The fastest way to find an air leak in your home is to hold a lit stick of incense or a smoke pencil near suspected gaps and watch for the smoke trail to waver or get sucked sideways. That flicker tells you air is moving through a crack you can’t see. But the real challenge isn’t the technique. It’s knowing where to look, because the biggest leaks aren’t around your windows. They’re hidden in your attic and basement.
Sealing those leaks can save up to 10% on annual energy bills, according to ENERGY STAR. Here’s how to track them down yourself and when it makes sense to call a professional.
Where Air Leaks Hide
Most people assume their drafty windows are the main problem. Windows and doors do leak, but the most significant air leaks in a typical home are hidden in the attic and basement, where gaps in the building structure create direct pathways between your conditioned living space and the outdoors.
In the attic, the top offenders include:
- Wiring and plumbing holes: Every wire, pipe, and vent that passes through the ceiling into the attic leaves a gap around it. These small holes add up fast.
- Open soffits: The boxed-in areas that hide recessed lights or ductwork often have open tops, letting warm air pour straight into the attic.
- Recessed lights: Older can lights that aren’t rated for insulation contact act as chimneys, venting heated air upward.
- Attic hatches: The pull-down staircase or access panel is rarely weatherstripped or insulated.
- Furnace flues and duct chaseways: The hollow shafts that conceal heating ducts or chimney pipes often have unsealed gaps at the top where they meet the attic floor.
- Kneewalls: In rooms with sloped ceilings, the short walls (kneewalls) separating living space from the unfinished attic behind them frequently lack any air barrier on the back side.
In the basement, the most common leak point is along the rim joist, where the home’s wood framing sits on top of the concrete or block foundation. That seam runs the entire perimeter of the house, and it’s almost never sealed in older homes. Plumbing penetrations through the basement ceiling are another major source.
The Incense and Smoke Pencil Test
This is the simplest DIY method and costs almost nothing. A smoke pencil is a small pen-shaped tool with a wick that produces a thin, steady trail of white smoke when lit. You expose about 3/8 of an inch of wick, light it, and slowly move it along edges and seams. Unlike smoke candles that fill an entire room, a smoke pencil lets you test specific cracks, outlets, and seals with pinpoint accuracy. A stick of incense works the same way.
Hold the smoke source an inch or two from the area you’re testing. If the smoke trail stays vertical and steady, there’s no air movement. If it bends, flutters, or gets pulled toward (or pushed away from) a gap, you’ve found a leak.
To make this test more effective, create a slight pressure difference in your home first. Turn on all your exhaust fans (bathroom fans, kitchen range hood, dryer) and close all windows and exterior doors. This pulls air out of the house and forces outside air to rush in through any leaks, making them easier to spot with smoke. Work your way around every window frame, door frame, electrical outlet on an exterior wall, baseboards, and any visible penetration through ceilings and floors.
Using a Thermal Camera
A thermal imaging camera shows temperature differences on surfaces, making air leaks visible as cold streaks or hot spots depending on the season. During winter, cold air infiltrating through a gap around a window frame shows up as a blue or purple streak against the warmer wall. In summer, the pattern reverses.
For accurate results, you need at least a 20°F temperature difference between indoors and outdoors. A mild spring day won’t give you enough contrast. Early morning on a cold winter day, or a hot summer afternoon, works best.
You don’t need professional equipment for a basic scan. Smartphone thermal camera attachments are available for around $230 and plug directly into your phone. They won’t match the resolution of a $5,000 professional unit, but they’re more than capable of showing you where cold air is streaming in around rim joists, window frames, or attic access panels. If you only plan to use it once, some home improvement stores and tool rental shops rent thermal cameras by the day.
The Hand Test
On a cold, windy day, you can skip the tools entirely. Wet your hand slightly and move it slowly along window edges, door frames, electrical outlets, and baseboards on exterior walls. Your damp skin is surprisingly sensitive to even faint air movement. You’ll feel a cool draft as a distinct chill on the back of your hand. This method is free and surprisingly effective for larger leaks, though it won’t catch the subtle ones a smoke pencil or thermal camera would reveal.
The Professional Blower Door Test
If you want a complete, measured picture of your home’s air leakage, a blower door test is the standard. An energy auditor mounts a powerful fan in your front door frame, sealed with a canvas cover. After closing all windows and exterior doors and shutting off exhaust fans and HVAC equipment, the fan pulls air out of the house until the indoor pressure drops to 50 pascals below outdoor pressure.
At that pressure, outside air rushes in through every leak in the building. The auditor then walks through the house with a thermal camera, and the leaks that were subtle on a calm day become obvious under the exaggerated pressure difference. You can feel drafts pouring through gaps you’d never notice otherwise.
The test produces a number called ACH50: air changes per hour at 50 pascals. It tells you how many times the entire volume of air inside your home would be replaced by outside air in one hour under that test pressure. Modern building codes generally require homes to score 7 ACH50 or less. A home at 5 ACH50 or below is tight enough that mechanical fresh air ventilation becomes a requirement, not just a recommendation. Older homes commonly test at 10 to 15 ACH50 or higher, meaning there’s substantial room for improvement.
A blower door test typically costs $200 to $450 depending on your area and is often included as part of a full home energy audit. Many utility companies subsidize or fully cover the cost.
A Safety Note on Tight Homes
If your home has a natural-draft gas furnace, gas water heater, or wood-burning fireplace, sealing air leaks aggressively can create a real safety hazard. These appliances rely on the natural buoyancy of hot air to vent combustion gases up and out through their flue pipes. When a home is sealed tightly (roughly 4 ACH50 or below), exhaust fans and even a strong wind can depressurize the house enough to reverse that airflow, pulling carbon monoxide back down the flue and into your living space.
Carbon monoxide at 400 ppm becomes life-threatening in two to three hours. At 800 ppm, it can be fatal in two hours. These aren’t abstract numbers. Backdrafting in a poorly ventilated space depletes oxygen and creates a cascading combustion problem where the appliance produces more carbon monoxide precisely because it’s starved for air.
This doesn’t mean you shouldn’t seal leaks. It means that if you have natural-draft combustion appliances and plan to do significant air sealing, have a professional perform a combustion safety test afterward. The long-term fix is replacing natural-draft appliances with sealed-combustion or electric models that pull their combustion air directly from outside through a dedicated pipe, making them safe regardless of how tight the house is.