Inspired by a helpful YouTube guide. This walk-through draws on the popular video "Double Glazing Unit Condensation / Blown DIY Repair", which shows the full process of diagnosing a failed sealed unit and working through repair options. It is particularly useful on the difference between condensation that sits on the glass surface and moisture that is actually trapped inside the unit - two problems that look similar but need completely different fixes.
1. Confirm the unit is genuinely blown
Run your finger across both surfaces of the glass. If the misting wipes away, it is condensation on the outside or inside of the room - not a failed seal. That is a ventilation issue, not a glazing problem.
If the fog or white deposit does not shift, the unit seal has failed. Moisture has got into the cavity between the two panes and is now trapped there permanently. You cannot wipe it off because you cannot reach it.
2. Understand what a sealed unit actually is
Modern double glazing is not two loose sheets of glass. It is a factory-sealed sandwich: two panes separated by an aluminium or steel spacer bar, with the cavity filled with dry air or inert gas (argon is common). The edges are sealed with butyl and silicone.
When that seal fails - usually due to age, UV degradation, or movement in the frame - warm damp air gets into the cavity, condenses on the cooler pane, and leaves mineral deposits behind. That milky film you see is the mineral residue, not just water vapour.
3. Measure the unit before you do anything else
You need the height, width, and thickness of the existing unit. Thickness is usually 24 mm or 28 mm for standard UK double glazing, but measure yours - do not guess.
Write the measurements down. You will need them whether you are ordering a replacement unit or a repair kit.
4. Try a DIY repair kit for light cases
For units where the seal has failed but the glass itself is still clear, repair kits are available. They work by drilling small holes through the spacer bar, injecting a desiccant to absorb the remaining moisture, and sealing the holes back up.
Results vary. On newer failures - where moisture has not yet left heavy deposits - the window can clear up nicely over a few days. On older, badly stained units, the mineral residue will not lift no matter how dry the cavity becomes. Be honest with yourself about which you are dealing with before you spend money on the kit.
5. Order a replacement sealed unit for persistent cases
If the unit is badly stained, cracked, or the frame itself is in good condition, replacing just the glass unit is the most reliable fix. You keep the existing uPVC, timber, or aluminium frame and simply swap the pane.
Local glass merchants or online glazing suppliers will cut a sealed unit to size. Give them your measurements and specify whether you need low-e glass (for energy efficiency), toughened glass (required by building regulations in certain locations), or any other specification.
6. Remove the old unit carefully
Pop off the internal beading using a stiff putty knife or a specialist glazing tool. Work around the perimeter gently - the beads are usually held by friction and a little pressure, not adhesive. Keep them safe; you will reuse them.
The unit will be heavy. For anything above a small side window, have a second person ready to take the weight as it comes free. Glass is unforgiving if it drops.
7. Clean the rebate and fit the new unit
Clear out old setting blocks, debris, and dried sealant from the frame rebate. Fit new plastic setting blocks at the correct positions - typically at quarter points on the bottom rail - to support the unit's weight and keep it off the frame.
Lower the new unit in square, press it back gently, and refit the beading by hand. Start at the corners. No hammer needed on uPVC beads; a rubber mallet tap is fine if they are tight.
8. Check the drainage and reseal
Most uPVC frames have small drainage slots or holes along the bottom rail. Make sure they are clear after fitting. A blocked drainage slot is often what caused the original seal failure.
Run a thin bead of exterior frame sealant along any visible gap between unit and bead if needed. That said, a properly fitted unit with good beads should not need additional sealant. If it does, something is not sitting right.
When to call a handyman
Call Richard if the frame itself is damaged, distorted, or rotten. A perfect new glass unit in a failed frame will blow again within a year. Similarly, if the unit is large, above ground-floor level, or you are not confident lifting heavy glass safely, it is worth getting professional help. A dropped unit can do far more damage than the cost of a call-out.
Need help with your windows?
The Sandwich Handyman can replace blown sealed units, refit beading, clear blocked drainage slots, and advise on when a full window replacement makes more sense than a repair.
Contact Richard