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.
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.)
Although there are many variations, here are some common attic bypass situations:
- Dropped, or soffited, ceilings are common in kitchens above cabinets, in bathrooms over showers or vanities, and sometimes over stairways (A in illustration).
- Plumbing chases (D in illustration) can be much larger than the pipes that run through them.
- Chimney chases often run all the way to the basement of a house (C in illustration).
- 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).
- Top plates and exterior walls may have significant openings into the attic from below (G, H, and I in illustration).
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
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.
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 www.epa.gov/radon for more information.
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.
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.
Transitions in exterior wall framing
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.
Material adapted from Build Like a Pro: Insulate & Weatherize, published by The Taunton Press, 2012; used by permission.