Condensation between double glazing: the failed seal

You wipe the glass after a cold night and the smear is still there. The cloth slides over a dry room face, and the grey patch stays put, low in the pane or creeping in from a corner. That is condensation between double glazing: water in the cavity of the sealed unit, where a cloth cannot reach. Foam and mirror film leave it there. The edge seal has failed, so the useful next step is the guarantee and a glazier.

In short

Condensation between double glazing that you cannot wipe is water inside a failed sealed unit. Water on the room face comes from the room, and water on the outside can mean the outer pane is cold because the unit is insulating. A draught strip and a film roll leave the cavity wet, so replace the unit or claim on the guarantee, and treat a frame draught as a separate job.

Look at which surface is wet

The diagnosis is which surface holds the water. Stand close on a dry day and decide whether it is on the room face, on the outside, or locked between the two panes. Anglian, the Window Supply Company's condensation sheet from June 2023, and Which? all use that split. Products Compared read those notes on 15 October 2026, did not mist a unit in a lab, and is not quoting a replacement price, because the sources do not give one figure for every frame.

Run a dry cloth over the room face. If it comes away wet and the glass goes clear, the water was on your side. If the cloth stays dry and the grey patch remains, you are looking at the cavity. Dew that sits only on the outer face, and clears as the morning warms, is a third pattern. Mixing the three up is how foam ends up on a fault it cannot touch.

Secondary glazing is the limit. A pane fixed inside an older window makes its own gap, and the Window Supply Company sheet treats that as a different cavity you may wipe or ventilate. A factory unit has a spacer bar around the edge and comes out of the sash as one piece. Mist inside that unit has no opening a cloth can reach.

Cavity mist means the sealed unit has failed

A sealed unit is two panes, a spacer, and an edge seal meant to keep the cavity dry. When the seal leaks, moist air enters, the desiccant in the spacer is eventually overwhelmed, and you get a misted double glazed unit. Anglian says condensation between the panes means the seal has leaked and the cavity is no longer sealed: replace the unit and check the guarantee.

Which? says the seals of the unit have failed, and lists a draught at the frame as its own fault. Blown double glazing shows as a fog that stays when the room is warm.

A hair dryer can move the mist for an hour. It cannot remake the edge seal, which is why pieces that lead with the dryer, including Albany's, still end on a failed unit. Treat the clearer hour as proof the water is trapped. The Which? repairs guide notes firms that drill the mist out for around £45 plus VAT, and calls a clear result likely to be short-term, with the plug often loose again within six months. A new hole does not restore the edge seal. The lasting job is a new sealed unit in a sound frame.

Water on the room face comes from the room

Condensation on the inside of windows, on the face you can wipe, is moisture from the room on a pane colder than the air. Cooking, showers, clothes dried indoors, and a shut bedroom all add vapour. Which? links this to that moisture, and to homes sealed so tightly that vapour cannot leave. Covering every gap, including the vents, often leaves the glass wetter.

Use the extract fan while you cook or shower, and for a few minutes afterwards. Open a window briefly when the pane is streaming, and leave trickle vents open in winter. Approved Document F expects those background ventilators to stay usable. Energy Saving Trust says the same about draught-proofing: do not cover trickle vents.

If you wipe the room face dry and the mist is still between the panes, more ventilation will not empty the cavity. The bathroom may need the fan. The bedroom unit needs a glazier.

Water on the outside can mean the unit is working

External condensation on windows often means the unit is insulating. On a still, clear morning the outer pane stays cold because room heat is kept off it, and dew forms on the weather face. The Window Supply Company sheet from June 2023 describes that. The droplets usually go once the sun or a breeze warms the glass.

Outside dew is patchy, it sits on the outer surface, and it has often gone before you leave the house. Cavity mist is still there at noon, it looks the same from the garden and from the sofa, and a cloth on the room face does nothing.

The limit is a haze that seems to float behind the outer pane, or a patch that starts at a corner and spreads along the spacer. Outside dew does not explain that. If you can see both, note which is which before you phone.

Why tape and film leave the cavity wet

A stick-on seal fills a gap where the sash meets the frame, which is the job Energy Saving Trust describes. The coil never reaches the cavity, so it cannot dry condensation inside double glazing. The window seal comparison is for that frame gap only. The coil is a related buy, not a cure for the mist.

That top pick is the YANKUIRUI D-profile foam seal, 6 m, white: a self-adhesive EPDM D-profile, 9 mm by 6 mm, for a stated gap of about 3–6 mm, at about £4.49 on amazon.co.uk. Adhesive lifts on a dusty frame, and the same thickness can stop a tight door. A qualifying purchase through our amazon.co.uk links can earn us a commission at no extra cost to you.

A frame draught can sit beside cavity mist. Only the closing gap is a coil. A glazing-bead gasket under the bead is a fitter's part, because the bead has to come off. The foam or gasket guide separates those jobs. Neither replaces a failed unit.

Mirror film changes the view and leaves the water in place. Dark interior film is a poor match for ordinary double glazing you cannot name as toughened. Pilkington describes thermal stress as a hot centre and a cooler edge, and toughened glass resists that crack. A Zindoo listing warns against long sun on double glazing, and those rolls show no EN 410 result. Use the window solar film comparison as a warning, and the double glazing film note only when the glass is sound.

Who pays if the guarantee still runs

Start with the company that installed the windows. Anglian's note is to replace the sealed unit and check the guarantee. Find the invoice, name the room, and photograph the dry room face with the mist still visible, in daylight. Then write.

Which?, on your rights when buying double glazing, points to the Consumer Rights Act 2015: if the installation fails the contract, you can require repair or replacement in a reasonable time. The fitter chooses repair or replacement on cost, so say what you can see and set a date for a reply. If a subcontractor fitted the glass, keep the claim against the company you paid.

This page does not add a guarantee length the sources do not give. A neighbour's "ten years" is not your paperwork. If that paperwork has lapsed, a drill-and-plug visit is still a weak stand-in for a new edge seal.

Ask a glazier whether the frame can take a replacement unit. On many uPVC sashes the beads come off and the glass lifts out. The thickness has to match, safety glass is required where the opening needs it, and the unit has to sit so the new seal is not the next failure. If the sash whistles, measure the closing gap before you order a coil.