---
title: "Diagnosing a draughty window in ten minutes"
source: https://www.taim.io/home-repairs/diagnosing-a-draughty-window-in-ten-minutes
published: Sun May 10 2026 19:33:55 GMT+0000 (Coordinated Universal Time)
updated: Sun May 10 2026 19:34:58 GMT+0000 (Coordinated Universal Time)
description: "Most draughty windows come from three fixable problems. Use a simple candle test to find the leak, then match it to a cheap, targeted repair you can do yourself."
---

# Diagnosing a draughty window in ten minutes

Most “draughty windows” aren’t dying. They’re leaking in one or two very specific spots you can find in ten minutes with a candle and fix for a few pounds or dollars.

Most “draughty windows” aren’t dying. They’re leaking in one or two very specific spots you can find in ten minutes with a candle and fix for a few pounds or dollars.

## Key takeaways: fixing a draughty window without replacing it

- Most draughts come from three places: the sash seal, worn weatherstrip, or gaps where the frame meets the wall or sill.
- A slow candle or incense test will show you exactly where air is sneaking in so you can pick the right fix instead of guessing.
- Match your repair to the cause: silicone for frame joints, new weatherstrip for moving parts, and caulk/foam for wall gaps. Call a pro for rotten frames or failed glazing.

## Know your starting point (and whether this is your job)

Before you pick up a tube of sealant, work out what sort of job you’re standing in front of.

If the window opens and closes, isn’t rotten, and the glass is intact, this is usually safe DIY. You’re dealing with air, not structure. You shouldn’t be touching gas lines, electrics, or anything that needs building-control sign‑off.

Stand by the window on a cold, breezy day. Run the back of your hand slowly around the frame and along the wall joint. If you can feel a definite cold stream in one or two lines, you’re the right person to fix this.

If you see any of these, pause and reconsider:

- Soft, crumbling timber or flaking, blackened frame sections.
- Cracks wider than 5mm (3/16") between frame and wall.
- Moisture stains, mould, or blown plaster around the reveal.

That’s when you stop with the DIY sealant idea and consider a joiner, window specialist, or at least a landlord visit. You’re not just losing heat; the building might be taking on water.

## The ten-minute candle test: map the leak first

You can’t fix what you haven’t actually found. The candle (or incense) test is your quick survey.

You’ll need:

- A small candle in a stable holder **or** a stick of incense.
- A lighter or matches.

Turn off fans, close interior doors, and switch off any nearby HVAC vents. You want as little indoor air movement as possible.

1. Wait for a breezy or at least cool day. This works best with a real temperature difference between inside and out.
2. Close and latch the window as you normally would. Lock it if it has locks; that pulls the sash tight to the frame.
3. Hold the lit candle or incense about 2-3cm (1") from the frame, starting at the bottom corner.
4. Move slowly, about 2-3cm per second, along the joint where sash meets frame, then where frame meets wall and sill.

Watch the flame or smoke. You’re looking for **sudden sideways pulls, wild flickers, or spots where the flame nearly blows out**.

Use a pencil and lightly mark the frame or wall where movement is strongest. This is your leak map.

If the flame is dancing everywhere, not just by the window, the room has general draughts. Close doors to other rooms, wait a minute, and repeat. If you still can’t isolate movement to the window, switch to incense and watch where the smoke bends.

**Fire note:** Keep hair, curtains, and blinds well clear of the flame. Don’t do this with loose paper blinds or net curtains dangling near the test area. A jam jar over a tea light makes a safer, steady test candle.

## The three main causes of a draughty window

Almost every draughty window I’ve been called to look at has come down to one of three things.

  **1. Failed seal where sash meets frame**
  The moving part of the window (sash) no longer presses firmly against a seal. The strip is flattened, missing, or never installed properly. Air comes in all along that line when the wind hits it.
  **2. Gaps where frame meets wall or sill**
  The window unit itself is fine, but the join between frame and wall or sill has a gap. Old caulk has cracked, or the original installer stuffed in a bit of foam and walked away. You’ll see or feel this on the outside edge, inside edge, or both.
  **3. Failed seal at frame joints and beads**
  On timber, aluminium, or uPVC, the corners and glazing beads (the strips that hold the glass in) sometimes lose their seal. Tiny cracks open up, especially along the bottom glazing bead, and wind forces air through.

Very occasionally, the **glass unit itself** is the problem: blown double glazing or a cracked single pane. That usually shows as condensation between panes or an obvious crack, and it’s more a replacement job than a bead of caulk.

Your candle map will tell you which of these you’re dealing with. Continuous flicker along a moving edge points to weatherstrip. A sharp local pull where the frame meets plaster screams wall gap. A small but persistent leak right along a glazing bead suggests a bead or frame‑joint seal.

## Cause 1: Failed seal between sash and frame (weatherstripping)

If the candle danced along the whole length where the moving sash meets the frame, your weatherstrip (or built‑in seal) is done.

This is common on 10-30‑year‑old windows. The rubber compresses and stays flat, or old felt strips turn to fluff.

For **side-hung casement or tilt‑and‑turn windows**, look for a rubber gasket running around the sash. If it looks cracked, shiny‑flat, or has gaps at the corners, it’s not sealing.

For **sliding sash windows**, there may be brush strips in the channels or nothing at all. A missing or worn brush gives you that classic rattly, cold sash.

> Most people try to “caulk” everything they can see, then wonder why the window still leaks and now barely opens. Don’t glue your window shut. Replace the weatherstrip on the moving edge first. Caulk is for fixed joints; strip is for parts that move. Respect that line and you’ll fix the draught without creating a new problem.

### First fix attempt: add or replace weatherstrip

You don’t need a brand name. You need the **right profile and compression**:

- Measure the gap: close a strip of Blu‑Tack or soft putty in the window, then measure its squashed thickness. That’s your approximate seal gap.
- Buy self‑adhesive foam or rubber weatherstrip that compresses from that thickness down by about 30-50%. For most domestic windows that’s 3-6mm (1/8-1/4").

Clean the frame where the strip will stick with mild detergent and let it dry. Degrease with a bit of methylated spirit or rubbing alcohol if you have it.

Peel and stick the strip along the frame side where the sash closes, not on the sash itself if you can avoid it. Run one continuous length per side, pushing firmly as you go. Avoid stretching it; stretched foam shrinks back and leaves gaps.

Close the window gently. If you have to lean on it or the handle feels like it’s fighting you hard, the strip is too thick. Open back up and try a thinner profile.

**Good feedback:** the window still closes and latches cleanly, the handle isn’t straining, and the candle test now shows only a slight wobble or none along that edge.

**Bad feedback:** the sash won’t close fully, or you have to force the lock. Don’t muscle it; you’ll bend hinges or strip gearboxes. Peel back, drop down a thickness, or reduce the length in tight corners and retry.

## Cause 2: Gaps where frame meets wall or sill

If your candle or smoke pulls sideways **right where the frame meets plaster, trim, or sill**, the problem isn’t the window unit. It’s the join to the building.

This is common in older masonry buildings and in rushed modern installs. The original installer may have used expanding foam and never sealed over it, or interior caulk simply cracked over time.

You’ll often see a visible line or hairline crack along the reveal, especially under paint.

### Interior fix: caulk or silicone bead

For small gaps up to about 5mm (3/16"), a flexible decorator’s caulk (water‑based) or acrylic sealant is usually enough inside. Around timber sills or where there’s risk of the joint getting wet, a paintable **low‑modulus** silicone is better.

Tape off the joint with masking tape if you want a clean line. Cut the tube nozzle at about a 45° angle with an opening just bigger than the widest part of the crack. Run a steady bead, then tool it smooth with a wet finger or a proper profiling tool.

Don’t fill over rotten or soft timber; that just hides rot. If the frame feels spongy, get a joiner to look at it.

### Exterior fix: caulk or low‑expansion foam

If you can safely get to the outside and see daylight between frame and wall, that’s where the real air path lives. For gaps up to about 10-15mm (3/8-5/8"), use an exterior‑grade window and door sealant rated for movement and UV.

For bigger voids, you may need **low‑expansion window and door foam** first, then a sealant over the top once cured. Standard foam expands too much and can bow the frame. Always follow the can’s instructions and don’t overfill.

As a renter, stick to neat interior caulk and leave exterior foam to the building owner; they’re responsible for the weather side.

**Good feedback:** on your next candle test, the flame calms down right where you sealed, and you no longer feel a direct cold line along the wall joint.

**Bad feedback:** the flame still kicks hard at the same spot. Look for a hidden path, under the sill board, behind trim, or on the exterior side you couldn’t see before. Sometimes the real gap is outside, and the inside crack is just the symptom.

## Cause 3: Frame joints and glazing beads leaking

If your leak lines up with a **corner joint** of the frame or runs along a **glazing bead**, you’re dealing with smaller but persistent gaps.

On timber frames, joints can open a hair as the wood moves. On uPVC or aluminium, old sealant along the beads can crack and let air and sometimes water through.

Run your fingernail or a blunt tool gently along the bead or corner. If you can feel a groove or see a crack, that’s your path. The candle will usually flare briefly at one spot rather than along the whole side.

### Fix: targeted bead of silicone or sealant

Use a neutral‑cure silicone that’s compatible with your frame material. Check the tube; most will list timber, uPVC, or aluminium explicitly.

Clean the area well and dry it. Any moisture trapped under fresh sealant is asking for mould.

Cut a fine nozzle tip and run a small, continuous bead right into the crack. Tool it in so it bonds to both sides; a proud bead that only touches one side is decoration, not sealing.

Avoid gunking over existing weep holes or drainage slots at the bottom of the frame. Those are there to let water out; blocking them can cause worse problems than a bit of air movement.

**Good feedback:** the next time you test, that small hotspot is gone or much reduced, but the window still drains freely if you pour a little water along the outside bead.

**Bad feedback:** air movement is unchanged, or you’ve now trapped moisture (condensation or fogging around that bead). If in doubt, scrape back what you did and keep the weep paths clear.

## Very rare: when the glazing itself is the problem

If the candle never shows much at the frame edges, but you feel cold standing right next to the glass, the glass might simply be **poor at insulation**, not leaking air.

Old single glazing and failed double‑glazed units feel brutally cold in winter. You may see condensation between panes or a milky, fogged look if a sealed unit has blown.

You won’t usually feel a sharp “jet” of air from the middle of the glass. Cold radiates instead of flows. That’s the difference between bad insulation and a draught.

DIY fixes here are about **reducing heat loss**, not sealing air paths:

- Clip‑on or magnetic secondary glazing kits.
- Seasonal shrink‑film kits you heat with a hairdryer.
- Heavy, well‑fitted curtains.

Actual glass replacement is specialist work. In most jurisdictions, replacing a whole window unit or messing with fire‑rated glazing crosses into building‑regulation territory. In the UK, that’s Part L guidance; in many US states, energy‑code compliance hangs off the window spec.

If your candle map shows no real air leaks but the room is still cold, focus on these upgrades or talk to a pro about better glazing rather than chasing non‑existent gaps.

## First real attempt: fix one window, then re-test

Don’t try to do the whole house in one heroic weekend. You’ll rush the diagnostics and waste materials.

Pick the **one worst window** from your candle test, the one with the strongest flicker or the one you sit by and shiver.

1. Mark the leak points with pencil from your candle map.
2. Decide which cause they correspond to: sash seal, wall gap, or frame bead.
3. Buy the minimum materials to fix just that cause on just that window.
4. Do the repair slowly and cleanly. Cut back old, loose sealant instead of piling new on top.
5. Let any sealant cure as per the tube (usually 2-24 hours) with the window closed as it will normally sit.

Now re‑run the candle test in exactly the same way.

**Good signals:** the strong flickers are gone or much weaker, the room feels less “edgy” cold within a day, and you’re not seeing new issues like sticking handles.

**Poor signals:** the test still shows big movement in the same place, or you’ve simply moved the draught along a bit. That means the real gap is either longer than you thought (replace more weatherstrip) or is actually in a connected path (often outside at the frame‑to‑wall joint).

Adjust and retry:

- If your weatherstrip helped but didn’t cure it, run a continuous strip around the full sash instead of patching just the worst area.
- If caulk stopped one crack but not another, chase the joint a bit further, especially under trim or along the sill where hairline gaps hide.

Ten extra minutes on this second pass usually beats three more tubes of anything.

## When to call a pro (or your landlord)

Some window jobs cross the line from “half an hour and a tube of silicone” into “someone needs to take responsibility for this”. Know where that line is.

Call a pro or your landlord when:

- The frame is rotten, soft, or visibly deformed.
- Cracks between frame and wall are wide enough to see daylight easily or change size when you push the frame.
- There are signs of water ingress: stained plaster, bubbling paint, mould growth around the reveal.
- The glass is cracked or a sealed unit has clearly failed and you want it replaced, not just taped.

In many rentals, you’re allowed to apply **temporary, reversible** draught‑proofing (foam strips, shrink‑film, removable caulk) but not to alter frames or exterior faces. Check your tenancy agreement.

If you’re in doubt, document the draught with a short video of the candle test and send it with a clear note. It’s much harder for a landlord or property manager to ignore visible air movement than a vague “it’s a bit cold”.

If you’re in a flat with possible structural movement or exterior cladding issues, don’t start digging out exterior seals yourself. That’s beyond DIY; you want the managing agent and their chosen contractor on record.

## Tools, materials, and realistic costs

You don’t need a van full of gear to fix most window draughts. A simple kit covers almost all of it.

Here’s a realistic view of what you’ll use and roughly what it costs in 2026:

Item
Use
Typical price (UK)
Typical price (US/EU)

Self‑adhesive foam/rubber weatherstrip (5-10m roll)
Replacing sash seals
£3–£10
$4–$15 / €4–€15

Decorator’s caulk / acrylic sealant (310ml)
Interior frame‑to‑wall gaps
£2–£5
$3–$7 / €3–€7

Exterior window & door silicone (310ml)
Exterior frame joints, beads
£4–£10
$5–$15 / €5–€15

Low‑expansion window & door foam
Larger frame‑to‑wall voids
£6–£12
$8–$18 / €8–€18

Basic caulk gun
Applying sealant
£4–£10
$5–$12 / €5–€12

Utility knife / craft knife
Cutting strip, trimming sealant
£3–£8
$4–$10 / €4–€10

Masking tape
Clean sealant lines
£2–£4
$3–$5 / €3–€5

Tea light + jar or incense sticks
Draught diagnosis
£1–£3
$1–$4 / €1–€4

Add a cloth, mild cleaner, and a bit of patience and you’re in business.

Most single‑window fixes come in under £10–£20 / $15–$25 / €15–€25 in materials. A full professional sash service or serious repair can easily run £60–£150+ / $80–$200+, which is sometimes worth it – but only **after** you’ve confirmed the problem isn’t just a £5 strip or bead.

### Cheatsheet: diagnosing and fixing a draughty window fast

#### Three-cause diagnostic at a glance

Most draughts are one of three things:

1. **Sash-to-frame seal**: Candle flickers along the moving edge all the way or in long runs. Fix with new weatherstrip.
2. **Frame-to-wall/sill gap**: Candle pulls hard exactly where frame meets plaster, trim, or sill. Fix with interior caulk, exterior sealant, or low-expansion foam + sealant.
3. **Frame joints / glazing beads**: Small, localised flickers at corners or along the bead that holds glass in. Fix with targeted neutral-cure silicone, keeping weep holes clear.

#### ️ Candle / incense test quick steps

1. Pick a breezy or cold day. Close and latch the window fully.
2. Turn off fans and HVAC nearby; close interior doors to calm indoor air.
3. Light a small candle in a jar **or** an incense stick. Hold 2-3cm (1") from the frame.
4. Move slowly around the sash-to-frame joint, then frame-to-wall and sill joints, 2-3cm per second.
5. Mark any spots where the flame or smoke kicks sideways sharply, flares, or nearly blows out.
6. Use those marks to decide the cause and pick a specific fix instead of guessing or sealing everything.

#### Fix-by-cause quick table

Candle result
Likely cause
DIY fix

Continuous flicker along opening edge
Worn/missing weatherstrip
Measure gap, fit correct-thickness self-adhesive foam or rubber strip along frame; retest.

Strong pull exactly at frame–wall or frame–sill line
Gap at frame–wall joint
Interior: decorator’s caulk or acrylic. Exterior: window & door silicone; use low-expansion foam for big voids before sealing.

Small hotspot at corner or along bead
Frame joint / glazing bead leak
Scrape loose sealant; apply neat bead of neutral-cure silicone. Keep drainage/weep holes open.

No real edge flicker but room feels cold
Poor glazing / general heat loss
Secondary glazing, shrink film, heavy curtains. Glass replacement or spec upgrade is a pro job.

#### Tools & prices pocket list

For one typical window:

- Weatherstrip roll (5-10m, 3-6mm thick): £3–£10 / $4–$15 / €4–€15.
- Decorator’s caulk (interior) + exterior silicone: £6–£15 total / $8–$22 / €8–€22.
- Low-expansion window foam (only if big wall gaps): £6–£12 / $8–$18 / €8–€18.
- Basic caulk gun: £4–£10 / $5–$12 / €5–€12.
- Knife, masking tape, cloth, cleaner: £5–£10 / $7–$15 / €7–€15. Budget £10–£25 per problem window for materials. Compare that to £80–£150+ for a pro visit so you know when DIY is worth a try first.

### FAQ: common questions about fixing a draughty window

#### Is it the window or the wall causing the draught?

Use your candle map. If the flame kicks right where the sash meets the frame, it’s the window’s own seal or weatherstrip that’s failing. If the flame or smoke bends sharply **where the frame meets plaster, trim, or sill**, it’s the wall joint leaking rather than the window unit. A third pattern is small hotspots at corners or beads, which points to frame joints. Treat those three cases differently: strip for sash edges, caulk/foam for wall joints, and bead sealant for frame corners. If everything around the window is calm but the room is still freezing, the wall insulation or overall glazing spec may be the culprit rather than any specific gap.

#### Will weather strip really fix a 30-year-old window?

Often, yes, if the frame and hardware are basically sound. On an older timber or uPVC window, the original seals get tired long before the frame is structurally done. Fresh, correctly sized weatherstrip can make a 30‑year‑old sash feel surprisingly tight again. What it won’t do is fix rot, warped frames, or worn hinges that stop the sash closing square. If you can close and latch the window without forcing it, adding new seals is usually worth a shot. If you have to lift, shove, or slam to get it shut, deal with the alignment or hardware first, or call someone who does sash services.

#### ️ Can I do this in winter, or is it too cold?

You can absolutely diagnose and seal in winter; in some ways it’s the best time because the temperature difference makes leaks obvious. The only real constraint is **sealant curing**: most acrylics and silicones have a minimum application temperature, typically around 5°C–10°C (40°F–50°F), so check the tube. If it’s colder, work indoors, close the window as soon as a skin forms, and expect slower curing. Weatherstrip has no such issue; it sticks fine in the cold as long as the surface is clean and dry. Have a jumper ready, move efficiently, and you’ll warm the room up afterwards with less heat loss than before.

#### Do I need to remove the window sash to fix draughts?

For basic draught‑proofing, no. Most weatherstripping and interior caulking can be done with the sash in place and simply opened and closed as needed. Removing sashes is a more advanced job and easy to get wrong, especially on modern tilt‑and‑turn or spring‑balanced sash windows. If you can access the full perimeter of the frame and sash with the window open, that’s enough for nearly all the fixes in this guide. The main time you’d remove a sash is for serious hardware work, repainting, or full refurbishment. At that point, you’re usually into joiner or window‑specialist territory. Don’t strip out balances or hinges just to stick foam on; work around what’s installed.

#### What if I’m renting, what’s safe to do without upsetting my landlord?

Most landlords are fine with reversible draught‑proofing that makes the place more liveable and doesn’t damage finishes. Foam or rubber weatherstrip on the sash, shrink‑film on the glass, and neat interior decorator’s caulk in hairline gaps are usually low‑risk. Big changes, like foaming exterior gaps, removing trim, or altering drainage holes, should be left to them or done with written permission. A good approach is to film a quick candle test, send it over with a short explanation, and offer to apply simple strip or film yourself if they don’t want to call a contractor. That way you’re clearly improving their asset, not bodging it behind their back.

### Wrapping up: one window at a time, not a full replacement

You don’t need to rip out all your windows to stop a cold draft sneaking across the room. You do need to stop guessing and actually see **where** the air is coming in.

A ten‑minute candle or incense test gives you a map: sash seals, wall gaps, or frame joints. Once you know which you’ve got, the fix is targeted and cheap – a roll of strip, a tube of caulk, maybe a careful bead of silicone.

If your first attempt only half‑works, that’s not a failure; it’s feedback. Adjust the thickness of the strip, chase the crack a bit further, or accept that you’ve hit the limits of DIY and call a pro.

Most people underestimate how much comfort you can buy with a £10–£20 spend and an hour’s attention to detail. Start with one bad window, get it properly tight, and then decide which one deserves your next tube of sealant.

### Next steps: from one draught to a warmer room

- Pick your coldest window and run the candle or incense test exactly as described, marking leak points with a pencil.
- Decide which of the three main causes your marks line up with, then buy only the materials for that one cause on that one window.
- Install the weatherstrip or sealant slowly and cleanly, then re‑test with the candle to confirm the draught has actually gone or reduced.
- If the fix works, repeat the process on the next‑worst window; if not, reassess whether the leak is actually at the wall joint or frame bead instead.
- Document any frames that are rotten, badly deformed, or leaking around blown glazing units, and plan a conversation with your landlord or a window specialist.