Misted or Foggy Double Glazing? You Can Replace Just the Glass

If your double glazing has turned cloudy, foggy or picked up a permanent smear of condensation between the panes, the good news is that nothing is wrong with your window frame. The sealed glass unit inside it has failed, and in most cases you can replace just the glass, cut to your exact size, for a fraction of the cost of a whole new window.

This guide explains why units mist up, how to tell whether you need new glass or a new window, and how to order a made-to-measure replacement unit online.

Why double glazing mists up

A double glazed unit is two panes of glass separated by a spacer bar and sealed around the edge, with the cavity filled with dry air or argon gas. That sealed cavity is what does the insulating. Over years of heating and cooling, the perimeter seal can eventually break down. Once it does, moist outside air works its way into the cavity, and when it cools it condenses on the inside of the glass. That is the misting you can see, and no amount of cleaning will shift it because it is trapped inside the unit.

It is worth being clear about what this means: the glass itself is fine, and so is your frame. It is only the sealed unit that has reached the end of its life, which is exactly the part you can swap out.

Do you need a new window, or just new glass?

Nine times out of ten, a misted unit only needs new glass. If the frame still opens, closes and locks properly and is not rotten or bowed, replacing the sealed unit restores the window completely. You keep the frame, the handles and the fit, and you avoid the disruption and cost of a full window replacement.

A new window makes more sense when the frame itself is damaged, draughty or you want to change the style or colour. If that is the situation, it is worth looking at a full made-to-measure aluminium window instead. But if it is only the glass that has gone, a replacement glass unit cut to size is the quicker, cheaper fix.

How to tell your unit has failed

• Cloudiness, haze or a milky film that stays even after cleaning both sides.
• Water droplets or condensation that appear between the panes, not on the room-side surface.
• A visible tide mark or streaking inside the glass.
• Rooms feeling colder near that window, as the failed cavity loses its insulating value.

Condensation on the inside face of the glass, in the room, is a different thing entirely. That is usually just humidity in the room and does not mean the unit has failed.

Measuring and ordering a replacement unit

Ordering a replacement is straightforward once you have three numbers: the width, the height and the overall thickness of the existing unit. Most modern units are 28mm thick overall, made up of two 6mm panes and a 16mm cavity, but it is always worth checking. Measure the visible glass and add on the amount hidden behind the frame rebate so the new unit sits correctly.

Our step-by-step measuring and ordering guide walks through exactly where to measure from and the small deduction to allow for a snug fit. Once you have your sizes, you enter them straight on the product page and the price updates instantly.

Which replacement spec should you choose?

You have a choice to either match what you had, or upgrade while the glass is out anyway:

• 6mm Clear / 6mm Clear is the standard clear double glazed unit, a like-for-like match for older glazing (U-value around 2.7).
• 6mm Clear / 6mm Low-E is the upgrade most people take. The Low-E coating pushes the U-value down to around 1.0 and reflects roughly half the heat back into the room, while still letting about 67% of the light through. If you are paying to have the glass replaced, this is the version that starts saving you money.
• Satin Privacy / Low-E is ideal for a bathroom, en-suite or overlooked window where you want obscurity without losing brightness.

If the window is somewhere you really feel the cold, or you want the best acoustic and thermal performance, it can also be worth stepping up to a triple glazed unit while the frame is open.

What replacement glass costs

Replacement units are priced by the square metre and cut to your exact size, so you only pay for the glass you need. That is why replacing a failed unit almost always comes in well below the cost of a whole new window. The quickest way to get an accurate figure is to pop your width and height into the relevant glass unit page and read the live price, including VAT, before you order.

Get your replacement unit priced

Measure the failed unit, choose your spec, and enter the size to see the price. Every unit is made to measure and delivered across the UK mainland. Start with the double glazing range, or browse all cut-to-size glass if you are after triple, privacy or solar control.

Frequently asked questions

Can misted double glazing be fixed without replacing the glass?
Not reliably. "Defogging" services drill and dry the cavity, but the failed seal remains, so misting usually returns. Replacing the sealed unit is the proper, lasting fix.

Can I replace just one pane of a double glazed unit?
No. A sealed unit is a single factory-made component, so the whole unit is replaced. The frame and everything else stays.

How long do replacement units last?
A quality sealed unit with a warm-edge spacer typically lasts many years. Ordering the right size for a proper fit in the rebate helps the seal last.

Do I need the same thickness as my old unit?
Yes, match the overall thickness (often 28mm) so the new unit fits the frame. Measure the old one or check the spacer bar.