What Is a Blown Double Glazed Unit and How Do You Fix It?

If your double glazed windows have gone misty, hazy, or are showing a white smear between the panes that no amount of cleaning will shift, you are dealing with what glaziers call a blown unit. It is one of the most common window problems in UK homes, and the good news is that it is much simpler and cheaper to fix than most people expect.

What does "blown" actually mean?

A double glazed unit is made up of two panes of glass sealed together with a gap between them. That gap is filled with air or an insulating gas such as argon, which is what gives the window its thermal performance. Around the edge of the unit sits a seal, and when that seal fails, the insulation is lost. Warm, moist air from your home creeps in between the panes, condenses, and leaves a cloudy deposit you cannot reach to wipe away. That is a blown unit.

The frame around it, whether uPVC, timber, or aluminium, is almost always completely fine. The problem is the glass unit itself, not the window.

Is a blown unit just cosmetic, or does it affect performance?

Both, unfortunately. The visual haziness is annoying, but the bigger issue is that a failed seal means the insulating gas has escaped. Your window is no longer performing the way it should, which means more heat is getting out and your heating is working harder to compensate. Over time, blown units contribute to higher energy bills and draughtier rooms, particularly noticeable in older properties with several failed panes.

How do you fix a blown double glazed unit?

The fix is a glass-only replacement. Rather than replacing the entire window, a glazier removes the failed sealed unit from inside the existing frame and fits a brand new one. The frame stays exactly where it is. There is no building work, no mess, and because only the glass is being changed, no FENSA certificate is required.

At The Glass Doctor, we manufacture our own A+ rated sealed units in our North Wales factory. Every unit is made to measure, so it fits your existing frame perfectly. Most replacements are completed within a few days of your call, and all our work comes with a 10-year guarantee as standard.

What will it cost?

Less than you might think. Glass-only replacement is a fraction of the cost of a full window installation, and with over 20 years of experience and in-house manufacturing, we keep our prices sharp. We also back every quote with our Price Match Promise: bring us any written quote from another glazier and we will match it. Free fitting is included on every job, and your old unit is recycled rather than going to landfill.

We offer free, no-obligation home visits across North Wales and the North West, so you will have an exact price before committing to anything.

If your windows have gone cloudy, do not put it off. Visit theglassdoctor.org or call us on 0800 7747684 for a free quote today.