Not turning up the thermostat: the lesser‑known draught fix around windows that keeps rooms warmest

Ryan Collins • December 10, 2025 16:40

Cold air has a talent for finding you. You can be sat in a room that’s officially 20°C, socks on, jumper zipped, and still feel that needle‑thin strip of chill running along your ankles whenever you walk past the window. The usual reflex is to turn the thermostat up and hope for the best.

Yet in a lot of British homes, the real fix is sitting a few centimetres away from that dial, in a place most of us barely notice: the hairline cracks around the window frame itself. Not the glass. Not the curtains. The skinny gap where frame meets wall and sill.

Seal that properly, and the whole room changes temperature without the boiler working any harder.

We’re talking about a thin, invisible ring of seal around the window that quietly stops the worst draughts before they even enter the room.

The forgotten gap that makes rooms feel colder than they are

Stand by a chilly window for a minute and watch what happens. Warm air from the radiator rises, brushes past the cold glass, drops like a curtain and slinks back across the floor. Add a sneaky gap around the frame, and that loop turns into a mini wind tunnel.

The thermostat might say 20°C, but fast‑moving air can make your skin feel two or three degrees colder.

Most of us try to fight this with heavier curtains, thicker socks, maybe even new glazing. Yet a surprising amount of heat is lost not through the glass, but through:

The join between the frame and the plastered wall
The thin line where the sill meets the frame
Cracks behind the window board or trim

Builders are meant to seal those joints. On older properties, that sealant has often cracked, shrunk or fallen out. On newer ones, it was sometimes rushed or skipped entirely. The result is a series of hairline tunnels that link your living room to the outside air.

That’s the draught you feel, even when the windows look fine.

The “invisible ring” that keeps the warmth in

The lesser‑known fix is simple enough to sketch: imagine drawing a continuous loop around the inside edge of every window, sealing every tiny gap where air can sneak through. That loop might be made of flexible decorator’s caulk, a neat bead of sealant, or slim foam strip. The point is continuity.

Think of it as fitting a gasket to the room side of the window, not just trusting what’s buried in the wall.

Done well, that ring:

Slows cold air before it can spill into the room
Calms the draughts that make a space feel “leaky”
Lets your existing heating actually stay in the room longer

You’re not adding bulk insulation; you’re removing the wind‑chill effect. That’s why the room often feels dramatically different even though the thermostat setting hasn’t moved.

For many people, it’s the difference between “I can’t sit by that window in winter” and “this is where I drink my morning tea”.

How to tell if your windows are leaking heat

You don’t need a thermal camera to spot the culprits, although they are oddly addictive if you ever borrow one. A few low‑tech checks usually do the job:

Back‑of‑hand test
On a breezy day, slowly trace the back of your hand around the frame, sill and corners. Our skin is good at picking up fast‑moving cold air.
Tissue or incense test
Hold a strip of loo roll or a stick of incense close to the frame. If it flutters or the smoke stream bends and wobbles, you’ve found a leak.
Torch at night
If you can get someone outside safely, shine a bright torch around the frame. Light seeping through cracks or under the sill is often a sign of a poor seal.
Noise and dust
Excess road noise and fine dust around the reveal can also hint at gaps between frame and wall.

If the draught feels like it’s coming through the glass or around the opening sections, you may need additional draught‑proofing strips on the sashes as well. But even then, that frame‑to‑wall joint is worth tackling; it’s the bit most people skip.

Step‑by‑step: sealing the frame without wrecking the paintwork

For small to moderate gaps (up to roughly 5 mm), a tube of flexible decorator’s caulk is often all you need. It’s paintable, forgiving and designed for exactly this job.

What you’ll need

Flexible decorator’s caulk (or acrylic frame sealant)
Caulking gun (if not a squeezy tube)
Craft knife or scraper
Damp cloth or sponge
Masking tape (optional but handy)
Paint to match, if you want an invisible finish

1. Prep the area

Scrape out any cracked, loose or crumbly old filler between the frame and wall. You want a clean, dry gap. Vacuum or brush away dust; sealant doesn’t stick well to fluff and plaster crumbs.

If you’re nervous about messy lines, run masking tape a few millimetres away from the joint on both the frame and the wall. It gives you a neat channel to work in.

2. Apply a continuous bead

Cut the nozzle at a slight angle for a thin bead. Starting in a corner, slowly squeeze caulk into the gap, keeping the nozzle moving so you don’t pile it up. The aim is one unbroken line around the entire frame.

Corners and the line where the sill meets the frame are where most leaks hide. Don’t forget under the overhang of any internal window board if you can reach it.

3. Smooth and clean

With a damp finger or sponge, gently smooth the bead so it keys into both surfaces and loses the sharp ridge. Work in sections before it skins over. Wipe away any smears with a damp cloth.

Peel off the masking tape while the sealant is still soft for a crisp edge.

4. Let it cure, then paint if needed

Most decorator’s caulks are touch‑dry in an hour or so, but check the instructions before painting. Once over‑painted, the seal almost disappears, leaving a plain wall that just happens to be much less draughty.

If you rent and don’t want to paint, choose a white or off‑white caulk that blends with existing trim. It usually looks tidier than a cracked gap.

What if the gaps are bigger – or the window is really leaky?

Sometimes you peel back the curtain and discover proper holes: big cracks, visible daylight, or a frame that’s clearly shrunk away from the plaster. In that case, you may need to go a step up.

Options include:

Backer rod + sealant
A squishy foam cord pushed into the gap first, with sealant over the top. It fills depth without wasting product and gives the caulk something to bite against.
Low‑expansion foam (used carefully)
For large, hidden voids behind trim, professionals often use low‑expansion foam from the outside, then seal the inside neatly later. It’s easy to overdo from indoors and make a mess, so if in doubt, get a joiner or window fitter in.
Self‑adhesive draught tape for renters
If you can’t touch the paintwork, slim foam or rubber draught‑proofing tape around the inside of the frame and along opening sashes is a removable compromise. It won’t fix structural gaps, but it will soften minor leaks.

Whatever you do, don’t block drainage or permanent ventilation:

Those little slots or caps at the bottom of the outside frame are weep holes; they let rainwater escape. Never seal them.
Trickle vents at the top of windows are there for background ventilation. You can close the movable flap in cold snaps, but sealing them permanently risks condensation and mould.

Common mistakes that make rooms colder, not warmer

Sealing around windows sounds foolproof, but a few missteps can backfire.

Over‑sealing a damp room
Bathrooms, kitchens and bedrooms still need some fresh air. If you block every crack and never use extractor fans or trickle vents, moisture will look for the next cold surface – often behind wardrobes or in corners.
Using the wrong product
Standard silicone bathroom sealant doesn’t paint well and can peel from plaster. Look for “decorator’s caulk” or “frame sealant” that specifically says paintable.
Ignoring the sill and lower corners
Many people run a neat line up each side, then stop. The worst draughts usually creep in where frame meets sill and in the bottom corners.
Stopping halfway round
That “invisible ring” only works if it actually forms a ring. Sealing one side of the window and not the rest just diverts the draught to another crack.

If a window feels rotten, loose, or visibly out of square, no amount of caulk will make it truly snug. In those cases, professional repair or replacement is the warmer – and safer – option.

How this small job stacks with other low‑tech heat savers

Sealing the perimeter is not a silver bullet, but it’s one of those rare jobs where a tube of £3–£5 goop can upgrade an entire room. It also makes your other efforts work harder.

Area to tackle	Simple action	What it changes
Frame‑to‑wall gap	Continuous caulk ring	Kills cold draughts, boosts comfort
Opening sashes	Self‑adhesive foam strip	Stops whistling and rattling
Curtains/blinds	Close at dusk, use lined fabrics	Cuts radiant heat loss at night

Layered together, these tweaks often shave real money off winter bills. More importantly, they make the house feel habitable in the corners we usually avoid – the chair by the bay window, the desk near the patio doors.

You’re not upgrading to triple glazing. You’re simply letting your existing heating count.

FAQ:

Will sealing around my windows cause condensation or mould?
Done sensibly, no. You’re stopping uncontrolled draughts, not eliminating all ventilation. Keep trickle vents and extractor fans in kitchens and bathrooms working, and open windows briefly when cooking, drying clothes or after showers.

Is this worth doing if I already have double or triple glazing?
Yes. High‑performance glass doesn’t fix gaps between the frame and wall. Many “cold room” complaints in newer homes trace back to poorly sealed reveals rather than the glazing units themselves.

Can I do this in a rented property?
Usually, yes for reversible options like draught‑proofing strips. For permanent sealant along walls or frames, it’s best to ask your landlord first – especially if it means painting afterwards. Many are happy as it protects their building and can lower energy costs.

Will this make the room quieter as well as warmer?
Often. The same gaps that let in cold air also leak traffic noise and dust. A good continuous seal can noticeably soften outside sounds, particularly high‑pitched noise.

How long does the fix last?
Quality decorator’s caulk can last years if the frame isn’t moving around excessively. You might notice fine hairline cracks appear again over time; a quick touch‑up every few winters is usually enough to keep the seal effective.