Why it happens
The perimeter seal keeps the argon in and the moisture out. Over fifteen to twenty years, heat cycling, ultraviolet exposure and standing water in the frame rebate break that seal down. Once air can enter, so can water vapour, and it condenses on the cold inner surface.
Drainage is the accelerator. Frames whose drainage slots are blocked hold water against the bottom edge of the unit, which is exactly where the seal fails first. Clearing those slots on your existing windows is a genuinely useful five minute job.
Glass only or whole window
Glass only makes sense when the frame is square, the hinges and locks work smoothly, the gaskets are still supple and the profile is not chalked or brittle. Whole window makes sense when two or more of those fail, because you would otherwise be putting new glass into a frame that needs replacing in three years.
| Check | Points to glass only | Points to a new window |
|---|---|---|
| Frame square, sash closes cleanly | Yes | |
| Handles and hinges smooth | Yes | |
| Gaskets soft and intact | Yes | |
| Draught felt at the sash | Yes | |
| Profile chalky or brittle | Yes | |
| Frame over 20 years old | Usually yes |
What the work involves
A sealed unit swap is normally under an hour per window. We remove the beading, take out the failed unit, check and clear the drainage, then fit the new unit on fresh packers and refit the beading.
New units go in A-rated with a warm edge spacer as standard, so a glass only job still improves the thermal performance rather than simply restoring the view.
Find out which one you need
We check the frame, the hardware and the drainage before quoting, and recommend glass only whenever glass only will do.