'Blown' is trade slang for a sealed unit that's lost its seal. Here's what's actually happened and how it's fixed.
A blown double-glazed unit has lost the airtight seal around its edge. Moist air has entered the cavity between the two panes and condenses on the inside faces of the glass — the visible mist. It hasn't literally exploded; the phrase just means the sealed unit has failed.
Every sealed unit has a primary seal (butyl) and a secondary seal (silicone or polyurethane). Both age. Typical lifespan is 15–25 years depending on quality of manufacture, exposure to UV and thermal cycling. South- and west-facing windows fail sooner because of higher temperature swings.
Yes. A blown unit has lost its Argon fill (which improves U-value by around 15%) and the moisture inside acts as a thermal bridge. You'll notice colder glass, more condensation on the room-side, and a higher heating bill. It's not just cosmetic.
'Defogging' the unit (drilling, drying, resealing) is a widely offered short-term fix but performs poorly and doesn't restore the Argon fill. A replacement sealed unit is the correct fix — same frame, new glass. Typical cost £80–£200 per unit, 30 minutes' work per window.
Choose warm-edge spacers, Argon fill and a 10-year insurance-backed guarantee on any replacement unit. Keep the external frame drainage clear so water doesn't sit against the sealant. Well-installed modern units routinely last 20+ years without failure.
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