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Weatherization: How to Seal Air Leaks and Insulate for Energy Efficiency

A woman standing at a double-hung window on a snowy day, with condensation on one pane

Weatherization means two jobs: sealing the air leaks in your house, then adding insulation where there is not enough. The U.S. Department of Energy calls air leaks one of the biggest sources of energy loss in a home and says sealing uncontrolled leaks can save 10% to 20% on heating and cooling bills. DOE also calls air sealing one of the most cost-effective ways to improve a home's comfort and energy efficiency. A homeowner or renter can do much of it in a weekend with a caulk gun and weatherstripping.

Seal first to stop drafts; insulation then slows heat moving through walls and ceilings. DOE compares insulation to a sweater and air sealing to a windbreaker. You want both.

A man adjusting the sash of a double-hung window beside an interior louvered shutter

Find the air leaks first

The obvious spots are around doors and windows. DOE also lists walls and ceilings, light and plumbing fixtures, switches, electrical outlets, and anywhere pipes, ducts or wires pass through a wall or floor. In the attic, warm air rises into the space above your rooms, so gaps there matter. In summer the same gaps let hot air in and raise cooling costs.

DOE suggests a simple test on a windy day. Close the windows and exterior doors, then hold a lit incense stick next to window frames, door frames, outlets, fixtures and the attic hatch. If the smoke blows sideways or gets pulled in, you have found a leak.

Seal what you found

  • Caulk for cracks that do not move: where window and door trim meets the wall, and where plumbing, ducts or wiring come through walls, floors and ceilings.
  • Weatherstripping for parts that move: around doors and between window sashes and frames. Foam tape, felt, tension seals and rubber are all common types.
  • A door sweep along the bottom of each exterior door. ENERGY STAR notes it cuts drafts and also helps keep out water and insects.
  • Foam gaskets behind outlet and switch plates on exterior walls, and spray foam for larger gaps around trim and baseboards.
  • The fireplace damper. Make sure it closes fully when the fireplace is not in use.
  • The attic hatch or pull-down stairs, which DOE says to check for leaks; for leaky attic stairs it suggests an insulated cover box.

One caution. Around chimneys, furnace flues and gas water heater vents, DOE says to use fire-resistant materials such as sheet metal and furnace cement caulk, not ordinary caulk or foam, and recommends a professional for that work. Gaps under exterior doors can also let in mice, so our guide on keeping mice out of your home pairs well with this job.

Then insulate

Sealing is the cheap part of saving energy at home. Once the leaks are sealed, check whether you need more insulation, starting with the attic. ENERGY STAR says sealing and insulating together can save up to 10% on annual energy bills. Our post on insulation material options compares materials by R-value and environmental impact.

Keep the air healthy

A tighter house needs the right amount of fresh air. DOE warns that if gas or oil furnaces, water heaters, stoves or fireplaces do not get enough air, they can run poorly and release carbon monoxide into the home. If you have fuel-burning appliances, test your carbon monoxide alarms and consider a professional home energy assessment before a big sealing project. A certified assessor (DOE names BPI and RESNET HERS certifications) uses a blower door test and an infrared camera to find leaks you cannot see, including ones buried under attic insulation, and checks ventilation at the same time. You get a report listing the most energy-efficient upgrade options for your house.

Free weatherization for income-qualified households

The DOE Weatherization Assistance Program pays for weatherizing homes of low-income households. DOE says it serves about 32,000 homes a year with its own funds and that households save an average of $372 or more a year.

  • Who qualifies: households at or below 200% of the federal poverty guidelines, or that receive Supplemental Security Income. Some states use the LIHEAP limit of 60% of state median income instead.
  • Who goes first: households with older adults, people with disabilities or children, and those with high energy use or energy costs.
  • Renters can apply with the landlord's permission.
  • How to apply: find your state's weatherization office on energy.gov, contact the local provider for your county, and bring proof of last year's income. Eligible homes go on a waitlist, then an energy auditor visits.

If you do not qualify, DOE suggests asking your utility and retailers about rebates, and see our signs it's time to replace your windows if drafts persist after sealing.

Sources: U.S. Department of Energy Energy Saver Guide and Consumer Guide to Air Sealing the Home; DOE Weatherization Assistance Program eligibility and application pages; ENERGY STAR Seal and Insulate guidance and home sealing fact sheet.

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