this is to install a new exterior door at the foundation wall inside the bulkhead.
Sills and band joists The tops of foundation walls are often far from flat. The gaps may be small, but spread around the perimeter of a home, they really add up. Even recently built homes that have a foam sill sealer between the sill and the concrete may leak a lot of cold air there.

Non-combustible material like sheet metal is used when sealing around a chimney. Photo Credit: Randy O’Rourke

Sometimes huge holes can be found in your attic, where plumbing, wiring, lights, or in this case a chimney, emerge from the house below. Photo Credit: Randy O’Rourke

Safety First

Although air-sealing a home can theoretically increase radon levels, it is unlikely. Sealing leaks and slowing the stack effect is more likely to reduce the amount of soil gas— including radon—that is pulled into the house from underground. The only way to tell whether you have dangerous radon levels is to test your home. Because of the potential health risks, it is a good idea to test for radon whether or not you do air-sealing. If elevated radon levels are confirmed by at least two tests, find a certified professional to install a mitigation system. See for more information.

A blower door is used to measure how airtight a house is. It’s also useful in helping identify where the air leaks are. Photo Credit: John Curtis

Blower-door testing

Air-sealing is a job that requires much more labor than material, which often suggests a do-it-yourself approach. However, air-sealing does require attention to detail and (in many homes) a willingness to squeeze into difficult places. If these qualities don’t describe you, it may be worth hiring a building-performance professional. Experienced professionals use a blower door to help find air leaks and measure the tightness of the building. The fan blows air into (or out of) the house, measuring how well the building enclosure contains pressure. This can be very handy, especially in a house with complex air-leakage paths.

It’s also good to keep the big picture in mind: If you are planning on renovating or adding onto your house, you will have a unique opportunity to seal leaks while the building is opened up. It may not make sense to manually seal leaks now that you know you will deal with them more effectively at the next stage, especially leaks that are small or hard to reach. Of course, if the next stage is a few years off, it may still be well worth hitting the big leaks at a minimum. A building performance professional can help you set priorities and make a long-term plan. 

Sidewall Air-Sealing


Exterior walls are not typically the leakiest parts of a house. Leaks occur in walls where services enter the home, where the framing of the house makes transitions, around rough openings for windows, and through and around window sashes.

Service penetrations

Sealing cable, wiring, plumbing, and telephone service penetrations is fairly easy. Such openings are usually accessible from the exterior and are often small because some effort is made to weatherproof them during installation.

Transitions in exterior wall framing

One common example of a wall transition is a cantilevered floor, typically an overhang of 1 ft. to 3 ft. that is common on raised ranches and Colonial-style homes. Often, the plywood soffit at the bottom of the overhang is not sealed tightly, and air can leak through these gaps, which are easy to caulk. In some cases, these overhangs don’t even have a plywood covering; they may have vinyl siding soffit covering, which is hardly an air barrier at all.

Weatherstripping Doors and Hatches


Most modern doors and windows have good weatherstripping built in, but older ones usually don’t. In addition, many homes have at least one less-finished, or even makeshift, door or hatchway as part of the thermal boundary. For doors and hatches where there are noticeable gaps, it is worth installing good-quality weatherstripping and/or door sweeps as appropriate, but it will not usually save all that much energy compared with the big attic bypasses.

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