How to replace a sealed double glazing unit

Inspired by a helpful YouTube guide. This walkthrough is based on the UK video "Replacing a sealed unit in a double glazed window", which shows the full process from removing the old misted unit to fitting the replacement. It is clear on the beading removal technique and on using the correct glazing packers - the two bits that trip most people up first time.

1. Confirm it is the sealed unit that has failed

Condensation between the panes - not on the glass surface but in the gap between the two panes - is the sign. It might look like a haze, smearing, or actual droplets inside the unit. If you can wipe it off, it is on the glass surface and the unit is fine. If you cannot, the seal has gone.

Also check the frame itself. Damaged, rotten, or misaligned frames mean the job is bigger than just swapping a unit, and it may be worth getting a glazing company in rather than doing it yourself.

2. Remove the beading

The sealed unit is held in the frame by plastic beading - usually four strips that click in around the perimeter. Start at a corner with a flat pry tool or a stiff putty knife. Do not use a screwdriver, which will mark the frame. Work the bead away from the frame carefully. Most beading is push-in, not screwed.

Keep the beads in order. They are often cut to different lengths and mitred at corners. Mixing them up when refitting is a common and annoying mistake. Lay them out on the floor in the positions they came from.

3. Note the glazing packers

Before lifting out the old unit, look at the bottom edge of the glass inside the frame. You should see small plastic packers - typically two at the bottom, positioned about a third in from each corner. These packers take the weight of the glass and keep it centred. Note where they are and keep them for reuse, or replace them with new ones of the same size.

4. Remove the old unit

With the beading off, the sealed unit should be free. Have a second person ready - even a small sealed unit is heavier than it looks, and a large one is very heavy. Tilt the top edge gently toward you first, then lift it out. Keep your hands on the face of the glass, not the edges. Stand it upright against a wall - never lay double glazing flat.

Wear safety glasses throughout. Old sealed units sometimes have hidden cracks that give way when the beading comes off.

5. Measure for the replacement unit

Measure the opening in the frame carefully - the rebate (the recess the glass sits in) rather than the outer frame. Measure width and height in at least three places each and use the smallest measurement. Double glazing is made to specific tolerances and needs a small clearance gap all round - typically 3–5 mm. Your supplier will know the standard deductions for your frame type if you give them the rebate size.

Also note the unit thickness - usually 24 mm for a standard 4–16–4 unit (4 mm glass, 16 mm gas gap, 4 mm glass), but older or specialist windows may differ. Take a photo of the edge of the old unit before ordering.

6. Order and check the replacement

Supply the measurements and unit specification to a local glazing company or an online sealed unit supplier. Lead times vary - usually a few days to a week for standard sizes. Check the new unit carefully when it arrives: right size, no chips on the edges, and the spacer bar sitting square. Do not accept a damaged unit.

7. Fit the replacement unit

Place the bottom packers back in position in the frame. Lift the new unit in with a helper, bottom edge first, settling it onto the packers. Check it is centred in the frame - equal gap on both sides. Push it gently back until the rear face sits flat against the rebate.

Refit the beading in the reverse order it came out. Press it firmly from the centre of each strip outward - it should click back without tools. Check the corners are tight. If any bead has split or is too short to fit back properly, it is worth ordering replacement beading from a window hardware supplier rather than leaving a gap.

When to call a handyman

Call Richard if the window is above ground floor, if the frame looks to be in poor condition, or if you are not confident handling large sheets of glass. A large sealed unit in a first-floor window is genuinely a two-person job and the glass is worth treating with respect.