04 / HIDDEN LEAKS
Cold air coming back through the dryer vent
Every dryer duct is a four-inch hole through an exterior wall. Something is supposed to close it when the dryer stops, and very often nothing does.
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Go outside and look at the vent hood while the dryer is off. Its flap should be closed. If it is stuck open with lint, cleaning it is the whole fix and costs nothing. If the flap is missing or broken, replace the hood or fit an inline backdraft damper in the duct behind the dryer.
A dryer duct is a genuinely large opening — four inches across on almost every residential install — running straight through an exterior wall. It has to be open when the dryer runs, so the only thing standing between your laundry room and outside is a flap that is supposed to fall shut afterwards.
That flap lives outdoors, in the weather, in the path of everything the dryer blows at it. It fails constantly, and when it does you have a permanently open hole that nobody thinks to check.
Step 1: look at the hood from outside
- 1With the dryer OFF, go outside and find the vent hood.
- 2Look at the flap or louvers. They should be sitting closed under their own weight.
- 3If they are propped open, look at why: lint packed behind the hinge is the usual culprit, followed by a bent louver and a nest.
- 4Clean out what is there. On a lot of houses the draft stops here and you have spent nothing.
- 5If the flap is missing, cracked, or will not fall shut when you lift and release it, the hood needs replacing.
Step 2: if the hood cannot be fixed, add a damper
An inline backdraft damper sits in the duct behind the dryer. It is a gravity flap: the dryer's airflow pushes it open, and it falls shut when the dryer stops. It gives you a second closure in a warm, accessible, indoor location instead of relying on a weathered plastic flap outside.
- Four inch is the standard residential size. Measure your duct before ordering; almost all of them are 4 in, but not quite all.
- Fit it where you can reach it. The point of an indoor damper is that you can clean it. One buried in a wall cavity is worse than no damper.
- It is not a substitute for a working hood. Fix the outside flap as well if you can — two closures are better than one, and the hood also keeps birds and rodents out.
Quick picks
Ranked for fit, not for hype. Tap a row to jump to the full write-up, or go straight to the listing.
| # | Product | Best for | Gap range | Price |
|---|---|---|---|---|
| 1 |  Deflecto Dryer Vent Draft Blocker, 4 in Inline Backdraft DamperA gravity flap that sits inline in a 4 inch duct: the dryer's own airflow pushes it open, and it falls shut when the dryer stops. | Cold air coming back down the dryer duct | Fits standard 4 in dryer duct | $7.99 · CHECK ON AMAZON |
| 2 |  Dundas Jafine DB4ZW Dryer Vent Draft BlockerThe same gravity-flap idea from the other established duct-accessory brand, worth knowing as a second source. | The alternative if the Deflecto is out of stock | Fits standard 4 in dryer duct | $4.99 · CHECK ON AMAZON |
| 3 |  fowong Open Cell Foam Weather Stripping, 1 in x 1/2 in x 13 ftHalf-inch-thick open-cell foam for the gap nothing else reaches — an AC surround, a loose sash, a warped frame. | Large, irregular gaps | Published 1/2 in thick; fills gaps of roughly 1/8 in to 3/8 in | $16.99 · CHECK ON AMAZON |
#ad · how we are funded · price as of October 17, 2026
Cold air coming back down the dryer duct
Deflecto Dryer Vent Draft Blocker, 4 in Inline Backdraft Damper
Published gap range
Fits standard 4 in dryer duct
#ad · how we are funded · price as of October 17, 2026
A gravity flap that sits inline in a 4 inch duct: the dryer's own airflow pushes it open, and it falls shut when the dryer stops.
Where the number comes fromDeflecto publishes the duct diameter (4 in) and the mechanism (a gravity flap for one-way airflow). There is no gap dimension involved — the thing you are closing is the duct itself, and 4 in is the standard residential dryer size.
- Published duct size
- 4 in
- Mechanism
- Gravity flap, one-way airflow
- Install
- Inline in the duct
- Material
- Plastic
- Also blocks
- Dust and outdoor debris, per the maker
- Mounting
- Inline in the duct run, behind the dryer
What it does well
- Closes a duct that is otherwise a permanently open hole to outside whenever the dryer is off
- Standard 4 in size fits the overwhelming majority of residential dryer runs
- No power, no wiring, nothing to fail except a hinge
Where it fails
- It is one more restriction in a duct run that needs to stay clear — do not fit one to a long or already-restricted run without checking airflow
- Lint can foul the flap and hold it open; it needs cleaning with the rest of the duct
- Plastic, so treat it as a maintenance item rather than a permanent fitting
Who this is forA laundry room that is noticeably cold when the dryer is off, with an accessible duct behind the machine.
The alternative if the Deflecto is out of stock
Dundas Jafine DB4ZW Dryer Vent Draft Blocker
Published gap range
Fits standard 4 in dryer duct
#ad · how we are funded · price as of October 17, 2026
The same gravity-flap idea from the other established duct-accessory brand, worth knowing as a second source.
Where the number comes fromDundas Jafine publishes this as a dryer vent draft blocker with a flap that opens when the dryer starts and closes when it stops. The retail listing publishes no further dimensions, so we quote none.
- Mechanism
- Flap, opens on dryer airflow
- Install
- Inline in the duct
- Model
- DB4ZW
- Material
- Plastic
- Spec status
- Listing verified by brand; its title does not print the model code
- Mounting
- Inline in the duct run
What it does well
- Second source for the same job from an established duct-accessory brand
- Same no-power, no-wiring simplicity as the Deflecto
- Useful when the first choice is unavailable
Where it fails
- The retail listing carries the Dundas Jafine brand and the right part, but its title does not print the DB4ZW model code — that code comes from Dundas Jafine's own catalog
- Same lint-fouling maintenance requirement as any duct flap
- Same airflow caution on a long or restricted duct run
Who this is forAnyone whose first choice of damper is out of stock.
Large, irregular gaps
fowong Open Cell Foam Weather Stripping, 1 in x 1/2 in x 13 ft
Published gap range
Published 1/2 in thick; fills gaps of roughly 1/8 in to 3/8 in
#ad · how we are funded · price as of October 17, 2026
Half-inch-thick open-cell foam for the gap nothing else reaches — an AC surround, a loose sash, a warped frame.
Where the number comes fromfowong publishes 1" wide x 1/2" thick x 13 ft. Open-cell foam compresses much further than rubber, which is why it covers a wider range — and also why it is not a long-life exterior seal.
- Material
- PU foam, PE cover
- Published width
- 1 in
- Published thickness
- 1/2 in
- Published length
- 13 ft
- Mounting
- Strong adhesive
- Mounting
- Self-adhesive
What it does well
- Half an inch of squash covers gaps rubber sections cannot
- The PE facing sheds water better than bare foam
- Cheap enough to over-buy and cut down
Where it fails
- Open-cell foam holds water if the facing is cut or torn
- Shortest life of any seal here in direct weather
- Too thick for a door that latches tightly
Who this is forA portable-AC surround, a basement sash, or any gap you can push a finger into.
How we picked
- Diagnose the outside hood before recommending anything, because the free fix works more often than the paid one.
- Products placed by published duct diameter, and the airflow caution stated before the recommendation.
We have not tested these products and we do not claim to have. The full method, including what we refuse to claim, is on how we pick.
The gap around the duct is a separate leak
While you are behind the machine, look at where the duct passes through the wall. There is frequently an oversized hole with the duct running loosely through the middle of it, and that annulus is its own air path — independent of whether the flap works.
Seal it from inside with foam or caulk, sized to the gap. The method is the same as every other penetration in the house and it is covered in spray foam vs caulk vs backer rod. Do not seal the duct itself shut, obviously — you are sealing the gap around it.
Once the laundry room is dealt with, the same logic applies to every other vent and penetration in the house. How to find drafts in your house walks the whole building with an incense stick.
Questions people actually ask
- Why is cold air coming through my dryer vent?
- Because the flap on the outside vent hood is not closing. It is usually held open by lint, sometimes bent or broken, and occasionally missing entirely. With the dryer off, that leaves a four-inch open hole through an exterior wall.
- How do I stop cold air coming in the dryer vent?
- Clean or replace the outside vent hood so its flap closes under its own weight. If that cannot be made to work, add an inline backdraft damper in the duct behind the dryer — but clean the duct run first and check the run is not already restricted.
- Are dryer vent backdraft dampers a good idea?
- They work, and they add a restriction to a duct that needs to stay clear. On a short accessible run they are a sensible second closure. On a long or already-restricted run, fixing the outside hood is the better answer.
- Can I just cover the dryer vent in winter?
- No. Blocking a dryer duct traps moisture and lint in the run and, on a gas dryer, exhaust gases. Never cap a vent that serves an appliance you use.
Sources
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.
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