Clouded Glass, Sound Frame: The Case for Sealed-Unit-Only Replacement

A misted double-glazed window is often treated as a cue to replace the whole thing. In many homes that is an unnecessarily large piece of work. The frame may be perfectly sound; the fault is confined to the sealed unit — the factory-made glass cassette inside the frame. Replacing only that unit can restore clarity and reduce heat loss without disturbing brickwork, plaster or decoration. This guide explains how to tell a failed unit from a frame or ventilation problem, what to specify, and what the work should cost in 2026.

What has actually failed inside a misted unit

A sealed unit is not a single sheet of glass. It comprises two or more panes separated by a spacer bar, with primary and secondary seals around the perimeter and a desiccant — a drying agent — inside the spacer. The cavity is filled with air or a gas such as argon. When the edge seal is breached, moisture-laden air enters. The desiccant absorbs water until it is saturated; after that, vapour condenses on the cooler pane. The result is fogging that cannot be wiped away from either side.

Two related phenomena are often confused with this. Surface condensation on the room-side pane points to indoor humidity and poor ventilation, not a failed unit. External condensation on the outside pane can occur on well-insulated glazing in spring and autumn and is usually normal. The test is simple: mist between the panes means the unit has failed; mist on a surface you can touch means it probably has not.

Diagnosing the fault: unit, frame or ventilation

Before accepting a quote for new windows, make three checks.

First, confirm the mist is internal. A unit that clears in warm sunshine and fogs again on a cold evening is showing classic seal failure.

Second, inspect the frame. Timber can rot at the sill or around the rebate; uPVC can crack, distort or lose its gaskets. Look for water staining, blocked drainage slots and draughts around opening sashes. If the frame is sound, square and watertight, reglazing is usually viable. If it is soft, warped or poorly drained, a new window may be the better long-term investment.

Third, rule out ventilation. Condensation on the room side, mould in corners and lingering humidity after cooking or showering point to inadequate extract or trickle ventilation. New glass will not solve that. In bay windows, check that the structure is sound and measure each unit individually, since sizes often vary.

When reglazing is the sensible route — and when it is not

Reglazing suits a frame that is structurally sound, where the failure is confined to one or more units, and where the windows have character worth retaining — Georgian bars, leaded lights, an unusual colour or a shape that would be costly to reproduce. It is also the lower-cost option when work is being phased and the budget is better spent on the units that have actually failed.

Full replacement deserves consideration when frames are rotten, warped or thermally poor — early aluminium frames without a thermal break, for example. It is also sensible if the hardware is obsolete, the configuration needs to change, or the existing frame is single-glazed and the whole assembly is being upgraded. Reglazing will not improve the thermal performance of the frame, so a sound but inefficient frame will still limit the whole-window U-value.

Specifying the replacement unit

Several details determine whether a new unit performs well and looks right.

Size and thickness come first. The new unit must fit the existing rebate and glazing beads. Many modern frames take a 28mm unit, but older frames may require 20mm or 24mm; slim units with krypton fill are available where depth is tight. Measure the rebate, not just the visible glass.

For thermal performance, ask for the declared centre-pane U-value (Ug). Around 1.0 to 1.1 W/m²K or better is a reasonable target for a modern double-glazed unit. Where the work falls within the scope of Part L of the Building Regulations in England and Wales, the whole window should not fall below the current standard of 1.2 W/m²K. Your installer should confirm the specification in writing.

Choose a warm-edge spacer rather than a traditional aluminium spacer; the warm-edge design reduces thermal bridging at the perimeter and therefore edge condensation. Specify an argon fill for standard cavities, and krypton where the cavity is narrow. Low-emissivity coating is now standard, but confirm it is on the correct face.

Match the appearance carefully. Obscure glass patterns, Georgian bars (internal or external), leaded lights and sightline dimensions all affect the look of the window. Supply a photograph and, where possible, a sample. If the glazing beads are being replaced, match the colour and profile. Safety glazing must be maintained: toughened or laminated glass is required in critical locations such as doors, side panels and low-level glazing under Part N.

Guarantees and paperwork on glass-only work

A sealed unit is typically guaranteed for five to ten years, with workmanship covered for one to two years. Ask for an insurance-backed guarantee where available, and keep the unit specification and any labels. Check the original window warranty before proceeding: some manufacturers state that glazing work by a third party voids the frame guarantee, so obtain written confirmation if that is a concern.

Replacing sealed units is generally not notifiable under the Building Regulations in England and Wales, but the glazing must still comply with safety and thermal requirements. FENSA does not cover glazing-only work, because it certifies the replacement of complete windows and doors. Ask your installer for a written declaration that the units meet the relevant requirements.

Practical pitfalls: older frames, bays and conservation areas

Older timber frames may have putty fronts rather than modern beads, and the rebate may be shallower than expected. The installer should allow for back-bedding, drainage and ventilation of the cavity. On bay windows, access is often the largest variable: scaffold or a tower may be needed, and each unit is bespoke. Allow for the extra labour.

In conservation areas and listed buildings, like-for-like replacement is often the most straightforward route, but consent requirements vary. Slim units, coloured spacer bars and traditional glazing bar layouts can help match the original appearance. Check with the local conservation officer before ordering, and use a specialist leaded-light contractor if the window has decorative lead.

What it costs in 2026

Prices vary by region, specification and access, but these ranges are realistic for 2026. A standard sealed unit of around 1,200mm by 900mm, 28mm thick, with low-e glass, argon fill and a warm-edge spacer, typically costs £90 to £170 supplied. Fitting is commonly £45 to £85 per unit, with a minimum call-out charge on small jobs. Toughened glass, obscure patterns, Georgian bars or leaded lights add roughly £30 to £100 per unit.

A bay window with four to six units might cost £600 to £1,100 installed. By comparison, a complete replacement window is commonly £600 to £1,200 or more per window, installed, depending on size and frame material. Access equipment can add £150 to £400 where required. These are estimates, not quotations; obtain three itemised quotes and confirm what is included.

A decision framework

Ask five questions. Is the mist between the panes? Is the frame sound and square? Will a new unit fit the existing rebate and beads? Does the specification meet safety and thermal requirements? And what is the position on guarantees? If the first two answers are yes and the others can be resolved, reglazing is likely to be the sensible, lower-cost answer. It is a repair, not a compromise, when the frame still has plenty of life left in it.