If you’re reading this, you’ve probably got a window that’s fogged up, has a crack, or is just not insulating like it used to. The term everyone uses is “blown double glazed windows,” and the first question is always: can I just fix the seal, or do I need a whole new unit?
Honestly? It depends. And anyone who gives you a one-size-fits-all answer hasn't dealt with enough edge cases. I'm a purchasing specialist at a building supply company in the UK. In the last four years, I've processed over 300 emergency glazing orders—everything from a single 300mm x 300mm unit for a landlord to a full facade overhaul for a commercial client. My experience is mostly with standard double glazing (4mm-6mm glass, standard argon fill) and laminated safety glass. If you're working with heritage triple-glazing or vacuum-glazed units, your options (and costs) will be very different.
So, let’s break this down into the three most common scenarios I see.
Scenario A: The Seal is Blown, But the Glass is Fine (The ‘Foggy’ Window)
This is the classic “blown” window. You see condensation or fog between the panes. The glass itself isn’t cracked, but the seal has failed, letting moisture in. This is what most people imagine.
The common advice you’ll hear: “Get the seal repaired for £50-£80. It’s cheaper than a new unit.”
My advice (based on experience): Don’t bother. Save yourself the headache. Here’s why.
I’ve tried seal repair kits three times—twice on a client’s property and once on a friend’s window. Every time, the fix lasted less than 18 months. The problem is that standard sealant repair kits don’t address the root cause. They plug the visible gap but don't replace the desiccant (the drying agent) inside the spacer bar that’s already saturated. So, the seal fails again, often worse.
In March 2024, a client called me at 10 AM needing a solution for a blown unit in a rental property. The tenant was threatening to withhold rent over a “broken” window (i.e., condensation). The landlord wanted the cheapest fix. We spent £65 on a seal repair kit. By August 2024, the fog was back. We then had to pay for a full replacement unit anyway, plus an extra call-out fee. Total cost? Over £250. Replacing it first would have cost £120 for the unit and saved everyone the hassle.
When I would recommend repair: Only if you’re selling the house in the next 3 months and need the cheapest cosmetic fix to get through the survey. Or if the unit is an odd size that costs a fortune to custom-order (over £400). Even then, I'd warn you about the risk.
Scenario B: The Glass is Cracked or Broken (The ‘Safety’ Issue)
This is different. The glass itself is damaged—a crack, a chip, or a full spiderweb pattern. This is a safety issue, not just an insulation issue.
The common advice you’ll hear: “You need to replace the whole sealed unit.”
My advice (based on experience): You absolutely need to replace the whole sealed unit. But this is where the “clear tempered safety glass” or “laminated” spec becomes critical. This is also where I see the most dangerous DIY mistakes.
I cannot stress this enough: do not try to just glue a broken pane back into the frame. I once had a client who did that to a 1.5m x 1m pane of tempered glass in a living room window. A month later, the pane exploded during a hot day (tempered glass is under tension). The safety glass did its job—it shattered into small, relatively harmless cubes—but it fell out of the frame onto the patio. No one was hurt, but it meant an emergency call-out at double the cost.
When you order a replacement, make sure you specify clear tempered safety glass (for the inner pane) if it’s in a location near a door, low to the floor, or in a bathroom. For shower doors, privacy glass shower doors and textured glass shower doors are often the only legal option due to building regs. I’ve seen people try to use standard annealed glass to save £15—it’s a violation, and it’s dangerous. Don’t do it.
The cost of a standard replacement unit (e.g., 600mm x 900mm, 24mm overall, with one pane of laminated glass) is typically £120-£180 for the unit itself. Installation is another £50-£80 if you’re not doing it yourself. That’s the cost of safety.
Scenario C: You Want an Upgrade (The ‘I’m Bored of This Builder-Grade Look’ Scenario)
This is the scenario most guides miss. The window isn’t blown. The glass isn’t broken. You just want to change it. You’re redecorating, or you want stained glass frame patterns for a porch, or you want to upgrade to professional double laminated glass for better soundproofing.
The common advice you’ll hear: “Just buy a sticker or a film.”
My advice (based on experience): If you have the budget, replace the whole sealed unit with the spec you want. It’s worth it. The cost difference is often smaller than you think.
Many people assume that custom glass—like a stained glass frame pattern or textured glass—is prohibitively expensive. A client of ours just replaced two standard clear units in their front door with a custom stained glass frame pattern sealed unit. The cost of the custom sealed unit from a specialist fabricator was £320 each, compared to £110 for a standard clear unit. So, £420 extra for a front door that now looks like it cost £5,000. Worth every penny, in my opinion.
For soundproofing, upgrading from standard 4mm/4mm float glass to professional double laminated glass (e.g., 6mm/6mm with a PVB interlayer) can cut traffic noise by half. The unit cost is about 40% more, but it’s way cheaper than retrofitting secondary glazing after the fact.
One word of caution: make sure your frames can handle the extra weight. Laminated glass is heavier. I had a situation where a client ordered 10mm laminated units for a large casement window without checking the hinges. Let’s just say the fitters had to replace the hinges on the spot (note to self: always verify weight specs before approving an order).
So, How Do You Decide?
Ask yourself these three questions:
- Is the glass physically broken? Then replace it. No question. Safety first.
- Is it just a failed seal? Consider replacement unless you are selling in the next 3-6 months. The long-term cost of repair is usually higher.
- Do you want a different look or performance? If the frame is sound, a custom sealed unit is a fantastic upgrade that adds real value.
My honest take? If you have a blown unit, bite the bullet and replace it. The peace of mind that comes from a properly sealed, 10-year-guaranteed unit is worth the extra £50-£100 over a temporary repair. I’ve learned this the hard way—three times. I should have learned after the first.
