Searching for how to find air leaks in house usually means one thing: you want a cheap test you can run tonight, not a sales pitch. Here is the direct answer: turn on the exhaust fans and a box fan set in a window to pull air out, then run a lit incense stick or a smoke pencil along windows, outlets, the attic hatch and the rim joist. Smoke that streams sideways or gets pulled toward a gap marks a real leak. This test needs no special equipment and no attic crawl beyond opening a hatch, though the attic floor and the basement rim joist, not the windows, usually hide the biggest losses.
Before You Start: Fire, Carbon Monoxide and Fall Hazards
This test uses an open ember and changes the air pressure inside an occupied house, so read this paragraph before the how-to. A lit incense stick or a smoke pencil is a small open flame or ember: never hold it near curtains, paper, sawdust, spray foam, or loose attic insulation, and never leave it burning unattended. Keep a phone or a second person nearby if you plan to test the attic. Second, pulling air out of the house with exhaust fans lowers the indoor pressure, and a gas water heater, furnace or fireplace that is running can backdraft, pulling combustion exhaust and carbon monoxide back down its flue instead of venting it outside. The U.S. Department of Energy's guidance on detecting air leaks (read 2026-09-18) recommends turning off combustion appliances before this kind of depressurization test, and every home should already have a working carbon monoxide detector regardless. Third, attic access means a ladder and joists you cannot see under insulation: step only on joists or a plywood walkway, wear closed shoes, and if the pull-down ladder or its hardware feels loose, stop. The CPSC recalled more than a million Louisville Ladder pull-down attic stairway ladders in 2026 after the support bolts were found to break during normal use, which is a reminder that even a built-in ladder needs a visual check before you trust your weight to it.
Set Up the Window-Fan Depressurization Test
Close all windows and exterior doors, and close the fireplace damper. Turn off the furnace, water heater and any other combustion appliance for the duration of the test. Turn on every fan that exhausts outside: bathroom fans, the kitchen range hood, and a dryer if it is running. Then set a box fan into an open window, facing outward, to pull additional air from inside the house to the outside. This drops the indoor pressure below the outdoor pressure, so outside air gets pulled in through every gap in the shell, which is exactly what makes small leaks easier to feel and see. This is a homeowner's low-cost version of the same principle a professional blower door test uses, just with far less pressure difference and far less precision.
How to Find Air Leaks in House, Test by Test
With the house depressurized, walk the perimeter inside with a lit incense stick or a smoke pencil held an inch or two from each suspect: window and door frames, electrical outlets and switch plates on exterior walls, the attic hatch or pull-down stairs, recessed lights, the fireplace surround, and any pipe or wire penetration through an exterior wall. Watch the smoke stream, not the incense tip. If it wavers, bends sharply toward the gap, or gets pulled flat against the surface, air is moving through that spot. A steady vertical rise means no leak at that exact point. Work slowly; a smoke pencil reacts to any draft in the room, including one from an open door or a running fan elsewhere in the house, so close interior doors between rooms as you test each one to keep the reading local to that room.
Where to Aim the Smoke First
Windows and doors get the attention because they are visible, but they are rarely the biggest loss. Start where the house actually leaks most: the attic floor around the hatch, top plates and any chimney chase; the basement or crawl space rim joist, where the wood framing sits on the foundation; and knee walls in a bonus room, which back onto unconditioned attic space on the other side of a thin partition. If the smoke test finds a gap at the rim joist, that is a job for rigid foam or spray foam against masonry, covered in our guide to insulating a rim joist. A knee wall that pulls smoke sideways usually means the air barrier behind the drywall is missing, which is the exact problem in knee wall insulation for a bonus room. In a vented crawl space, expect more air movement than in a sealed one; the difference in what you will find is explained in crawl space insulation, vented versus encapsulated. Once you have located and marked a gap, matching the sealant to the size of it, caulk, foam or backer rod, is covered in choosing a sealant for air sealing.
Backup Tests With No Tools: Paper, Dollar Bill and Flashlight
If you would rather not use an open flame indoors, three low-tech checks work without one. Close a door or a window on a sheet of paper or a dollar bill; if you can pull it out without any drag, the seal at that point is not doing its job. Run a bare, damp hand slowly around a window or door frame on a windy day; moving air feels cool against damp skin even when it is too faint to see or hear. After dark, have a helper shine a flashlight around the outside of every door and window frame while you watch from inside for light coming through; this catches larger gaps well but misses hairline cracks, which is exactly where the smoke test does better.
Where These DIY Tests Mislead You
Be honest with the reading, not just the method. A smoke pencil tells you a spot is moving air right now, under whatever pressure your fan setup created, not how much energy that spot costs you over a winter; a dramatic-looking wisp at an outlet cover can be a smaller loss than a quiet, wide gap along the whole length of a sill plate that barely moves the smoke at all. A flashlight test at night can also flag light leaking around a window that is well-sealed for air but poorly caulked cosmetically, and it will miss any leak that is not a straight line, which describes most real gaps. None of these home tests measure the house's overall air changes per hour, so they cannot tell you how the house compares to a code target, only where the visible weak points are. Treat every result as "seal this" evidence, not as a report card.
How to Find Air Leaks in House When DIY Tools Fall Short
A professional home energy audit uses a blower door: a calibrated fan sealed into an exterior door frame that depressurizes the whole house to a measured level and reports a number, typically ACH50 (air changes per hour at 50 pascals), alongside a room-by-room smoke or infrared scan at that controlled pressure. ENERGY STAR's guide to locating air leaks (read 2026-09-18) notes that sealing and insulating the air leaks a professional audit finds can save as much as 15% a year on heating and cooling costs. Reading the report is general information, not a shopping list: the audit will rank leaks by size, not by how easy they are to fix, and it is not this article's place to tell you which contractor, sealant brand or insulation product to buy off that report. Get more than one written quote, confirm each contractor's licensing and insurance yourself, and treat the report as the starting point for your own decision, not an instruction to act on immediately. If your DIY smoke test turns up leaks at the attic floor along with signs of ice damming in winter, that combination usually points to sealing the attic floor before adding more insulation, not to a roof repair.
FAQ
What is the cheapest way to find air leaks in a house?
An incense stick and a box fan set in a window, both items most households already own or can buy for a few dollars. Turn off combustion appliances first, run the fans, and walk the smoke around windows, outlets and the attic hatch.
Can I use a candle instead of an incense stick to find drafts?
An incense stick or a purpose-made smoke pencil is safer than an open candle flame indoors, because the smoke trail is easier to read and the ember is smaller. Whatever you use, keep it away from curtains, paper and insulation and never leave it burning unattended.
Where do most air leaks in a house actually happen?
Not at the windows in most homes. The attic floor, the rim joist in the basement or crawl space, and knee walls in finished attic rooms typically leak more air than window and door frames, according to the U.S. Department of Energy's home energy assessment guidance.
Is a professional blower door test worth the cost?
It gives you a measured number (ACH50) instead of a guess, and it finds leaks a smoke pencil cannot reach safely, like inside a wall cavity. For a DIY test that already found several sealable gaps, a blower door mainly confirms how much is left; for a house with no obvious leaks but high bills, it is often the only way to find out where the air is going.
Can I run the smoke or fan test with the furnace or water heater running?
No. Depressurizing the house while a combustion appliance is running risks pulling its exhaust, including carbon monoxide, back into the house instead of up the flue. Turn combustion appliances off for the test and make sure a working carbon monoxide detector is installed in the home either way.