Cold Air Coming Under Your Door? Find the Gap Before You Buy Any Draught Strip

By the FaultToFix team | Published: 31 October 2026 | Last updated: 31 October 2026

DIY level: Easy · Cost: £0 to find the gap; £5–£20 for the right strip or sweep once you know the size · Time: 10 minutes to locate; 20–40 minutes to fit

Quick Answer: Cold air under a door is a gap problem with a specific address — find it before you buy anything. Run the back of your hand around the closed door, then pinpoint the leak with a tissue or incense test: most door draughts come from one place, usually the threshold gap at the bottom, not from all round the door. Then check the door actually closes square — a dropped door defeats any strip, crushing it on one side and gaping on the other. Align first, measure the gap in millimetres, and buy the strip that matches that size: brush for uneven gaps up to about 15–20 mm, foam only for small, even gaps of 1–3 mm, blade or rubber for even gaps of about 3–6 mm. The wrong strip for the gap either does nothing or stops the door closing.

Key Takeaways

Locate before you buy: the hand test finds the zone; the tissue test finds the exact leak. One strip in the right place beats strip all round the door.
Align the door first: a dropped or crooked door crushes strip on one side and leaves a gap on the other. Hinge screws before draught strip, every time.
Match the strip to the measured gap: foam (1–3 mm, even gaps only), blade/rubber (about 3–6 mm, even), brush (uneven gaps, up to roughly 15–20 mm at a threshold).
The threshold is the usual culprit: the bottom gap is the biggest and the most commonly missed — a door sweep or threshold brush is often the whole fix.
Remember the letterbox and keyhole: on external doors, two small holes in the middle of the door leak all winter while everyone stares at the edges.

Jump to a Section

Confirm the diagnosis · Symptom → cause table · You need · Find the gap, step by step · Align the door first · Choose the right strip by gap size · The threshold and door sweeps · Letterbox and keyhole draughts · External vs internal doors · Common mistakes · If that didn’t fix it · When to stop and call a professional · Cost table · US / UK term box · FAQs · Checklist

Safety box

Do not fit draught strip, sweeps, or seals to a fire door, and do not adjust its closer or hinges to stop a draught: a fire door’s seals and self-closing action are part of how it protects you, and altering them is a competent professional’s job. If a fire door is draughty, report it (renters: to your landlord, in writing) rather than sealing it yourself. Take care with the incense test: use it away from curtains, paper, and anything flammable, keep it moving, and never leave it unattended — a tissue strip does the same job with no flame at all, and is the safer default. When trimming strip or a sweep to length, cut away from your body and mind the sharp aluminium carrier edges.

First, confirm the diagnosis (two free tests)

Test 1 — the hand test (1 minute). On a cold or windy day, close the door and run the back of your hand slowly around it — top, both sides, and along the bottom. The back of the hand feels moving air far better than the palm. Note every spot where you feel a flow, not just the first one.

Test 2 — the tissue test (2 minutes). Tape a single sheet of tissue or a strip of lightweight paper to the top of the door frame so it hangs against the closed door, or hold a thin strip loosely near the edges: it flutters and lifts exactly where air is pushing through. An incense stick does the same job with smoke (see the safety box) — watch where the smoke bends or streams. Work around the whole perimeter, including across the letterbox and keyhole on an external door.

Air only at the bottom: threshold gap — the common case. Go to the threshold section after the alignment check.
Air along one side, strongest at one corner: the door is not closing square — a hinge or alignment problem first, strip second.
Air all round, evenly: the door sits loose in its frame — worn or missing strip all round, or a door that has shrunk. Measure the gaps; strip all round is genuinely the fix here.
Air at the letterbox or keyhole only: middle-of-door leaks — see the letterbox section; edge strip will not help at all.
No air movement anywhere, but the hall still feels cold: the draught is not this door — check the next suspects (windows, another door, vents) rather than sealing a door that is not leaking.

Symptom → cause table

What you notice	Most likely cause	First check
Cold air at floor level, under the door	Threshold gap too large or no sweep fitted	Measure the bottom gap; check for an existing brush/sweep
Draught down one side, worst at a corner	Door dropped or out of square — gap uneven	Hinge screws; even-gap check before any strip
Draught all round the edges, evenly	Worn/perished strip, or door shrunk in frame	Measure gaps; strip type matched to size
Draught in the middle of the door	Letterbox or keyhole leak (external doors)	Tissue test across the letterbox flap and keyhole
Strip fitted last year, draught back	Foam strip crushed flat — foam has a short life under compression	Replace with blade/rubber or brush for a lasting fix
Door will not close after fitting strip	Strip too thick for the gap	Measure; fit the size down, not the size up
Draught plus daylight visible under door	Large threshold gap	Brush sweep rated to the measured gap
Draught from a fire door	Seal/closer fault on a fire door	Stop — professional; do not seal it yourself

You need

A strip of tissue or lightweight paper — for the leak test (incense optional, tissue is safer).
A tape measure or ruler — to measure each gap in millimetres before buying.
A screwdriver — Phillips/Pozidriv No. 2 for hinge screws and sweep fixings.
Draught strip matched to your measured gaps (buy after measuring — see the choosing section): self-adhesive foam tape, blade or rubber strip (often sold as V-strip or weatherstrip), or brush strip on an aluminium carrier.
A door sweep / threshold brush — for the bottom gap, screw-fixed or self-adhesive, cut to the door’s width.
Hacksaw (junior size is fine) and scissors — to cut carriers and strip to length.
A pencil — to mark cut lines and screw positions.
Letterbox draught cover / brush and a keyhole escutcheon cover — only if the tissue test found middle-of-door leaks.

Find the gap, step by step

Run the hand test and note the zones where you feel air.
Pinpoint with tissue. Hold or tape the tissue strip at each zone in turn; the flutter marks the exact run of the leak — a corner, a 30 cm stretch of one side, the full bottom.
Measure every leaking gap. With the door closed, measure the gap between door and frame (or floor, at the threshold) at several points along the leak. Write the numbers down: gaps vary, and the variation decides the strip type as much as the size does.
Do the alignment check (next section) before ordering anything. If the door is dropped, the gap sizes you just measured will change once it is aligned — measuring first and aligning second wastes a strip.
Buy once, by the numbers. Take your measurements to the strip types in the choosing section: size, evenness, and position (side edges vs threshold) each point to a different product.

Align the door first

A draught strip seals a gap of the size it was made for. A door that has dropped on its hinges has a gap that is 1 mm at one end and 7 mm at the other, and no single strip seals both ends — it is crushed flat where the gap is small (and stops the door closing) while air pours past it where the gap is large. So: open the door and tighten every hinge screw, top hinge first. Repair any screw that spins without gripping with a longer screw or the matchstick-and-glue fix (both covered in our sticking-door guide). Close the door and check the gaps are now roughly even all round. Only then measure for strip. The same rule applies to a door that will not latch cleanly — alignment faults and draught faults are usually the same fault wearing two coats, and our hinge and latch guide covers the latch side of it.

Choose the right strip by gap size

Illustration: Match the strip to the gap.

Gap (measured, door closed)	Strip that suits it	Notes
1–3 mm, even	Foam tape (self-adhesive)	Cheapest and easiest; compresses to seal small even gaps. Shortest-lived — expect to renew it after a season or two of heavy use. Useless on uneven gaps.
About 3–6 mm, even	Blade / rubber strip (V-strip, or rubber seal on a carrier)	The durable all-rounder for door edges. Springs against the door rather than relying on crush, so it lasts for years and tolerates small variation.
Uneven gap, varying by several mm	Brush strip (pile on a carrier)	The bristles flex to follow the variation — the only type that seals a genuinely uneven edge. Slightly less airtight than a perfect blade fit, vastly better than foam on the same gap.
Threshold gap up to about 15–20 mm	Brush door sweep / threshold brush	Screw-fixed to the door’s bottom face; bristles sweep the floor. For rigid sweeps (rubber blade type), the gap must be even and smaller — measure before choosing.
Gap over about 20 mm at the threshold	Raised threshold / threshold bar, or door repair	Beyond strip territory: a fitted threshold bar, or the door itself needs attention (see “if that didn’t fix it”).

Fit edge strip on the frame (the stop the door closes against), not on the door edge, so the door compresses or meets it as it closes. Work in one continuous run per side where you can, butt joints neatly at corners, and test-close the door after each side: if the door suddenly needs a shove to latch, the strip is too thick for that gap — go a size down rather than forcing the door for a winter.

The threshold and door sweeps

The bottom gap is usually the largest on the door — doors are hung with clearance to clear carpets and floors, often 5–15 mm — and it leaks at floor level, exactly where you feel cold first. Options, cheapest first: a brush sweep screwed to the inside face of the door at the bottom, its bristles just sweeping the floor — forgiving of uneven floors and the standard fix; a rubber-blade sweep, which seals better on smooth, even floors but drags on rough ones; a threshold bar fixed to the floor under the door (often combined with a brush or blade on the door), which also keeps rain out of an external door; and the humble sausage draught excluder as a tonight-only measure for internal doors — it works while it is in place and is not a fitted fix. Fit a sweep with the door closed where possible: position it so the brush or blade just touches the floor along its whole length, mark, then fix. Test the door’s swing over any rug or floor change — a sweep set for the hallway boards may drag on the living-room carpet.

Letterbox and keyhole draughts

External doors have two holes in the middle that edge strip cannot touch. A letterbox (mail slot) whose flap spring has weakened leaks a surprising stream of air: fixes are a brush-lined letterbox cover or an internal flap/draught cover fitted over the inside of the slot — measure the slot first, as sizes vary (common UK letter plates are around 254–305 mm / 10–12 in long). A keyhole leaks a thin but constant jet: a covered escutcheon — a small plate with a swinging cover — is a two-screw, two-minute fix. Run the tissue test across both after fitting; the flap should sit flat and the cover should swing freely over the hole.

External vs internal doors

External doors fight wind and rain as well as cold: they deserve the durable options (blade/rubber strip, brush sweeps, a proper threshold bar), all gaps including letterbox and keyhole checked, and strip fitted to the frame all round — and remember an external door that will not align may have a multipoint lock that must still engage, so never fit strip so thick that the lock has to be forced. Internal doors are about comfort and noise between rooms: foam tape is often enough for their smaller, even gaps, a sausage excluder or a brush sweep handles the bottom, and there is no weather to keep out — but do not seal an internal door so completely that a room with a gas appliance loses its ventilation; rooms with boilers, fires, or gas hobs need their air supply, and if in doubt, leave that door’s sealing to professional advice. (That is about ventilation, not a DIY job on the appliance — never touch the appliance itself.)

Common mistakes

Buying strip before finding the gap. Strip all round a door whose only leak is the threshold is wasted money, a harder-closing door, and the same cold ankles.
Sealing a dropped door. Crushed strip on one side, open gap on the other. Align first — hinge screws are free.
Foam on an uneven gap. Foam only works in the narrow size band it can compress into; on a varying gap it seals the tight spots and waves at the wide ones.
Strip so thick the door will not latch. If you have to shove the door, the strip is wrong. A door left unlatched leaks more than the gap did.
Forgetting the letterbox and keyhole. Two open holes in the middle of an external door, leaking all winter, while the edges get sealed twice.
Setting a sweep to drag. A blade dragging on carpet wears out in months and makes the door a chore; brush sweeps forgive uneven floors.
Sealing a fire door yourself. Its seals and closer are life-safety parts — professional only, and renters report in writing.

If that didn’t fix it

You aligned the door, measured, fitted the matched strip, and air still gets in. Next causes, in order: the gap is at the frame-to-wall joint, not the door at all — run the tissue test around the outside of the frame where it meets the wall; cracked or missing sealant there is a different (and simple) fix, re-sealing the perimeter. The frame itself is twisted or has moved, so the door meets it at the top and stands off it at the bottom beyond any strip’s range — that is a carpenter’s assessment, not thicker strip. The door has shrunk or warped beyond strip tolerance (an old timber external door can bow several millimetres) — a carpenter can advise whether the door can be adjusted, or whether replacement is the honest answer. And if the draught arrives together with new wall cracks or other doors starting to stick, stop treating it as a door problem: read the “professional now” signs in our cracks guide and get the building assessed.

When to stop and call a professional

Stop and get help if: the door is a fire door (any sealing or closer adjustment); the frame is cracked, twisted, rotten, or has visibly moved; the threshold gap is over about 20 mm even after alignment; an external door’s multipoint lock will not engage without force after strip is fitted (locksmith or door specialist — never force it); the wall around the frame is damp or the plaster soft; or the draught comes with new cracks or several doors and windows misbehaving at once (building assessment first).

Cost table

Item	Typical cost
Hand test + tissue test (DIY diagnosis)	$0 / £0
Self-adhesive foam strip, per door	$4–$8 / £3–£6
Blade / rubber strip kit, per door	$8–$18 / £6–£15
Brush strip on carrier, per door	$10–$22 / £8–£18
Door sweep / threshold brush	$10–$25 / £8–£20
Letterbox brush cover + keyhole escutcheon	$8–$20 / £6–£16
Threshold bar (fitted DIY)	$15–$35 / £12–£28
Carpenter to align a door and fit seals	$75–$200 / £50–£150, typical

US / UK term box

US term	UK term used in this guide
Drafty door	Draughty door
Weatherstripping	Draught strip / draught excluder
Door sweep	Door sweep / threshold brush
Mail slot	Letterbox
Keyhole cover	Escutcheon (covered)
Threshold	Threshold / sill
Foam tape	Foam draught strip
Draft stopper (fabric)	Sausage draught excluder

FAQs

Why is the gap under my door so big compared with the sides?

Doors are deliberately hung with clearance at the bottom to swing over carpets, rugs, and uneven floors — 5–15 mm is common, against 2–3 mm at the sides. That is why the threshold is so often the main leak, and why it needs its own fix (a sweep or threshold brush) rather than edge strip.

Foam, brush, or blade — which should I actually buy?

Measure first, then match: foam only for even gaps of 1–3 mm (cheap, short-lived); blade or rubber for even gaps around 3–6 mm (durable, the usual best choice for edges); brush for uneven gaps and for thresholds up to about 15–20 mm, because the bristles follow the variation. Buying by gap size, not by price, is the whole trick.

Can I stop a draught without removing the door?

Yes — almost everything in this guide is done with the door in place: alignment is screw-tightening, edge strip fits to the frame, sweeps screw to the door’s face, and letterbox covers fit from inside. The door only comes off if a hinge itself needs replacing.

My door will not close properly since I fitted draught strip. What went wrong?

The strip is too thick for the gap it is in — very common when strip is bought by guess. The door should latch with its normal gentle push. Fit the next size down, or switch that run from foam to a blade type, and never solve it by shoving the door: a door left unlatched leaks more than it did before the strip.

Should I seal the internal door to my boiler room or the room with the gas fire?

Be careful. Rooms with gas appliances need ventilation, and sealing their doors tightly can be unsafe. Do not fully seal a door to a room containing a boiler, fire, or gas hob without professional advice — and never adjust or block vents in such a room. That caution does not apply to ordinary bedrooms and living rooms.

Checklist

[ ] Hand test done — leaking zones noted (bottom / side / all round / middle)
[ ] Tissue test pinpointed the exact leak runs
[ ] Door aligned first — hinge screws tight, stripped holes repaired, gaps evened
[ ] Every leaking gap measured in mm before buying
[ ] Strip matched to gap: foam 1–3 mm even · blade/rubber ~3–6 mm even · brush for uneven and thresholds
[ ] Threshold fitted with a sweep or brush that just touches the floor
[ ] Letterbox and keyhole tested and covered on external doors
[ ] Door still latches with a normal gentle push after fitting
[ ] Fire door left alone and reported instead, if that is what the door is