How air leakage wastes energy
A house is never perfectly airtight. Air moves in and out through small gaps, a process called infiltration when outdoor air comes in and exfiltration when indoor air goes out. In winter, heated air escapes and cold air replaces it, and the heating system must warm the new air. In summer, the process is reversed, and humidity can come along with it.
Two forces drive the movement. Wind pushes air through gaps on the windward side. The stack effect occurs because warm air rises: in cold weather it leaks out near the top of the house, which pulls colder air in through lower openings. Taller buildings and bigger temperature differences strengthen the effect.
The energy cost of leakage depends on the house, the climate and the weather. In many homes it is a significant part of heat loss, comparable to what is lost through poorly insulated surfaces, but there is no single number that fits every building.
Where leaks tend to be
Drafts are not always where people expect. Common leakage points include:
- Around windows and doors, at frames, sashes and thresholds.
- The attic, including hatches, around chimneys, and where wires, pipes and ducts pass through the ceiling.
- Recessed lights that are not designed to be airtight.
- Plumbing and electrical penetrations in walls, floors and under sinks.
- The basement or crawl space, where the wall meets the foundation (the rim joist is a common weak spot).
- Electrical outlets and switches on exterior walls.
- Fireplaces and chimneys, especially dampers that do not close fully.
- Ductwork leaks, which send conditioned air into unheated spaces.
Often the largest leaks are hidden in the attic and basement rather than around windows, which is why an inspection can be surprising.
Finding the leaks
- Simple observation. On a windy day, feel for moving air around frames, or hold a lit incense stick or tissue near suspected gaps and watch for movement. Take care around flames and flammable materials.
- Daylight check. Light visible around a closed door is a gap.
- Professional testing. A blower door test depressurizes the house to measure leakage and make it easier to find sources, often combined with an infrared camera. This is usually part of a home energy audit.
General ways to reduce leakage
Weatherstripping seals moving parts such as doors and operable windows. It comes in foam, rubber, felt and metal forms, and wears out, so it needs replacing now and then.
Caulk seals narrow, fixed gaps, such as around window trim and where different materials meet. Choose a product suited to the surface and the temperature range, and follow the directions.
Spray foam and sealants fill larger gaps around pipes and wires. Some foams are flammable or must be kept clear of heat sources, so check the product warnings and local rules.
Door sweeps and thresholds stop air at the bottom of a door.
Outlet gaskets are inexpensive foam pads behind cover plates on exterior walls. Take care when removing cover plates, and switch off the circuit first. If you are not comfortable working near wiring, skip it or call an electrician.
Attic work. Sealing around penetrations and hatches before adding insulation tends to be more effective, and some of this work involves fire-safety clearances around chimneys and flues. Professionals typically handle it.
Chimney dampers and fireplace covers can reduce loss when a fireplace is not in use.
What about windows?
Windows have two weaknesses: they leak air at the frame and they conduct heat through the glass. These can be addressed separately.
- Sealing and maintenance fix leaks cheaply.
- Coverings. Heavy curtains, cellular shades and seasonal interior or exterior storm windows can reduce heat loss and drafts. Plastic film kits are inexpensive but temporary.
- Replacement windows. Modern double or triple glazing and low-emissivity coatings reduce heat flow through the glass, and well-fitted frames reduce leakage. But replacement is a large expense, and the energy payback can be long, particularly when existing windows are in reasonable shape. Comfort, noise reduction, condensation and maintenance may justify replacement even if the energy savings alone would not. Labels showing U-factor and solar heat gain help compare products, and the right balance depends on climate.
The cheapest effective steps often come first: seal leaks, then consider coverings, then think about replacement when the windows are failing or other work is planned.
Do not seal without thinking about ventilation
Fresh air is necessary. A very tight house can trap moisture, cooking fumes, combustion gases and pollutants such as radon in some regions. Fuel-burning appliances need a proper supply of air and venting, and sealing around them can cause backdrafting, which can push dangerous carbon monoxide into living areas. For this reason, sealing a home thoroughly is often combined with planned ventilation, such as exhaust fans or heat-recovery ventilators, and with carbon monoxide detectors. If your house has gas or oil appliances, a wood stove or an open fireplace, get professional advice before extensive sealing, and test your alarms.
Combined with insulation
Sealing and home insulation work together. Insulation slows heat conduction but does not reliably stop air flow, and sealing stops air flow but adds little resistance to heat conduction. Doing both usually works better than either alone.
Setting expectations
Savings from sealing depend on how leaky the house was to begin with. A very leaky older home often benefits noticeably, while a newer, tighter one may gain little. Estimates vary, and promises of a fixed percentage should be treated skeptically. Even so, sealing is often among the lower-cost improvements, and fewer drafts usually make a house feel more comfortable at a given thermostat setting.