Replace a misted double-glazed sealed unit

Order a made-to-measure sealed unit, then remove the old glazing bead, swap the misted pane, and reseal it into the frame.

GZ-01Confirm it's a failed seal, not condensationMisting between the panes that won't wipe away from either the inside or outside face means the seal has failed and moist air has got trapped between the panes. Condensation on the room-side face only is normal and clears with ventilation, not a repair job. Check the spacer bar edge, usually etched near a corner, for the make date and glass spec - you'll need this when ordering the replacement.

GZ-02Measure the unit and check for safety glassMeasure the width and height at the frame rebate, and measure the unit's overall thickness from outer glass face to outer glass face with a small ruler or vernier caliper at the edge. Note the spacer bar colour and check for a small toughened or laminated stamp etched into a corner of the glass. Tell your supplier the exact location of the window in the house.

RULES

KNOW THE RULES

Building Regulations (Approved Document K/N) require safety glass in critical locations - below 800mm from floor level, in doors, and next to doors. If the original pane was toughened or laminated, the replacement must match.

GZ-03Order the new sealed unitGive a local glass supplier the width, height, overall thickness, spacer bar colour and glass spec, plus whether it needs to be safety glass. Made-to-measure units typically take several days to arrive, so treat this as advance prep, not part of the same working session as the fitting.

1 each · Made-to-measure double-glazed sealed unit (approx 600x900mm)

Buy:ScrewfixB&QToolstationAmazon

GZ-04Set up the work areaLay dust sheets under the window inside, and outside too if you'll be working from that side. Put on cut-resistant gloves before handling any glass, old or new. Get a second person to help if the unit is larger than roughly 600x900mm - it's heavy and awkward for one person.

1 each · Dust sheet

Buy:ScrewfixB&QToolstationAmazon

1 pair · Cut-resistant gloves

Buy:ScrewfixB&QToolstationAmazon

GZ-05Remove the glazing beadOn uPVC or aluminium windows, start at the middle of the longest bead and lever it out gently with a flat glazing tool, working outward - it clips in and should pop free without much force. On timber windows, the bead is usually pinned; ease it away near each pin with a wide chisel or putty knife, then pull the pins out with pliers. Older timber windows may use putty instead of a bead - score along the putty line with a knife first, then chip it away carefully.

1 each · Flat glazing tool / pry bar

Buy:ScrewfixB&QToolstationAmazon

GZ-06Lift out the old sealed unitWith the bead clear, the old unit should lift straight out of the frame rebate. Use suction cups on larger panes so it doesn't slip, and lift with your legs, not your back. Stand the old unit on its edge somewhere safe, wrapped in cardboard, until you can dispose of it.

1 pair · Suction cup glass lifter (pair)

Buy:ScrewfixB&QToolstationAmazon

GZ-07Clean the rebate and check the setting blocksBrush and wipe out the frame channel where the unit sat, removing old debris and any perished sealant. Check the small plastic setting blocks at the base of the rebate, which hold the unit at the right height and stop glass touching the frame. Replace any that are cracked or missing before fitting the new unit.

1 pack · Glazing setting blocks/packers (pack)

Buy:ScrewfixB&QToolstationAmazon

GZ-08Fit the new sealed unitLower the new unit into the rebate onto the setting blocks, keeping it square and centred. Check there's an even gap all round between the glass edge and the frame, which lets the unit move slightly and stops it rattling. Add packers at the sides if the gap isn't even.

GZ-09Refit the glazing beadOn uPVC or aluminium, press the bead back in by hand starting at one corner, then tap it fully home with a rubber mallet along its length. On timber, refit with new glazing pins tapped in at an angle every 150-200mm, or reapply putty and smooth it to a neat angled line if that was the original method.

1 each · Rubber mallet

Buy:ScrewfixB&QToolstationAmazon

1 pack · Glazing pins (pack)

Buy:ScrewfixB&QToolstationAmazon

GZ-10Reseal and finishIf the frame has a sealant bead running along the join between bead and glass, replace it with a matching frame-grade silicone for a weatherproof finish. Wipe away any excess with a damp cloth before it cures. Leave putty, if used, to skin over for a few days before painting it.

1 tube · Frame-grade clear silicone sealant

Buy:ScrewfixB&QToolstationAmazon

GZ-11Check the window operates properlyOpen and close the window fully to check nothing binds or rattles, and that any locking mechanism still lines up. A little stiffness settling in over a day or two is normal on timber windows as the new unit beds in.

GZ-12Dispose of the old unit and tidy upWrap the old sealed unit in cardboard or heavy plastic before moving it, since a failed edge seal can mean the panes aren't bonded as firmly as they look. Most councils don't take double glazing in kerbside recycling, so take it to a household waste site or ask your glass supplier if they'll take it back. Clear away dust sheets and check the area for glass splinters.

Some links may earn us a small commission at no extra cost to you.

Save this as a project

Back to all guides