7 Steps to Replace a Failed Sealed Unit in an Aluminium Frame with Pilkington K Glass

Read the Frame Before You Touch the Glass

Aluminium frames from the 1990s onward mostly use externally beaded systems, where the glazing bead clips onto the frame around the outer face of the glass. Older thermally broken profiles from suppliers like Smart Systems or Kawneer sometimes bead internally, which changes the whole approach because you work from inside the room and the wind cannot help you. Before ordering anything, identify which type sits in front of you by looking for the fine seam of the bead and testing which face it releases from.

The sealed unit itself is a factory-made sandwich: two panes separated by a spacer bar filled with desiccant, sealed at the edge with polyisobutylene and a secondary polysulphide or silicone. When the edge seal fails, argon escapes and moisture-laden air enters, so the desiccant saturates and misting appears. No repair short of replacement addresses this. The Pilkington K coating sits on face 3 in a standard sealed unit, the inner pane’s outer surface, reflecting long-wave heat back into the room while staying near-invisible.

Measure for the Replacement Unit

Get the sight size wrong and the unit either will not seat or rattles in the rebate. Remove one bead first so you can measure the actual glass, not the aperture, then deduct the standard edge cover. For most aluminium systems the tight size is the visible glass width and height plus roughly 10 mm on each edge, giving 5 mm of edge cover per side, but confirm the rebate depth before committing.

Record four figures the glass merchant needs. The overall unit dimensions in millimetres, width by height. The overall thickness, commonly 24 mm or 28 mm, made up of a 4 mm outer pane, a 16 mm or 20 mm spacer, and a 4 mm Pilkington K inner pane. The spacer type, since a warm-edge spacer such as Swisspacer Ultimate outperforms the older aluminium box bar at the edge by several degrees. And any toughening requirement, which applies to glass in critical locations under BS 6262 Part 4, meaning doors, low-level glazing, and side panels.

Hand the K-glass orientation to the merchant explicitly. State that the coated pane must face the cavity as the inner leaf, because a unit built with the coating on the wrong surface performs like ordinary float glass and voids the thermal benefit you paid for.

Strip Out the Old Unit

Start with the shortest bead and work around. A stiff nylon glazing spatula or a plastic-tipped bead knife lifts the bead lip without scoring the powder coat; a steel chisel will mark aluminium permanently, and scratched frames corrode at the exposed edge. Ease the tool behind the bead near its midpoint, spring it out, then walk along to each corner. Keep the beads in order and labelled, because they are cut to length per side and swapping them leaves gaps.

Underneath the beads sit the glazing packers and setting blocks. Note their positions before removing them. The setting blocks, small plastic or nylon pads, carry the weight of the glass and normally sit at the two bottom corners about 100 mm in from each end, on a horizontally pivoting or fixed light. Location packers hold the unit central in the rebate. Photograph the layout so the new unit sits identically.

With the beads off and packers noted, break the perimeter seal. On externally drained systems the unit lifts straight out once the internal gasket releases. Two people and a set of suction lifters handle a unit over about 0.8 square metres safely; a 4/20/4 unit at one square metre weighs close to 20 kg, and the edges are lethal if it slips.

Prepare the Rebate and Drainage

Clear every fragment of old sealant, dried gasket, and glazing tape from the rebate with a plastic scraper and a vacuum. Any debris left behind bridges the unit to the frame and blocks the drainage path. Aluminium windows shed water through weep slots milled into the outer face of the frame; check these are open with a piece of wire, because a blocked weep sits water against the new unit’s edge seal and starts the next failure early.

Inspect the internal gasket. EPDM or santoprene wedge gaskets harden and shrink over fifteen to twenty years, and a hard gasket will not compress against the new glass to hold it. Renew the gasket while the frame is open if it has lost its spring; it pulls out of the groove by hand and the replacement pushes back in, mitred at the corners. Order the correct profile for the system, since a Smart Alitherm gasket does not fit a Kawneer groove.

One Point on Handling K Glass

The Pilkington K coating is a hard pyrolytic layer fired onto the float ribbon, so it tolerates handling far better than soft-coat products, but the merchant still builds it into the sealed unit at the factory. You never touch the coated surface yourself.

Set the New Unit and Re-bead

Drop the setting blocks back into their photographed positions first. They must sit under the unit’s edge so the load transfers to the frame, not the drainage line, and on an aluminium frame they go on the fixed sill for a top-hung casement. Lower the sealed unit onto the blocks using the suction lifters, then centre it with the location packers so the edge cover is even all round; an off-centre unit shows as a fat gasket line on one side and a thin one opposite.

Check the unit for square and for rock. Press diagonal corners; any movement means a packer has shifted. On a 1200 mm by 900 mm casement you want 3 mm to 5 mm of cavity between glass edge and rebate base all round, filled by the packers at load and location points, left clear elsewhere for drainage. The unit should stand vertical in the frame with no lean into the room.

Re-fit the beads in their labelled order, longest sides usually last. Each bead clips over the gasket and snaps into the frame channel with hand pressure or a soft mallet against a timber block; hammering directly on aluminium dents the powder coat. Work the corners tight so the mitres close. Once all four beads seat, run a finger along the internal gasket to confirm even compression against the glass, and check the sight lines from outside.

Externally beaded aluminium rarely needs additional silicone, since the gasket does the sealing and the weep slots do the drainage. If the system is a wet-glazed or capped commercial profile, the low-modulus neutral-cure silicone specified by the frame maker goes in as a continuous bead, tooled once, never re-worked after it skins.

Verify the Result

Run water down the glass from a hose or bottle after fitting and watch the weep slots discharge outside within a minute. Water pooling on the internal sill points to a blocked weep or a unit sitting on the drainage channel, both correctable before the sealant cures. Check the misting has gone, obviously, and that the new unit sits flush with no proud corner.

The thermal test takes a cold morning. Condensation forming on the inside face of the new Pilkington K unit, where the old one stayed clear, would indicate the coated pane faces the wrong way and the room-side glass now runs colder than before. Correctly built, the inner pane sits warmer to the touch than a plain unit, and surface condensation appears later and clears faster.

One question the swap does not settle is whether a single failed unit signals more to come. Sealed units from the same batch, glazed the same year into the same elevation, tend to fail within a few years of each other once the first goes, and the south and west faces that take the most solar cycling usually lead. Whether to replace the neighbours now or wait for the misting depends on how many beads you are willing to lift twice.