04 / HIDDEN LEAKS
The drafts you cannot see
Doors and windows are the ones you notice. The attic hatch, the wall cavity behind an outlet and the joint behind the trim are the ones doing the damage.
Start with how to find drafts in your house — five tests, no equipment, about an hour.
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Everything in hidden leaks
Roundups first, then the type comparisons, then the how-to guides.
Roundup / Best outlet gaskets
The best outlet gaskets, and an honest word on what they fix
Two minutes and a screwdriver per outlet — genuinely worth it on exterior walls, and honestly small.
Top pick: Frost King OS6H Foam Outlet and Switch Sealers, 2-1/2 in x 4 in
CHECK PRICE
Once the doors and windows are sealed and the room is still cold, this is where the rest of it is. Not in one dramatic hole, usually, but in a dozen small ones plus one large one nobody has looked at: the hatch into the attic.
The reason these matter more than their size suggests is the stack effect. Warm air rises and escapes at the top of the house; an equal volume of cold air is pulled in at the bottom to replace it. Which means the draft you feel at ankle height in the hallway can be caused by a hole in the ceiling. Seal the bottom and the same volume still comes in through a different gap. Seal the top and the whole pump loses pressure.
Find them first — it is an hour with an incense stick
- 1Close every exterior window and door, and close the fireplace damper.
- 2Turn off the furnace and any other combustion appliance.
- 3Run the bathroom extractor fans and the range hood. The house is now slightly depressurized and every gap is pulling inward.
- 4Move a lit incense stick slowly along door and window frames, the attic hatch, baseboards, exterior-wall outlets, and where pipes enter under sinks.
- 5Watch the smoke column, not the stick. Where it bends, air is coming in. Mark it with tape and keep going.
What you will find, and what closes it
| Leak | Typical size | The fix |
|---|---|---|
| Attic hatch or pull-down stair | Often the biggest in the house | Perimeter bulb seal plus a latch or weight |
| Fireplace damper, open or warped | Very large when open | Close it; check it seats. A chimney draft stopper if the fireplace is unused |
| Joint behind door and window trim | Medium to large | Backer rod and caulk |
| Rim joist in the basement | Large, and rarely looked at | Minimal-expanding foam or rigid board plus sealant |
| Outlets and switches on exterior walls | Small, numerous | Outlet gaskets |
| Pipe and cable penetrations | Small, numerous | Caulk, or foam for larger holes |
| Recessed lights in a top-floor ceiling | Small each, badly placed | Airtight-rated fittings; not a DIY seal-over job |
The attic hatch deserves its own paragraph
On a great many houses it is a sheet of plywood resting in a frame, with no seal and no insulation, sitting at the exact point where the stack effect is strongest. It is the closest thing to a free win in this whole category: a perimeter bulb seal so the panel closes onto something, and a latch or a weight so it stays down.
A dedicated attic section — hatch covers, stair tents, and the pull-down stair problem — is being built and will ship with product picks when it is complete rather than as a thin page now. Until then, treat the hatch as a door: measure the gap around the panel and fit a bulb seal to the frame it rests on. The same parts work.
And the fireplace
An open damper is a permanent opening to the sky, and a warped one is nearly as bad. Check yours closes and seats. If the fireplace is genuinely never used, a chimney draft stopper is a real fix — with one absolute rule: leave an unmissable reminder that it is in place, because lighting a fire under an inflated chimney balloon is dangerous. A full fireplace section with product picks is planned rather than shipped, for the same reason as the attic: we would rather publish nothing than publish a thin page.
What not to seal
- Combustion air supplies. Anything that feeds a furnace, boiler or water heater.
- Deliberate ventilation. Trickle vents, extractor ducts, crawlspace vents where they are required.
- Inside electrical boxes. Seal around the box, never inside it.
- Weep holes in window and door frames, and the drainage slots in a sliding door track.
If you have any fuel-burning appliance and you are sealing seriously, a carbon monoxide alarm is not optional. The goal is to close the accidental holes, not the intentional ones.
Questions people actually ask
- How do I find air leaks I cannot feel?
- Depressurize the house by running the bathroom and kitchen extractor fans with the windows shut, then trace a lit incense stick around frames, the attic hatch, baseboards and outlets. The smoke shows movement your hand cannot feel.
- Are outlet gaskets worth fitting?
- On exterior walls and walls shared with an unheated garage, yes — cheap, quick and a real air path. On interior walls between heated rooms they do nothing. They are also a small fix compared with the attic hatch or the garage door.
- What is the biggest hidden air leak in a house?
- Usually the attic hatch or loft access: unsealed, uninsulated and sitting exactly where the stack effect pulls hardest. The rim joist in the basement is the common runner-up.
Sources
- Frost King — outlet and switch sealers, published material and UL listing
- Frost King — caulk saver backer rod, published sizing
Every specification on this page traces to a published manufacturer or retailer listing. Where a maker publishes no working gap range, we say so rather than estimating one.