Most houses have far more air leaks than their owners think. It's not one big hole; it's two hundred small ones — around pipes, wires, light fixtures, and framing — that add up to the equivalent of leaving a window open in every season. The good news is that finding them costs under $2 (an incense stick) and sealing the worst of them is a weekend of caulk and foam, not a contractor job.
Pick the right weather. A windy day is best, and a cold day works well too — any day when indoor and outdoor air want to trade places. Still, mild afternoons give weak results and you'll walk away thinking the house is tighter than it is.
Shut the house down. Close all windows, exterior doors, and the fireplace damper. Turn off the furnace, air conditioner, and anything with a pilot or burner — combustion appliances shouldn't be fighting the pressure changes you're about to create.
Depressurize slightly (optional but useful). Turn on the kitchen and bathroom exhaust fans. This pulls the house under slight negative pressure, so leaks pull air inward — and your incense smoke streams toward them instead of drifting lazily.
Walk the suspects. Hold the incense an inch or two from each spot and watch the smoke. Rising straight up means still air. Streaming sideways, flattening, or getting sucked toward the trim means a leak worth sealing. Mark each one with painter's tape so you can seal everything in one pass afterward.
The attic hatch or pull-down stairs. Frequently the single biggest leak in the house — an uninsulated, unsealed door straight into the attic, and warm air rises right to it. Test all four edges.
Recessed can lights. Older fixtures are basically chimneys into the attic, with holes in the housing by design. Test around the trim ring. (Sealing these needs care — see the warning below.)
Plumbing and wiring penetrations. Every pipe and cable that passes through a top plate, floor, or exterior wall has an oversized hole around it. Check under sinks, behind the tub access panel, where pipes enter the basement ceiling, and around hose bibs. Those gaps are pest highways as well as air leaks — a mouse fits through a hole the width of a pencil — so if droppings rather than drafts sent you hunting, our guide to getting rid of mice starts with this same list of entry points, sealed with copper mesh and steel wool instead of caulk alone.
Rim joists in the basement. The band of framing where the house sits on the foundation is often bare, uninsulated wood with gaps at every joint. Cold air pours in here in winter — it's also a big part of why floors above basements feel cold, and sealing it pairs well with cutting basement humidity, since the same incoming air carries moisture in summer.
Exterior-wall outlets and switches. Hold the incense at the slots of the plug. Electrical boxes in exterior walls are punched full of holes and sit in a wall cavity that connects to the attic and basement.
Windows, doors, and the fireplace. Test around the sash and the trim — trim leaks mean the gap between the window frame and the rough opening was never sealed. At the fireplace, test with the damper closed; a damper that doesn't seal is an open flue all winter. When a door's own seal is the leak, the fix is a strip swap rather than caulk — see how to replace weatherstripping on a door for the types and the dollar-bill test that proves the new seal works.
| Gap Size | Use | Notes |
|---|---|---|
| Under 1/4 inch | Caulk | Latex for painted trim indoors; silicone where water is present |
| 1/4 to 1 inch | Backer rod + caulk, or low-expansion foam | Backer rod keeps caulk from sinking into deep cracks |
| Over 1 inch, irregular holes | Spray foam, trimmed flush | Standard foam for framing holes; low-expansion around windows and doors |
| Anything that opens and closes | Weatherstripping | Attic hatch, exterior doors, operable windows — sealant would just tear |
One warning on the table: use the low-expansion "window and door" foam around window and door frames. Regular foam expands hard enough to bow the frame and turn a leak fix into a window that no longer latches.
Attic first. Warm air rises and escapes at the top of the house, which pulls cold air in at the bottom — so attic leaks (hatch, top plates, penetrations, can lights handled safely) give the biggest return. Weatherstrip the hatch and glue rigid foam board to its attic side. Our step-by-step on how to insulate an attic hatch covers the foam board, weatherstrip, and latch details that make the seal hold.
Then the basement rim joist. Cut rigid foam board to fit each joist bay, set it against the rim, and foam or caulk all four edges. Bare rim joists can be foamed directly with a thicker coat instead — check what your local code allows for leaving foam exposed.
Then the living-space details. Foam gaskets behind exterior-wall outlet and switch plates (power off at the breaker first), caulk around plumbing escutcheons, and seal the gaps your incense test found around trim. This is also a natural add-on to a fall winterizing routine, before heating season starts billing you for the leaks.
Retest one spot or two. Light the incense again at your worst marked leaks. Still air means you're done; any remaining wavering shows you which bead missed.
This whole sequence is the backbone of a bigger seasonal push, too — the same attic and rim-joist work shows up in any thorough guide to winterizing your house, because sealing and insulating do their best work together.
Sealing windows shut with caulk. Every year someone caulks operable sashes closed. Weatherstrip moving parts; caulk only the fixed joints and the exterior trim line.
Testing on a calm day and concluding there's nothing to fix. No wind, no pressure difference, no smoke movement. Wait for weather.
Doing the easy leaks and skipping the attic. Caulking around baseboards feels productive, but the physics of rising warm air means top-of-house leaks drive most of the loss. Start where the air leaves.
Foaming a flue. Worth repeating: hot flues and chimneys get clearance, flashing, and high-temp sealant — never a bead of foam.
A $2 incense stick and a windy afternoon will show you more about your house than most energy audits' first hour. Seal the attic hatch and penetrations, foam the rim joists, weatherstrip what moves, and respect the two no-foam zones. Most homes have enough leakage that this one weekend measurably changes how the house feels — fewer drafts at ankle level in winter, and an air conditioner that isn't cooling the attic all summer.
Eventually, yes — homes need some fresh air, and very tight houses add mechanical ventilation on purpose. But most older homes leak so much that a weekend of caulk and foam won't come close to that line. If you have gas appliances and seal aggressively, working carbon monoxide detectors on each floor are non-negotiable, and a combustion-safety check by a heating tech is money well spent.
Neither — they're for different gaps. Caulk suits cracks under about 1/4 inch and anywhere that will be painted. Foam expands to fill big, irregular holes around pipes and framing. Most sealing days use two or three cans of foam and two tubes of caulk.
Usually the attic — the hatch, plumbing and wiring holes through the top plates, and recessed lights together. Because warm air rises, attic leaks also pull cold air in through every basement and first-floor gap, which is why sealing the top often calms drafts at the bottom too.
Seal first. Insulation slows heat transfer through surfaces but does almost nothing to stop moving air — and air moving through insulation undercuts its rating. Sealing the attic floor before adding insulation there is the correct order, and it's much harder to do afterward.
It varies too much by house to promise a number, but in a leaky older home, sealing is consistently ranked among the cheapest energy upgrades per dollar — materials for the worst leaks usually run under $100 total. The comfort change (no more cold draft at the desk, more even room temperatures) is usually what people notice first.