Seal Air Leaks to Cut Home Energy Costs

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Applying caulk around windows lowers home energy costs by stopping air leaks, which otherwise allow heat to escape in the winter.
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Weatherstripping for doors is typically made of foam or metal. Foam is usually cheaper and easier to install, while metal is more durable.
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Foam weatherstripping products are usually the least expensive, and the easiest to install. 
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The main advantage of metal weatherstripping is its durability. 
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As home improvement projects go, sealing air leaks is amazingly easy and inexpensive.
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Sealing air leaks not only saves energy — it leads to a healthier, more durable home.

Air leaks cost us dearly. According to the Energy Star program, most people could save about 20 percent on their heating and cooling costs by sealing up air leaks. Most older homes are riddled with holes in the “building envelope,” which is made up of the outside walls, roof and foundation. These leaks range from large, obvious holes — such as broken or missing windowpanes in the basement — to tiny, almost invisible cracks. Small cracks can have a big impact. An eighth-inch-wide, 6-foot-long crack between a door and a doorjamb, for example, is equivalent to a 9-square-inch opening!

On cold winter days, leaks in the building envelope let heated air escape and cold air enter. In the summer, cool air slips out and hot air seeps in. Air leaks also allow moisture to enter walls and ceilings, which may make them breeding grounds for mold. Eventually, water in walls may lead to structural damage, because framing, if constantly wet, begins to rot. Replacing rotted framing is expensive, as a neighbor of mine recently discovered. He found moisture damage in his home’s framing members, and fixing the mess cost him a whopping $125,000! Sealing air leaks not only lowers home energy costs, it leads to a healthier, more durable home. And as home improvement projects go, this one is amazingly easy and inexpensive.

Find Those Air Leaks!

To seal air leaks, you first need to identify them, either on your own or with the help of a professional energy auditor. To identify leaks yourself, begin by looking for large openings in outside walls, then search for smaller, less visible openings. On windy days, you can find these leaks by feeling around doors and window frames, at the base of walls, and anywhere else with an opening from outside to inside walls. You can also detect leaks with a stick of burning incense — air leaking into a home will deflect the smoke. Be sure to check around electrical outlets and light switches (even those on interior walls.) Ceiling fixtures — especially recessed lighting and wholehouse fans — are other major sources of heat loss in the winter.

After you’ve found the air leaks in your home, you can hire a professional retrofitter to seal them, or you can do the work yourself. If you choose the latter, you’ll need some inexpensive supplies and simple tools, including clean rags, rubbing alcohol, a caulk gun, clear or paintable caulk, liquid spray foam (expanding foam), weatherstripping, foam gaskets for sockets and light switches, a utility knife or scissors, a screwdriver, and a stepladder.

Start in the Basement

Basements are a major source of air leaks, so they’re a good place to start. Begin by sealing the largest and most visible cracks in your basement or crawl space. Replace any broken or missing windowpanes, or install rigid foam insulation over the openings. Seal any gaps — such as those around dryer vent exhausts — with caulk or expandable liquid foam, which we’ll discuss shortly.

Next, seal cracks between the top of the foundation and the wall. These are often so large that light shines through them. Turn off the basement lights and look at the walls from inside to locate gaps, then seal them with caulk or foam. You can usually seal cracks from inside, though some may be easier to access from outside. This is also a good time to insulate the cavity formed by the floor joists and the rim joist. Place batt or blanket insulation in the cavity, or use rigid board insulation, which can be cut to size and friction-fit into the space. Another option is to use expanding foam insulation.

For large cracks (those wider than three-eighths of an inch and deeper than a half inch), professionals often use a tubular foam called backer rod, which you can purchase at a hardware store. Vacuum or brush dust and dirt out of openings, then stuff the backer rod into gaps with a screwdriver or small putty knife.

You also can use liquid foam insulation to seal large gaps. The foam quickly expands to fill the gap. After it’s dry, trim off any excess. Exposed foam can be sanded and painted. Like backer rod, liquid foam seals large openings, prevents airflow, and adds insulation, further reducing heat loss or gain. Liquid foam insulation is available at home improvement centers and hardware stores. Two types of foam are available: polyurethane and latex. Polyurethane expands rapidly and extensively. It sticks to your hands and clothing and is difficult to clean up. Latex foam expands less and is a bit easier to clean up.

For openings wider than a half inch, I use Dow’s Great Stuff Big Gap Filler. This polyurethane insulating foam sealant fills, seals, and insulates. For cracks less than a half inch, I use Dow’s Great Stuff Gaps and Cracks polyurethane insulating foam sealant. Note that these products work best if you use the whole can at once instead of storing some of the sealant for later use. Smaller cracks also can be sealed with a clear or paintable caulk (if you want the caulk to blend in).

Caulk comes in small tubes or cartridges for use in caulk guns. When caulking, cut off the plastic tip of the cartridge at an angle (a small opening is better) using a utility knife, then insert the cartridge in the gun, push the plunger forward and engage it. Pierce the seal at the tip and advance the plunger. (To save partially used caulk cartridges, wrap the tip with electrical tape.) Caulk comes in three basic varieties: pure silicone, a silicone/modified polymer formulation and latex. Silicone and silicone/modified polymer are the best products. Although they cost a bit more, they’re more flexible and can expand and contract with changing temperatures without cracking. This, in turn, means they last longer than the less expensive latex caulk. Latex caulk is a bit easier to apply, but again, it is less flexible and won’t last as long.

When sealing with caulk or foam, be sure to clean the surfaces first. Vacuum or blow out dust, then use a wet rag to remove the remaining particles. You can also wipe the surface with rubbing alcohol (isopropyl alcohol). After the surface has dried, fill the gap with caulk or foam. Apply caulk slowly and carefully so you deposit a steady, consistent bead. I use my finger to smooth out caulk, or you can try smoothing it with the back of a plastic spoon. Wipe up any excess immediately.

Be sure to seal heating, cooling, and ventilation ducts that run through the basement, as well as openings in the foundation wall where plumbing pipes exit. When sealing leaky ducts, many heating and air conditioning professionals use mastic sealant, which is by far the best product on the market for sealing leaky ductwork. Mastic is a paste that’s painted over the seams between sections of metal ductwork. It creates an excellent seal and outlasts the cheaper, less durable, metallic tape and duct tape. (The only thing duct tape doesn’t stick to is ducts!) Unfortunately, duct mastic may be difficult to locate at home improvement centers or hardware stores. Be sure to get mastic for sealing ducts, not for laying tile floors — these are entirely different materials. If you have trouble locating mastic, try calling professional energy auditors, or heating and air conditioning installers, to ask where they purchase theirs.

Walls and Ceilings, Windows and Doors

Walls in living spaces often have numerous cracks, typically around doors and windows, and along baseboards. You can usually seal these openings with clear or paintable caulk. Air leakage is common at the base of walls, so pay special attention to these areas.

To seal electrical outlets and light switches, use foam gasket insulators made especially for this purpose, which you can find at hardware stores and home improvement centers. Be sure to turn off the power at the breaker box before you remove the screws holding the cover plate in place, then insert a foam gasket and replace the cover plate. Block the slots in electrical outlets that are not being used by inserting child-safety plugs.

Be sure to check the ceiling for air leaks. Recessed ceiling light fixtures and whole-house fans are notorious for leakage. Leaky can lights should be replaced with new, airtight, recessed light fixtures, which is a job you may want to delegate to a professional electrician. For whole-house fans, buy or make an airtight cover to place over the opening in winter, when it’s not in use.

To seal up leaks around windows, use self-adhesive vinyl V-strip weatherstripping, following installation directions on the package. Another way to seal leaky windows is with rope caulk, a removable, puttylike cord that comes in a roll. Apply it before winter sets in and then remove it in the spring if you want to open the windows. If not, leave rope caulk in place during the summer to reduce leakage and cut air conditioning costs. It’s great for painted windows, but the oil in the caulk that keeps it flexible may stain wood.

Exterior doors are another big source of air leakage, which typically occurs between the door and the jamb. These leaks can be sealed with weatherstripping. This product comes in several varieties and is usually placed against the doorstop on the top and sides of the finished opening. Before applying, remove the old weatherstripping and clean the surface well. You may want to wipe down the area with rubbing alcohol to remove dirt and other residue.

The cheapest, easiest and most widely available weatherstripping consists of self-adhesive foam strips. Simply cut foam from the roll with a pair of scissors and then apply it to clean, dry surfaces, following the directions on the package. Although self-adhesive foam is easy to use, in my experience, it doesn’t last long. Another option is felt weatherstripping. It’s relatively inexpensive and easy to apply, but it’s the least durable and the least effective in stopping airflow. Metal V-strips, which are nailed down using tacks, are a considerably more durable option for doors. These can be difficult to find, but check hardware stores and home improvement centers, or call a local door and window installer.

Don’t forget to check the attic access door. Often located in back hallways or inside closets, this opening is usually uninsulated and unsealed, and is a major source of air leakage. Using a stepladder, remove the access panel. If uninsulated, glue a 2- to 4-inch layer of rigid polystyrene foam on the upper surface. (Blanket insulation is a poor choice for this application because it’s not easily held in place.)

Most home improvement centers sell 4-by-8-foot sheets and half sheets of rigid foam insulation. Of the polystyrene products, pink board or blue board offer the highest R-value per inch, and they’re easy to cut. After attaching the foam, apply a layer of self-adhesive foam weatherstripping along the perimeter of the access hatch that rests in the opening. If your attic access is through a pull-down staircase, you can purchase a cover for it at a hardware store or home improvement center. 

What Will It Cost?

The cost of an air sealing project varies widely depending on the size of your home, its age and condition, and whether you do the job yourself or hire a professional. You will need a variety of products, as mentioned throughout this article. Individually, many of them — such as weatherstripping tape or a can of foam sealant — cost $5 or less. To figure out material quantities, measure and add the dimensions of cracks and window and door perimeters that need to be weatherstripped, allowing a little extra for waste.

Cost estimate to caulk and weatherstrip a 2,000-square-foot home: This estimate of cost includes all windows, doors, and vertical trim boards, assuming no caulk is already in place. (Generally, the only time this amount of caulk and weatherstripping has to be applied is when the exterior of a home is repainted. If only some areas need replacing on your home, the job and the cost will be smaller.) The estimate also includes installation of vinyl weatherstripping and door sweeps.

Cost for materials only: $150

Contractor’s total, including materials, labor and markup: $750

Costs are national averages and do not include sales tax.  

If you have one contractor do several small projects, you’ll save money overall. 

What Will You Save?

How much you save depends on how leaky your house was in the first place. Sealing a home can result in savings in heating and cooling ranging from as little as 5 to 10 percent in a house that’s already well-sealed, to as much as 30 to 40 percent or even more.

This article is an excerpt from Green Home Improvement by Dan Chiras. Dan teaches workshops on renewable energy and green building through the Evergreen Institute.