Air Sealing and Insulation

Homeowners interested in making their homes more energy efficient may be considering expensive heating system upgrades or replacement windows. However, there are more cost-effective steps to take before considering these big-budget undertakings: air sealing and insulation projects that have relatively low cost, can often be done yourself, and will pay for themselves in reduced energy costs much more quickly.

Air sealing and insulation have been compared to a windbreaker and a sweater. Air sealing, the “windbreaker,” stops the flow of air through gaps and cracks, keeping warm air inside your home and reducing uncomfortable drafts. Insulation is the “sweater” that reduces heat being lost by conduction from ceilings, walls, and pipes.

Air sealing

Air movement can cause heat loss around the perimeter of the house—exterior doors, windows that don’t close tightly, and wall penetrations for TV cables, electrical wiring, etc. Air also gets in through seams around trim and through the largest common opening, the sill where walls meet the foundation. In addition, heat loss can be a problem in areas where air can move vertically. Warm air, which you’ve paid to heat, rises to the attic around a hatch door or plumbing vent, pulling cold air into the house from below. This is known as the “stack effect,” and it can occur within the interior core of your house.

The most common test for air leaks is a blower-door test. With all windows and doors closed, the blower door (a metal frame covered with nylon cloth with a powerful fan fitted into an exterior doorway) is activated, depressurizing the house. Replacement air finds its way in, allowing one to easily spot drafty areas.

Even without a blower door, a homeowner can search for areas of air infiltration using common sense and your senses. If you feel a draft, take a few minutes to trace it to its source. Sometimes smoke pencils are used for this, but incense sticks can also serve the purpose of making air flow visible. One technique is to use hand sanitizer to better feel leaks via evaporation on your hands.

Here are some examples of the stack effect we've seen in places one normally would not expect:

  • Stack-effect air movement from the bottom of a built-in china cabinet. Sub-flooring under the drawers may have gaps that can be closed up with expanding foam.
  • The fireplace, where the wood of the mantel meets the brick of the fireplace. Fire-rated caulk can be used here. At the top of the stack, where the chimney comes through the attic floor, the gap can be sealed with non-combustible materials: aluminum flashing and fire-rated caulk.

Drafts around doors can be fixed with a door weather-stripping kit and door sweep. For windows, there are a number of things to check. Is the window completely closed and locked? Is it weather stripped? Is the storm window closed tightly? If you have older double-hung windows with weights and pulleys, you will have air infiltration through the pulley opening. This can be tackled from the outside (by caulking the joint where the casing meets the sill) and from the inside (by blocking the opening with a piece of carpet pad or a pulley seal).

Gaps where the two sashes meet can be blocked with removable rope caulk. (If you’re interested in learning more about tuning up older double-hung windows to make them work better, consider taking our Window Rehab workshop.)

Another area to check for air infiltration is the basement. Look for the bottom of any stack-effect areas and seal them with caulk or foam. Also seal the rim joist—the part of the home’s frame that sits on top of the foundation. Expanding foam can be used at the top and bottom edges of the rim joist, or use a two-part foam with a wide spray to cover the whole area. Try looking for spider webs: spiders seek drafty areas to catch prey.


While air sealing stops heat loss due to air movement, insulation stops heat from being conducted through the structural materials of the house itself: wood, drywall, plaster, etc. Fiberglass batts and blown-in cellulose are the two main types of insulation for attics and walls. Currently, utility rebates are available to pay up to 75 percent of the cost of insulation work, with a maximum of $2,000. ( has more information about rebates.) If you live in, or own, a two- to four-family home in Boston, you may be eligible for an even greater rebate: 90 percent, up to $3,000, when all eligible units are weatherized together. This is offered through Renew Boston’s Whole Building Incentive.

Make energy-saving choices

We’d be remiss if we didn’t mention the many choices we can make to minimize our energy use. Using a programmable thermostat to turn the heat down when you’re away from home or sleeping, washing clothes in cold water, and switching to compact fluorescent light bulbs are just a start. The Low Carbon Diet book is full of practical suggestions for reducing your energy bills and your impact on the environment.

Boston Building Resources has been helping people weatherize their homes since 1978. Whether it’s through a workshop or by helping you finding the right materials for your project, we want you to stay warm within your budget this winter.

Resources for More Information has a “home energy yardstick” to measure your current level of energy use based on past utility bills, a downloadable DIY Guide to Sealing and Insulating, details about federal tax credits for energy efficiency, and more., a website sponsored by local utilities, can get you started with an energy audit and information about rebates.

The Massachusetts Climate Action Network and Boston Climate Action Network have classes, lifestyle tips, and opportunities for involvement.