How do I fit a replacement double glazed unit?

Swapping a misted sealed unit is one of the few glazing jobs a competent householder can do, and the mechanical part is easy. What decides whether the new unit lasts is the packing: a unit sitting directly on the frame, or standing in trapped water, will fail at its edge seal and mist again within a few years.

Safety

What you'll need

• Cut-resistant gloves and eye protection
• A glazing shovel or a stiff plastic wedge and a bolster
• Glazing packers in assorted thicknesses
• Glass suction lifters (optional)
• A second person for anything larger than a small pane
• A helper's flat, clear place to put the old unit

Step by step

1. Check the beads are on the inside.If the beads are on the outside, the unit is externally beaded and removing it from outside is straightforward but leaves the opening insecure while you work — and the same route is available to anyone else. Plan accordingly and never leave it part-done.
2. Remove the beads in the right order.Short beads first, then the long ones. Work a plastic wedge or a glazing shovel into the joint between bead and frame near the middle of the bead, lever gently, and work along. Mark each bead's position on the back with a pencil as it comes out.
3. Note where the packers are before removing the unit.Photograph the packers in place. Their positions are information you want when the new unit goes in, particularly on an opening sash where the toe-and-heel arrangement is not obvious.
4. Take the old unit out from the bottom.Tilt the top of the unit into the room, take the weight along the bottom edge, and lift it out. Two people on anything sizeable. Lay it flat somewhere safe — a broken unit at this point is dangerous and messy.
5. Clean and check the rebate.Clear out old packers, dirt and any debris. Check the drainage slots at the bottom of the rebate are clear — this is the single most important thing you will do in the whole job.
6. Set the load-bearing blocks first.On a fixed light, two setting blocks on the bottom rebate at roughly the quarter points. On an opening sash, use the toe-and-heel arrangement: a block at the bottom of the hinge side and one diagonally opposite at the top of the closing side, which braces the sash against dropping out of square.
7. Offer the unit in and check the fit.It should sit in the rebate with a small even gap all round. If it is tight anywhere, do not force it — a unit under compression from the frame will crack, sometimes weeks later.
8. Add location blocks and distance pieces.Location blocks at the sides and top stop the unit sliding within the rebate; they carry no weight. Distance pieces front and back hold the glass centred so it does not touch the frame anywhere, and so the air gap and the drainage path stay open.
9. Never block the drainage.The bottom of the rebate must be able to drain out through the frame's weep slots. Packers go on the rebate, not across the drainage path, and no sealant or foam goes in the bottom of a drained rebate.
10. Refit the beads, long ones last.Short beads back into their original positions first, tapped home with the heel of your hand or a soft mallet, then the long ones sprung into place. If a bead will not go, something is not seated.
11. Check the sash operates and locks.On an opening sash, the new unit's weight and the toe-and-heel packing set the sash's squareness. Open, close and lock it several times, and check the gaps. Adjust the packing rather than the hinges if it is not right.
12. Dispose of the old unit properly.Sealed units are not ordinary glass recycling. Most household waste sites take them separately, and many glaziers will take one away with the delivery of the new one if you ask.

Common mistakes

• Resting the unit directly on the frame or on a bead — The frame is not flat and the load concentrates at a point, which cracks the glass or breaks the edge seal. Setting blocks exist to spread the load.
• Sealing the bottom of the rebate — The unit then stands in trapped water and the edge seal fails, which is exactly the misting fault you are replacing it to fix.
• Taking the long beads out first — They are under tension and they distort. The short ones give the long ones room to come out cleanly.
• Working alone on a large unit — There is no way to recover a sealed unit that starts to slip, and the edges are sharp enough to cause serious injury.

Questions people ask