How to Replace a Sealed Unit in a uPVC Double Glazed Window

Your sealed unit may need changing for a variety of reasons, either the glazing is cracked or shattered or the sealed unit is “broken down” – when a double glazed window mists up between the panes (this is where the sealed unit has failed and let moisture into the cavity). The good news is that in most cases you don’t need to replace the whole window, you can replace just the sealed unit (the glass cassette) within the existing frame, which is faster, cheaper and far less disruptive.

This guide walks through the job for a standard uPVC bead-glazed window. The same principles apply to aluminium and timber, though the bead and gasket details differ.

Is it the sealed unit or the whole window?

If the frame, hinges and locks are sound and only the glass is misted, condensation-stained or cracked, a new sealed unit is all you need. Replace the whole window only if the frame is damaged, distorted, rotten (timber) or the hardware has failed.

One worthwhile point for your customers: replacing only the glass unit is not notifiable work under Building Regulations, so it doesn’t require FENSA, CERTASS or building control sign-off — unlike replacing the entire window. That makes it a quick, compliant job.

How to measure the sealed unit

Accurate measurement is what makes or breaks this job, so take your time here. The visible glass is smaller than the actual unit, because the edges sit hidden behind the beads in the frame rebate.

The accurate method — measure through a removed bead. For a precise figure, remove one bead and measure the true glass size including the part now exposed in the rebate, plus the rebate depth on each side. Better still, measure the old unit directly once it’s out — the edge of the spacer bar gives you the exact manufactured size. Always order in millimetres, width before height.

Thickness (overall unit depth). Measure across the edge of the existing unit. Standard units are commonly 24mm or 28mm — for example 4mm glass / 16mm or 20mm spacer / 4mm glass. This must be matched exactly: too thick or too thin and the unit won’t seat in the rebate or take the beads.

Specification. Record and match (or upgrade) the full spec: Low-E coating, argon fill, any toughened or laminated requirement, and — importantly — the spacer bar colour to ensure your new unit matches existing. Always advise the Sevenday counter of the spacer bar detail at the point of order so the new unit matches the existing units that remain in the property. Spacer bars commonly come in white, silver or black, and if the replacement doesn’t match the spacer colour of the adjacent windows it stands out badly — a mismatched bar on one pane in a run of windows looks awful and is exactly the kind of thing a customer notices and calls you back about. Check what’s fitted in the neighbouring frames before you order, not just the unit you’re replacing.

Safety glazing is a must in critical locations — within 800mm of floor level, or 300mm of a door edge up to 1500mm high — where toughened or laminated glass is required by Part K. Never replace toughened glass with standard.

Tools and safety wear to undertake the replacement

You’ll need: a glazing shovel or stiff putty knife, a plastic glazing paddle or wedge, glazing/setting blocks (packers), a suction cup or two for handling the glass, a soft mallet, and a sealant if the bead system requires it.

For safety wear, glass handling demands more than the basics: cut-resistant Kevlar sleeves to protect your forearms, cut-resistant gloves, and safety glasses. Sealed units are heavy and edges are sharp even when intact. Have a second pair of hands for anything above about 1m wide as they may be heavier than you expect.

Handling a cracked or broken unit

Take extra care if the unit you are removing is cracked or broken, as it can fail suddenly by shattering or splintering during removal and cause serious injury

Before you touch it, tape up the glass. Apply strong tape (such as duct or packing tape) in a criss-cross pattern across both faces of the unit. On a standard annealed unit this holds the broken pane together and stops shards dropping out as you handle it. If the glass is toughened (tempered), taping is even more important: toughened glass doesn’t crack into shards — when it lets go it shatters instantly into thousands of small fragments, so taping the whole face is vital and helps hold those fragments together and prevents them showering out during removal.

With a unit replacements, always wear your kevlar sleeves, cut-resistant gloves and glasses, keep your body to one side rather than directly in front of the glass, and support the weight fully as it comes free. If a pane has already shattered in situ, clear loose fragments carefully before attempting to lift the unit out.

Step-by-step

1. Remove the glazing beads. uPVC windows are almost always internally beaded (the beads are on the inside — a security feature). Starting with a long edge, work the glazing shovel or a stiff knife into the gap between the bead and the frame and gently lever the bead out. It’s clipped in, not glued, so it should pop free. Work along the length, then do the remaining three beads. Ensure you mark the beads in order and the right way round — they’re often handed and you’ll refit the same ones.
2. Note the packer positions — and understand toe-and-heel packing. Before the unit comes out, look closely at how it’s packed, because this is the single most important thing to get right on refit. The setting blocks (packers) around the glass do two jobs: they carry the unit’s weight and they brace the sash square so it opens and locks correctly.
   
   The technique is called “toe and heel” packing. The idea is that the glass unit is used to stiffen and square the sash frame, transferring the load diagonally across it. On an opening sash, packers are placed in opposite corners: the “heel” packer sits at the bottom hinge corner (taking the weight), and the “toe” packer sits at the top corner on the opposite, leading edge — diagonally across from the heel. This diagonal bracing pushes the sash back into square and stops it dropping on the latch side, which is what causes a window to bind, scrape or fail to lock.
   
   On a fixed (non-opening) pane the principle is gentler — the unit just needs supporting on its bottom edge with packers at roughly quarter points, kept clear of the drainage path — but on any opening sash, toe-and-heel is essential.
   
   So before you lift the old unit out, note exactly where each packer sits and its thickness, and ideally take a quick photo. You’ll replicate that pattern with the new unit. Getting the toe and heel right is the difference between a sash that closes with a satisfying click and one that needs to be rectified later when the window closing is problematic.

3. Remove the old unit. With beads off, the unit may simply tip forward at the top. Use suction cups to get a grip, support the weight, and lift it clear. If it’s stuck, a plastic paddle around the edge will free it. If the unit is cracked or broken, follow the taping and handling precautions above before you start — a damaged unit can give way without warning.
4. Prepare the rebate. Clean out the rebate, removing old packers, debris and any failed sealant. Check the drainage slots at the bottom of the frame are clear. A clean, dry rebate is essential.
5. Dry-fit and pack the new unit. Offer the new unit in and seat it on the bottom setting blocks. Ensure the sealed unit is installed the correct way around, if the glass has a soft coated pane then this should be on the internal of the house. Now reapply the toe-and-heel packing you noted in step 2, positioning the blocks to push the sash square and transfer the glass weight into the frame corners. Check the sash sits square before moving on — on an opening sash this is what stops it dropping and ensures it closes and locks cleanly. Take your time getting it right.
6. Refit the beads. Start with the shortest beads or follow the order you removed them. Locate one edge in the frame and tap the bead home with a soft mallet or the heel of your hand until it clips back into place along its length. Fit the longer beads last. They should sit flush with no gaps at the mitred corners.
7. Check operation and finish. Open and close the sash, check it locks correctly, and confirm the unit is held firmly with no rattle. Wipe the glass down and clear up — and if you’re working for a customer, point out the cleared drainage slots and the new spec.

Common pitfalls

The job that goes wrong usually does so for one of a few reasons:

• the unit was measured to the visible glass without allowing for the rebate, so it’s too small;
• the wrong overall thickness was ordered, so the beads won’t fit;
• the packing was skipped or guessed, so the sash drops or won’t lock;
• or safety glass was needed and standard was fitted.

Get the measurement, the thickness, the glass type and the packing right and it’s a straightforward job.

Need replacement sealed units?

Sevenday supplies replacement sealed units and the full glazing range to the trade and public across our local depot network across England and Wales. As an independent supplier we can match virtually any spec — Low-E, argon-filled, toughened or laminated safety glass, in the thickness you need. When you order, give your local depot the sizes, the full spec and the spacer bar colour (white, silver or black) so the new unit matches the existing windows in the property — getting the spacer colour right is what makes a single replacement pane blend in seamlessly.

As with all out our Sevenday How To guides, if you are in any doubt then please ask for advice. If you feel the job is too much for you to undertake then we work with many experienced installers and maintenance providers who we can line you up with to help.