Why Double Glazing 'Blows'
Double glazing works because the gap between the panes contains air or an insulating gas (typically argon) that slows the transfer of heat through the glass. The sealed unit — the two panes and the spacer bar between them — is designed to keep that gas in and moisture out for the life of the window.
When the seal fails, moisture enters the cavity and the insulating gas can escape. The misting you see is condensation forming inside the unit — moisture that got in through the failed seal and has nowhere to go.
The energy efficiency impact: thermal performance reduces, heat passes through the glass more easily, internal glass surfaces become colder, and the risk of condensation on the room-side surface increases. The window is still better than single glazing, but it's no longer performing as designed.
Do You Need a New Window or Just a New Sealed Unit?
This is where I see homeowners spend far more than they need to. Many assume that blown double glazing means the whole window needs replacing — frame, hardware, and all. In most cases, that's not true.
If the frame is in good condition — no rot, no warping, no failed hinges or handles — replacing just the sealed unit (the glass) is usually the right answer. A replacement sealed unit typically costs £80–£200 per pane including fitting, depending on size and glass specification. A full window replacement for the same opening typically costs £400–£900.
The sealed unit is a standard product. A glazier can measure the existing unit, order a replacement to the same specification, and fit it in an hour. The frame stays. The hardware stays. You get a new, properly sealed unit at a fraction of the cost of a full window.
Sealed Unit vs Full Window — Typical Costs
| Option | Typical cost | Notes |
|---|---|---|
| Replacement sealed unit only | £80–£200 per pane | Frame must be in good condition; glazier can usually fit same-day or next-day |
| Full window replacement (standard casement) | £400–£900 per window | Includes frame, hardware, and fitting; necessary if frame is rotten or warped |
| Full window replacement (bay or large) | £800–£2,000+ | Larger openings, more complex fitting; always get three quotes |
Cases Where Full Replacement Makes Sense
What Blown Glazing Does to Your EPC Rating
On an EPC assessment, window condition and specification are assessed. Blown double glazing is noted and affects the window's assessed performance. Replacing blown units with new sealed units of the same specification will restore the window's EPC contribution — you don't need to upgrade to triple glazing to recover the lost performance.
If you're selling or letting a property and the EPC rating is borderline, fixing blown glazing is one of the more cost-effective improvements available — particularly if multiple windows are affected.
Get a Glazier's Quote Before a Window Company's
Before you spend £500+ on a full window replacement, call a local glazier and ask for a quote on the sealed unit alone. In most cases where the frame is sound, you'll save several hundred pounds per window and get the same thermal performance back.
Energy Guardian is an independent UW Partner. Referral link: uw.co.uk/ref/v2/partner/AQ5419