Misted Double Glazing Repair London: Causes and Solutions

Misted double glazing makes windows look cloudy or foggy and the effect cannot be fixed by cleaning. If condensation appears between the panes and cannot be wiped away from either surface, the sealed glass unit has failed — and there is a straightforward solution that does not require replacing the whole window.

What This Guide Covers

• Misting between the panes is a sealed unit failure — the edge seal has broken down and moisture has entered
• The frame is usually unaffected — only the glass unit needs replacing in most cases
• Sealed unit replacement costs a fraction of full window replacement
• The replacement unit must match the existing dimensions, thickness, and glass specification
• Surface condensation on the room-facing pane is a different problem caused by humidity and ventilation

Introduction

London homeowners face unique challenges when it comes to glazing — from period properties with sash windows in conservation areas, to modern developments requiring structural glass installations. Getting the right advice before you commit can save you significant time and money.

At BuilderFix Glazing, we have completed thousands of glazing jobs across Greater London and Surrey — from emergency repairs in Kensington and Chelsea, to bespoke walk-on glass installations in Mayfair and Belgravia. This guide is based on real experience and the questions our customers ask us most often.

In Depth

The terminology around misted double glazing confuses a lot of homeowners. A double-glazed window consists of a sealed unit — two panes of glass with a gap between them — sitting inside a frame. The sealed unit is manufactured separately from the frame and can be removed and replaced independently. When the edge seal around the glass unit fails, the hermetic seal is broken and atmospheric moisture enters the gap. As the window heats and cools through the day, this moisture condenses on the inner surfaces of the glass, creating the fogged or milky appearance that cannot be wiped away.

This is a glass unit failure, not a frame failure. The frame — the uPVC, aluminium or timber surround with its hinges, handles and locks — has nothing to do with the seal failing. In the vast majority of misted window calls we attend in London, the frame is in perfectly serviceable condition. Replacing the entire window because of a failed sealed unit is the equivalent of replacing a car because of a flat tyre.

The sealed unit replacement process starts with measurement. The new unit must match the existing dimensions precisely — width, height, and crucially the thickness of the unit (typically 24mm, 28mm, or 36mm for more recent high-performance units). It must also match the glass specification: standard float glass, low-emissivity coated glass (which affects the order in which the coating faces inward), toughened glass in any location covered by Building Regulations Part N (low-level panes, side panels, doors), or laminated glass for overhead glazing. Fitting the wrong specification creates a compliance problem and potentially a safety issue.

Ordering and fitting the replacement typically takes one to two visits. In many cases we can measure the unit on a first visit and return with the glass once it has been manufactured — standard sizes are often available within 24–48 hours. Non-standard dimensions or specialist glass specifications may take longer. The fitting itself takes under an hour for a standard casement or sash unit.

Surface condensation on the room-facing surface of the glass is not the same problem and should not be confused with a sealed unit failure. Condensation forming on the interior surface in the morning — particularly in kitchens, bathrooms and bedrooms — is caused by warm, humid room air contacting the cold glass surface. The remedy is ventilation and controlled humidity, not new glass. A simple test: if the moisture wipes away from inside the room, the sealed unit is intact.

What Does This Typically Cost in London?

Costs vary depending on the type and size of work, the glass specification, frame material, and access. As a general guide, always obtain a free written quote before committing — this protects you from unexpected charges.

At BuilderFix Glazing, we provide free, no-obligation written quotes for all work. Our prices are transparent and fixed before we start — no surprises on the invoice.

What to Ask When Getting a Quote

• Is the price fixed, or could it change once work starts?
• What glass specification is included?
• Are all fixings, sealants and finishing included?
• What compliance documentation will I receive?
• What happens if there is an issue after completion?