The short answer
A drafty house in Northern Virginia usually isn’t one bad window. Warm air leaks out up high, cold air gets pulled in down low to replace it, and if your ducts leak, the heating system itself can pull more cold air in every time it runs. The draft you feel at the baseboard is often air the house sucked in through the basement or crawl space, not air coming through the glass.
The part most people don’t know
In a Florida Solar Energy Center study of 91 homes, outside air came in about four times faster with the air handler running than with it off: 0.93 air changes an hour on, 0.21 off. The cause was leaky ductwork. Return ducts that leak pull attic or crawl space air straight into the system. Supply ducts that leak push heated air out of the house, which leaves the inside at lower pressure, so it pulls cold air in through every crack it can find. In that study, return leaks averaged about 10 percent of all the air the air handler moved.
Fixing the ducts worked. In 25 of those homes, duct repair cut the outside air coming in with the system on from 1.13 to 0.54 air changes an hour, more than half. It’s a Florida study from 1990, so the houses and the climate aren’t ours, but the physics hasn’t changed: a leaky duct system turns your furnace or heat pump into a draft machine.
How much air is that? ENERGY STAR says about 20 to 30 percent of the air moving through a typical duct system is lost to leaks, holes and poorly connected ducts. An EPA and ENERGY STAR guide puts air leakage at 25 to 40 percent of the energy used to heat and cool a typical home.
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Why it feels worse on cold, windy days
Air moves through a house when the pressure inside and outside is different. The EPA guide lists what drives it: wind, the temperature difference between inside and out (called the stack effect), chimney and flue exhaust, appliances with exhaust fans like dryers and central vacuums, and bath and kitchen fans. On a cold, windy night all of them line up. Warm air rises and leaks out through the attic, and the house pulls cold air in low to make up for it. Every time the dryer or a bath fan runs, that air has to come back in from somewhere.
The Building Science Corporation, a building research firm, figures a third of the energy you buy probably leaks out through holes in the house. The Department of Energy’s air sealing guide shows where. Cold air comes in through the crawl space, the dryer vent, outdoor faucets, the sill plate where the house sits on the foundation, and vent fans. Warm air goes out through the attic hatch, dropped soffits over kitchen cabinets, duct registers, the plumbing vent stack, recessed lights and the top plates of interior walls.
What you can check yourself
- On a windy evening, with the heat off, feel around outlets, baseboards and the attic hatch with the back of your hand. Then turn the fan on and feel again. If the drafts get stronger with the blower running, the ducts are part of it.
- Cold floors in winter usually trace to the basement or crawl space. ENERGY STAR’s guide to finding problems lists them as a sign. Here’s what to know about crawl space vents in winter.
- One room much colder than the rest points at that room’s duct run, not the whole house. Here’s why one room runs cold.
- Don’t close registers to push heat somewhere else. It raises the pressure in the ducts and can make leaks worse. More in does closing vents save money.
Sealing the house has a catch
EPA estimates that sealing air leaks and adding insulation saves homeowners an average of 15 percent on heating and cooling. But a tighter house changes how air moves through it. ENERGY STAR’s guide warns that sealing can cause gas appliances like water heaters to backdraft, pulling exhaust back into the house instead of sending it up the flue, and that a tight home may need mechanical ventilation. So any big sealing job should come with a carbon monoxide alarm and a check of every gas appliance afterward. Here’s how water heater backdrafting happens.
What we check
Our heating check is $67 per system, through December 31: furnace, heat pump or boiler. If the draft gets worse when the heat runs, that points at the ducts, and our duct cleaning starts with camera photos of your ducts, free, so you can see what’s in there before you pay for anything. The cleaning is $598 instead of $798 for one system in a home up to 2,000 square feet.
Holding a quote already? Our free second opinion sends a tech to look at the system you were quoted on and tell you what it actually needs, with our price if you want it. You don’t owe us anything for looking.
Seeing a code or a blinking light on the furnace or thermostat? Our error codes and warning lights guide covers what each one means.
Book it
Book the heating check, call (703) 997-9757, or book online from goswiftpro.com/cookiesoffer and a box of warm cookies comes with the visit. Rated 5.0 stars across 1,138 reviews on Google, Angi, Yelp, the BBB and Facebook, Virginia Class A license 2705192276.
Questions people ask
Why is my house so drafty in winter?
Pressure. Warm air rises and leaks out through the attic, and the house pulls cold air in low to replace it. Wind, exhaust fans, the dryer and leaky ducts all make it stronger, so the drafts are worst on cold, windy nights.
Do leaky ducts make a house drafty?
They can. In a Florida Solar Energy Center study of 91 homes, outside air came in about four times faster with the air handler running, because of duct leaks. Repairing the ducts in 25 of the homes cut that by more than half.
Where do most drafts come from?
The Department of Energy lists the crawl space, dryer vent, outdoor faucets, sill plate and vent fans as places cold air comes in, and the attic hatch, recessed lights, duct registers, soffits and the plumbing vent stack as places warm air goes out.
Can sealing a drafty house cause problems?
Yes, if it’s not done carefully. ENERGY STAR warns that sealing can make gas water heaters and other gas appliances backdraft, and a tight home may need mechanical ventilation. Have a carbon monoxide alarm and get the gas appliances checked after a big sealing job.
