Domakihub How to Replace a Blown Double-Glazed Unit
The permanent fix for misted windows when wiping the glass just does not work.
Reviewed by Domakihub home & decor pros · 2026-06-23 · 5 min read
When condensation appears between the panes of a double-glazed window, the sealed unit has failed and no amount of cleaning will clear it. The video above walks through a real replacement job; this guide breaks the process into five clear steps so you can follow along or brief a glazier with confidence. Greek properties are particularly prone to this problem because the intense Mediterranean sun accelerates the degradation of the rubber seals and desiccant that keep moisture out of the air gap — a fact worth understanding whether you own a home, a holiday rental, or a hotel.
Why Double-Glazed Units Fail
A sealed double-glazed unit (IGU) consists of two panes of glass separated by a spacer bar filled with desiccant and sealed around the perimeter with a primary and secondary sealant. Over time, repeated thermal expansion and contraction — especially severe in Greece, where a south-facing window can swing from 10 °C in January to 60 °C on the glass surface in July — works the rubber seals loose and allows moist air to migrate into the cavity.
Once moisture enters, the desiccant saturates and condensation forms on the inner faces of the glass. At that point the unit cannot be repaired by injection kits or drill-and-seal methods that hold for long. Replacement is the only reliable solution.
Assessing the Damage Before You Start
Before ordering a replacement pane, identify the exact size and specification of the failed unit. Measure the overall width and height of the glass, then look at the edge for a stamp or label that gives the glass thickness, spacer width, and any coatings (low-e, solar control, or laminated). In Greece, solar-control or reflective coatings are common on south and west elevations and a matching specification is important for thermal performance.
Also check the condition of the surrounding frame. If the UPVC or aluminium frame shows cracking, water ingress, or the beading is brittle from UV exposure, it may need attention at the same time. A failed unit in an otherwise sound frame is a straightforward swap; a damaged frame changes the scope of the job.
How to Replace a Blown Double-Glazed Unit: Step by Step
- 1
Remove the glass beading
Start at a corner and work a plastic chisel or flat-blade screwdriver into the joint between the bead and the frame. UPVC beading clips into a channel and usually releases with light pressure; aluminium frames often use screwed or snap-in beads. Work around all four sides, keeping the beads in order so you know which length belongs where. Never use a metal bar directly against the glass.
- 2
Lift out the old unit safely
With the beading removed, the unit sits on rubber setting blocks at the bottom of the frame. Grip the glass from both sides using suction cups — a pair rated for at least 40 kg is ideal. Tilt the top edge toward you slightly, lift off the setting blocks, and carry the unit flat to a safe disposal point. Broken glass from a sealed unit can be surprisingly heavy: a standard 24 mm double-glazed unit for a 1 m x 1.2 m window weighs around 30 kg.
- 3
Clean the frame and replace the setting blocks
Remove any old sealant, dirt, and adhesive residue from the rebate (the groove the glass sits in). Use a plastic scraper and isopropyl alcohol on UPVC; avoid abrasives on aluminium. Fit new rubber setting blocks — two at the bottom, positioned roughly one quarter of the way in from each corner. These blocks carry the weight of the new unit and ensure it cannot rock or vibrate in the frame.
- 4
Install the new sealed unit
Lower the new unit onto the setting blocks, keeping it central in the rebate with equal gaps on all four sides. Check the unit is plumb and square using a spirit level. Some installers add a thin bead of frame sealant in the rebate before lowering the glass, though this is frame-dependent; follow the manufacturer's recommendation for your frame type. Do not force the unit or allow it to rest against the frame on any edge.
- 5
Refit the beading and check the seal
Press the beads back into position starting at the corners, working along each side and clicking them firmly into the channel. Check that the bead sits flush all the way round with no gaps. If the frame uses a glazing gasket rather than beads, press the gasket back into its groove and check for any splits. Finally, open and close the window to confirm it operates smoothly and the unit does not rattle — a rattle usually means a setting block has shifted or the beading has not seated fully.
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FAQ
- Can I inject the old unit to fix the misting instead of replacing it?
- Injection kits drill small holes in the spacer bar and inject a drying agent, then reseal the holes. In mild cases this can reduce fogging for a season or two, but the underlying seal failure remains. In Greece, where temperature swings are extreme, injected units tend to re-mist quickly. Full replacement is the only method that restores the original thermal and acoustic performance and is the approach shown in the video above.
- How long does a double-glazed unit replacement take?
- A single unit swap by an experienced glazier typically takes 30 to 60 minutes, not counting time to source the glass. Measuring, ordering, and delivery of a custom unit usually adds five to ten working days. For a holiday rental or hotel room, plan the work during a low-occupancy window to avoid disruption to guests.
- What glass specification should I order for a Greek property?
- For most residential windows in Greece, a 4-16-4 unit (4 mm outer pane, 16 mm argon-filled spacer, 4 mm inner pane) with a low-e coating on the inner pane gives good year-round performance. South and west-facing glazing exposed to full summer sun benefits from a solar-control coating on the outer pane to reduce heat gain. If you are replacing in a coastal area, consider a marine-grade secondary sealant, as salt air accelerates seal degradation further.