The answer
Yes. Misted rooflight glass can be replaced, because the fogging sits inside a sealed double-glazed unit that is made as a separate part. A new unit is measured, made to size and fitted into the existing frame. If the frame or the surrounding flashing has failed as well, the whole rooflight is the better answer.
Why the glass can be swapped on its own
A rooflight pane is rarely a single sheet. It is a sealed unit: two layers of glass held apart by a metal or composite spacer, with a drying agent inside the spacer and a sealed edge all the way round. The cavity between the layers is meant to stay dry and still. That still air, or a gas fill, is what gives the pane its insulation.
When the edge seal gives up, damp room air creeps into the cavity. The drying agent fills up and then cannot cope, and the moisture shows as a milky haze or beads of water that no cloth can reach. Nothing is wrong with the glass itself. The unit has simply stopped being sealed.
Because that unit is a manufactured part, it can come out of the frame and a new one can go in. That is what people mean by replacing misted skylight glass. You are not replacing the view or the opening, only the part that has failed.
What happens when we replace the unit
It starts with a measurement. A sealed unit has to match the frame it sits in to the millimetre, so we measure the failed unit, the depth of the frame rebate and the type of glass already there. If the rooflight carries a maker's label or a size code, we note that too. A full walk-through of unit or whole-rooflight replacement sets out how that decision is made.
The new unit is made to that size. It can be specified with the same build as before or with a better one: an up-to-date energy-saving coating, a different spacer, or glass that suits the room, such as toughened on the inner face for a rooflight over a hall or stairs. On a flat roof, that choice is worth making with the room in mind, not just the old specification.
On the day, the old unit is lifted out, the rebate is cleaned and checked, new setting blocks and seals go in, and the new unit is pressed home. The fitting itself takes a short visit. The inside is left tidy, and the new glass is cleaned before we leave.
When the whole rooflight is the better choice
Glass-only replacement assumes the rest of the rooflight is sound. Before booking it, look at what the misted pane is attached to.
- Frame: a timber frame that is soft or stained, or an aluminium frame with a split corner, will not hold a new unit for long.
- Weathering: if rain has been getting in around the edge, the flashing or upstand needs attention, and the unit is not the only problem.
- Plastic domes and older polycarbonate: these discolour and turn brittle as they age, and a dome is a single moulded part. A dome is replaced whole, not reglazed.
- Fit for the room: a rooflight that overheats in summer or was never the right size is a reason to change the whole thing.
None of this is a reason to panic. It is a reason to have someone look at the frame and the roof edge while the old pane is out, before a new sealed unit is ordered. At the free survey we check all of these and tell you which route fits.
Roof windows, flat rooflights and the fitting details
The answer shifts a little with the type of rooflight. A pitched-roof window from a maker such as VELUX has a sash that holds its glazing unit, and a replacement unit has to match that maker's range and size code. A flat-roof rooflight is more often a framed glass unit on an upstand, with the glass at a slight fall to shed rain, and the unit can be made to the frame.
Either way, the Building Regulations still apply to new work. For a replacement in an existing home, Approved Document L sets 1.4 W/m²K or WER band B for roof windows and 2.2 W/m²K for rooflights, and the glazing goes in to current Building Regulations, and any Building Control notification is ours to deal with. If you want to know how the glass is built up, our guide to why rooflights mist up explains the make-up of a unit.
Blinds, rain sensors and electric opening are separate from the glass. Where a roof window has them, they are checked and refitted or kept working after the new unit goes in.
What to check before you order
- Is the mist between the panes, or only on the inner face of the glass? Misting on the surface is condensation. Misting inside the glass is a failed seal. The condensation answer shows the difference.
- Is the frame sound, or is there a stain or a soft patch around it?
- Do you know the size, the age and the maker, or is there a label on the frame?
- Do you want the same glass, or a better specification?
Book a free survey, or send a photo through WhatsApp on 07880 284133 or to info@vertexskylightsmiltonkeynes.co.uk. You get a fixed written quote that does not change on the day, and a 10-year workmanship guarantee in writing covers the fitting. The wider picture is in our misted rooflights section, and there is a plain answer on whether a misted rooflight is a problem if you are wondering how soon to act.
Before you replace the glass
Can I replace misted glass in a VELUX window?
Yes. The sealed unit is a separate part of the sash and can be swapped for one made to match, as long as the sash and flashing are sound.
Is a new sealed unit as good as the old one?
It can be better. A new unit is made to a current specification, so the insulation and the glass type can be chosen to suit the room.
Does replacing the glass need Building Control?
Yes where it is notifiable. The new glazing goes in to current Building Regulations and the paperwork is ours to file.
