04 / HIDDEN LEAKS
The best fireplace and chimney draft stoppers
A warped damper is a chimney-sized leak that runs all winter. Plugging it is a genuinely good fix — provided you can guarantee nobody lights a fire on top of it.
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If the fireplace is truly disused, a wool flue plug sized to the flue seals best. If there is any chance of a fire being lit, fit a magnetic blanket across the opening instead — it is visible from the room, so it cannot be forgotten. Never block a flue that serves a live appliance.
A hole to the sky, all winter
An open chimney is the most effective air leak a house has, and it is effective because it is designed to be. A chimney works by stack effect: warm air inside the flue is lighter than the cold air outside, so it rises, and that rising column pulls replacement air in behind it. That is precisely what carries smoke away from a fire.
With no fire lit, the mechanism does not switch off. It carries on pulling heated air out of your living room and up the chimney twenty-four hours a day, and every cubic foot that leaves is replaced by cold air drawn in through every gap lower down in the house. A chimney does not just leak. It actively pumps.
The damper is supposed to stop that, and on a well-fitted modern one it largely does. On a cast iron plate that has been heat-cycled for decades in a masonry throat, it does not. We say in why is my house so drafty that a warped damper is a chimney-sized leak, and this page is the part of the site that does something about it.
Opening or flue: the decision that matters
There are two honest approaches and they suit different households. It is not really a performance question — it is a question about your house and the people in it.
- At the opening. A blanket or panel across the front of the firebox, usually held by magnets on a steel surround. Seals the room from the firebox. Visible every time you walk past, so the risk of someone lighting a fire on top of it is close to zero. The chimney above continues to breathe, which is good for the masonry.
- Up the flue. A plug pushed into the throat above the damper. Seals much closer to the source, so the whole firebox stops acting as a cold sink, and it is invisible. Depends entirely on a reminder being noticed months later.
Our position: a flue plug is the better seal and the opening blanket is the better product for most households. If the fireplace is decorative in a house with a wood store and a box of matches on the hearth, take the visible option and accept the slightly worse seal.
Match the part to your gap
Measure at the widest point. Then read across. If your measurement sits on a boundary, take the larger band — a seal that is slightly too big compresses, and one that is too small does nothing at all.
| Your gap | Part type | Our pick | Price |
|---|---|---|---|
| You want it visible and reversible | Magnetic opening blanket |  CADARA Magnetic Fireplace Draft Blocker Blanket, 33 x 29 inCovers the firebox opening, never goes near the flue, and cannot be forgotten because you are looking at it. | $21.99 · CHECK ON AMAZON |
| Round flue, 8-9 in | Wool flue plug |  Flueblocker Chimney Plug, Wool, 8-9 in Round FlueFor a lined chimney or a disused stove. Wool breathes, so the flue is not sealed airtight against moisture. | $42.99 · CHECK ON AMAZON |
| Large rectangular flue | Wool flue plug, 9 x 45 in |  Flueblocker Chimney Draft Stopper, Wool, 9 x 45 inPublished to a named flue size rather than sold as universal, which is what you want up a chimney. | $129.99 · CHECK ON AMAZON |
| Rectangular flue, tighter budget | Flue plug, 8 x 32 in |  Chimney Flue Plug for 8 x 32 in Rectangular FluePublished size at a fraction of the wool price. Publishes no material, so keep it out of a damp chimney. | $48.99 · CHECK ON AMAZON |
Measuring a flue
The number you need is the internal dimension of the flue above the damper, and there is no shortcut to getting it. Open the damper fully, take a torch and a tape, and reach up into the throat. Expect soot; wear something you do not mind ruining and put a sheet down first.
Flues are either rectangular — a masonry throat in an older open fireplace, commonly something like 8 by 32 in — or round, which means a metal liner, typically 5 to 9 in in diameter, serving a stove or a relined chimney. Plugs are published for one or the other, and a rectangular plug will not seat in a round liner.
If you cannot reach the flue, or you find soot deposits thick enough to flake away in your hand, that is a chimney that wants sweeping before it wants plugging — and the sweep can tell you the flue size while they are there.
The picks
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 | CADARA Magnetic Fireplace Draft Blocker Blanket, 33 x 29 inThe one that never goes near the flue: a magnetic blanket across the fireplace opening, which is the safer choice for anyone who does not want to reach past a damper. | Sealing at the opening, not up the flue | Published as a 33 in wide x 29 in high blanket | $21.99 · CHECK ON AMAZON |
| 2 | Flueblocker Chimney Plug, Wool, 8-9 in Round FlueThe round-flue version, which is what a wood stove or a lined chimney usually has rather than the big rectangular throat of an open fireplace. | A round metal flue | Published to fit an 8-9 in round flue | $42.99 · CHECK ON AMAZON |
| 3 | Flueblocker Chimney Draft Stopper, Wool, 9 x 45 inWool rather than plastic, made by Chimney Sheep, and published at a specific flue size rather than sold as one-size-fits-all. | A large rectangular flue | Published to fit a 9 x 45 in flue | $129.99 · CHECK ON AMAZON |
| 4 | Chimney Flue Plug for 8 x 32 in Rectangular FlueThe budget route to a plugged rectangular flue, published at 8 by 32 inches and priced well under the wool equivalents. | A rectangular flue at lower cost | Published to fit an 8 x 32 in rectangular flue | $48.99 · CHECK ON AMAZON |
| 5 |  Frost King EPDM Rubber Self-Stick Weatherseal Tape, D-Section, 5/16 in x 1/4 in x 17 ftEPDM in a hollow D — the profile that keeps sealing at minus ten where foam tape has already gone hard. | Medium frame gaps | Published 1/4 in thick; compresses to seal gaps of roughly 1/8 in to 1/4 in | $7.69 · CHECK ON AMAZON |
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Sealing at the opening, not up the flue
CADARA Magnetic Fireplace Draft Blocker Blanket, 33 x 29 in
Published gap range
Published as a 33 in wide x 29 in high blanket
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The one that never goes near the flue: a magnetic blanket across the fireplace opening, which is the safer choice for anyone who does not want to reach past a damper.
Where the number comes fromThis covers the fireplace OPENING rather than plugging the flue, so it is sized against the firebox front: published at 33 in wide by 29 in high. It stops the room air being drawn up the chimney, which is most of what a homeowner feels, without anything being pushed into the flue where it can be forgotten.
- Published size
- 33 in W x 29 in H
- Fixing
- Magnetic, plus hook-and-loop tape
- Covers
- The fireplace opening
- Goes into the flue
- No
- Finish
- Black
- Mounting
- Magnets and hook-and-loop tape across the fireplace opening
What it does well
- Visible at the hearth, so it cannot be forgotten the way a flue plug can
- No reaching up a sooty throat to measure or fit
- Cheapest pick here by a wide margin
Where it fails
- Magnets only grip if your surround is steel — on a brick or stone surround you are relying on the tape
- Seals the opening, not the flue, so the chimney above it still breathes
- Still must be removed before any fire is lit
Who this is forA fireplace with a steel surround, or anyone unwilling to put anything up the flue.
A round metal flue
Flueblocker Chimney Plug, Wool, 8-9 in Round Flue
Published gap range
Published to fit an 8-9 in round flue
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The round-flue version, which is what a wood stove or a lined chimney usually has rather than the big rectangular throat of an open fireplace.
Where the number comes fromPublished for round flues of 8 to 9 in. Round flues are typically a metal liner, and a liner is the case where getting the diameter right matters most — too small and it falls, too large and it will not seat. Measure the internal diameter of the liner.
- Published flue fit
- 8-9 in round
- Material
- Wool
- Maker
- Chimney Sheep
- Suits
- Lined chimneys and stove flues
- Smaller size listed
- 5-6 in round
- Mounting
- Pushed up into the round flue
What it does well
- Published diameter range rather than a vague fit
- A 5-6 in version is listed for narrower stove flues
- Same breathable wool as the rectangular plugs
Where it fails
- Same absolute rule: no fire with the plug in place
- Only right for a round flue — an open fireplace throat is usually rectangular
- Costs several times a fabric fireplace blanket
Who this is forA disused stove or a lined chimney with a round flue.
A large rectangular flue
Flueblocker Chimney Draft Stopper, Wool, 9 x 45 in
Published gap range
Published to fit a 9 x 45 in flue
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Wool rather than plastic, made by Chimney Sheep, and published at a specific flue size rather than sold as one-size-fits-all.
Where the number comes fromA chimney plug is sized to the FLUE, measured inside the throat above the damper — not to the fireplace opening. Wool is the material the maker publishes, and it matters: wool breathes, so a small amount of moisture moving through the chimney is not trapped against the masonry the way a sealed plastic plug traps it.
- Published flue fit
- 9 x 45 in
- Material
- Wool
- Maker
- Chimney Sheep
- Fitted
- Into the flue, above the damper
- Breathable
- Yes — wool, not sealed plastic
- Mounting
- Pushed up into the flue above the damper
What it does well
- Published for a named flue size rather than sold as universal
- Wool breathes, so the flue is not sealed airtight against moisture
- A real maker with a full size range behind the part
Where it fails
- **Never light a fire with a plug fitted.** Leave an unmissable reminder at the hearth — this is the whole safety case for the category
- The most expensive option here by a wide margin
- You must measure inside the flue, which means reaching up past a sooty damper
Who this is forA large unused fireplace where the damper no longer closes properly.
A rectangular flue at lower cost
Chimney Flue Plug for 8 x 32 in Rectangular Flue
Published gap range
Published to fit an 8 x 32 in rectangular flue
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The budget route to a plugged rectangular flue, published at 8 by 32 inches and priced well under the wool equivalents.
Where the number comes fromPublished for an 8 x 32 in rectangular flue, with other rectangular sizes listed separately. The trade-off against the wool plugs is breathability: the listing does not publish the material's moisture behavior, so in a chimney that has ever shown damp, the breathable option is the safer one.
- Published flue fit
- 8 x 32 in rectangular
- Other sizes listed
- 8 x 24, 15 x 45, 16 x 16 in
- Published material
- None stated
- Fitted
- Into the flue
- Also claims
- Blocks odors and debris
- Mounting
- Pushed up into the flue above the damper
What it does well
- A published rectangular size at well under the wool plug price
- A wide range of rectangular sizes listed, including some the wool range does not cover
- Blocks the soot smell as well as the draft
Where it fails
- **The listing publishes no material**, so its moisture behavior in a damp chimney is unknown
- Same absolute no-fire rule
- Unbranded reseller listing with no maker's data sheet
Who this is forA dry, unused rectangular flue where the wool plug is out of budget.
Medium frame gaps
Frost King EPDM Rubber Self-Stick Weatherseal Tape, D-Section, 5/16 in x 1/4 in x 17 ft
Published gap range
Published 1/4 in thick; compresses to seal gaps of roughly 1/8 in to 1/4 in
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EPDM in a hollow D — the profile that keeps sealing at minus ten where foam tape has already gone hard.
Where the number comes fromFrost King publishes the section dimensions exactly: 5/16" wide by 1/4" thick, 17 ft long. A hollow D seals across roughly half to full its stated thickness, which is where the 1/8-1/4 in working figure comes from.
- Profile
- Hollow D
- Published width
- 5/16 in
- Published thickness
- 1/4 in
- Published length
- 17 ft
- Material
- EPDM rubber
- Mounting
- Self-adhesive onto the door stop
What it does well
- EPDM stays flexible in cold, which is the entire point of a winter seal
- Published dimensions to three fractions — you can actually match it to a measurement
- Compresses without taking a permanent set the way open-cell foam does
Where it fails
- 17 ft is one door frame; a house needs several rolls
- Adhesive still needs a clean, smooth surface
- Thicker than a V-strip, so it can stop a tight door from latching
Who this is forA door stop or window sash with a visible, consistent gap you have measured.
What we would skip, and why
Nobody in this category ever declines to recommend something. That is what makes universal praise worth ignoring. Here is what we would not buy.
Any of these, if there is the slightest chance of a fire being lit
This is an absolute rule and it is why the blanket is ranked first. A plug left in the flue turns the next fire into a chimney full of smoke pushed into the room, and the plug itself into fuel. Every maker in this category supplies a reminder tag for the hearth and every one of them means it. If the fireplace is genuinely disused, fine. If it is lit twice a year at Christmas, or if guests or children have access, fit something visible at the opening rather than something hidden up the throat.
Plugging a flue that serves a live appliance
If a gas fire, a gas water heater, a boiler or a wood stove vents into that chimney — even if you never use the fireplace opening itself — the flue is not disused and must not be blocked. Shared and disused-looking flues are more common in older houses than people expect. If you are not certain what vents where, do not put anything up there.
Buying before you have measured inside the flue
A chimney plug is sized to the flue, measured above the damper, not to the fireplace opening you can see. Those are wildly different dimensions — a 36 in wide firebox may have an 8 by 32 in flue above it. Reaching up past a sooty damper with a tape measure is an unpleasant ten minutes and there is no way around it if you want a plug that seats.
Relying on the damper you already have
A cast iron damper plate sitting in a masonry throat was never an airtight seal, and after twenty years of heat cycling most are warped, rusted at the edges or propped on a lump of debris. Close yours and put your hand under it on a cold windy day. If you feel movement, the damper is not doing the job and no amount of forcing the lever will make it.
How we picked
- We rank safety of the failure mode first, which is unusual and specific to this category. A product at the fireplace opening is visible from the room, so the failure mode — someone lighting a fire — is very unlikely. A plug hidden up the flue seals better and depends entirely on a reminder tag being noticed. Visible first.
- Published flue fit second. These are sized to the flue, and a maker who publishes named sizes is a maker whose product will seat. A universal plug in a chimney is a plug that falls or does not go in.
- Published material third, because it decides moisture behavior. Wool breathes, so a small amount of moisture passing through the chimney is not trapped against the masonry. A plug whose material is unpublished is an unknown in a place that gets damp.
- We have tested nothing and we are not chimney sweeps. If your chimney has any history of damp, or you cannot establish what vents into it, that is a question for a professional and not for this page. How we pick.
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.
Complete the kit
The parts that decide whether the seal you just bought actually holds. Skip these and you will be doing the job again in March.
Medium frame gaps
Frost King EPDM Rubber Self-Stick Weatherseal Tape, D-Section, 5/16 in x 1/4 in x 17 ft
EPDM in a hollow D — the profile that keeps sealing at minus ten where foam tape has already gone hard.
Removing old weatherstrip adhesive
Goo Gone Pro-Power Adhesive Remover Spray, 16 fl oz, with Scraper
The step everybody skips: the new tape will not stick to the residue the old tape left behind.
Deep gaps before caulking
Frost King C21CP-C Caulk Saver Backer Rod, 3/8 in x 350 ft
Backer rod: the foam you push into a deep gap so the caulk over it is a seal rather than a bottomless hole.
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Why wool, and why it is not just marketing
The premium plugs in this category are made of wool, and there is a real reason for it beyond the price tag. A chimney is a masonry structure exposed to weather at the top, and a small amount of moisture moving through it is normal. A plug that seals it airtight traps that moisture inside the stack, where it has nowhere to go but into the brickwork.
Wool is breathable: it stops the bulk airflow that costs you heat while still letting water vapor pass. That is the difference between blocking a draft and sealing a chimney shut, and in a chimney with any history of damp it is the difference that matters. Where a listing does not publish its material at all, we say so in the pick's cons — because in this specific location an unknown material is a genuine unknown.
Once the chimney is dealt with, the other openings in the same room worth checking are the outlets and switches on external walls and any return vent nearby — both are small individually and both are usually unaddressed.
Questions people actually ask
- Do fireplace draft stoppers work?
- Yes, and the effect is larger than most draft fixes because an open chimney actively pulls heated air out of the room rather than passively leaking. A plug sized to the flue seals best; a blanket across the opening seals slightly less well but is visible, which matters for safety.
- Is it safe to block a chimney?
- Only if the flue is genuinely disused and serves no appliance — no gas fire, no water heater, no boiler, no stove. If anything vents into that chimney, it must stay clear. And whatever you fit, a fire must never be lit while it is in place, which is why every maker supplies a reminder tag for the hearth.
- How do I measure my chimney for a flue plug?
- Open the damper, and with a torch and tape measure the internal dimensions of the flue above it. That is a different measurement from the fireplace opening — a wide firebox often has a much smaller flue. Rectangular masonry throats and round metal liners take different plugs.
- Why are wool chimney plugs more expensive?
- Wool breathes, so it blocks the bulk airflow that costs you heat while still letting water vapor through the stack. In a chimney with any history of damp that is a meaningful advantage over a sealed plug, which traps moisture inside the masonry.
Read next
Why is my house so drafty?
One mechanism, a handful of holes, and why the fix for a downstairs draft is often in the ceiling.
How to find drafts in your house
Five tests, no equipment, an hour — ordered so you find the big leaks before the small ones.
Cold air coming from an outlet or light switch
The outlet is the exit, not the leak. Where the air really comes from, and the twenty-minute fix.
Cold air blowing from a return vent
A return should pull, not blow. What it means when it does, what to seal, and where to stop and call someone.