Question answered
Misted roof window glass
The short answer
Misted roof window glass means the seal round the double-glazed unit has failed, letting damp air in between the panes. It cannot be cleaned away. You can often replace just the glazed unit if the frame is sound and a matching unit is available, but an old or damaged window, or one that also leaks, is usually better replaced as a whole.
Fog or droplets trapped between the two panes of a roof window mean the sealed unit has failed. You cannot wipe it away, and the question is whether to replace the glazing or the window.
What a failed unit looks like
A sealed unit is two panes of glass held a few millimetres apart by a spacer, with a seal running round the edge and dry air or gas in the gap. When the seal fails, damp air creeps in and the gap fogs. At first it shows as a haze on warm days or at certain times, then as permanent cloudiness, droplets, or a mineral bloom you cannot reach to clean.
The test is simple. Wipe the inside face and the outside face with a cloth. If the mist stays put, it lies between the panes. If it wipes off the room side, it is condensation, which is covered in condensation on a roof window. A pane that has misted does not mean the window leaks, but a failing seal sometimes goes hand in hand with other signs of age, such as tired gaskets and hardened flashing sealant, so it is worth looking at the whole window.
Why roof glazing fails
Roof windows face the sky. They take direct sun, heavy rain and frost, and on this coast salt in the air, all at angles that bring water to the edge seal far more than a wall window would see. The glass swells and shrinks through each day, and every cycle tests the seal. Given enough years it can give up.
Poor drainage round the frame, a blocked channel, or water standing at the edge of the glazing speeds the process up. A unit set with its spacer against a damp frame is at more risk still. If several windows on one house mist at about the same time, the cause may be shared, such as age or a similar exposure, and a survey can tell you whether to expect more. Misting also says little about the roof itself, so a dry loft does not rule out a failed seal.
Replace the pane or the whole window
On many roof windows the glazed unit can be changed without taking out the frame. The old unit is released, and a new one made to the same size and specification is set in. The roof covering and flashing are left alone, which saves disturbing the slope. That route makes sense when the frame is sound, the flashing is in good order and the window is a type for which units are still made.
A whole new window is the better route when the frame is rotten or scarred, when the sash no longer closes properly, when the model has been discontinued, or when the flashing is failing as well. Fitting a new window means renewing the flashing at the same time, which deals with the likeliest sources of future leaks. Where the building is old or sensitive, the choices are different again, and our page on roof window sizes helps you check what will fit an existing opening.
What Part L means for a replacement
Approved Document L sets limits for new and replacement roof windows in existing homes, with a U-value of 1.4 W/m²K or an energy rating in Band B. It also expects a replacement element to perform at least as well as the one it replaces. If you change the whole window, those limits apply to the new unit, and a modern window will often outperform the old one.
Whether a glazing-only change counts as replacing the element is a question for your building control body, so ask before ordering and do not assume. A new window also brings choices: laminated or toughened glass, solar control coatings, panes tuned to dull the sound of rain. Your roofer or the window supplier can advise on which suits the room. If you are weighing a motor or rain sensor at the same time, electric and solar roof windows explains what can be added.
Next steps and what we look at
Take a photograph of the window in different light, inside and out, and note the make and size, which are usually printed on a label in the frame. Those details tell a supplier whether a matching unit is still available. If the window is high, leave the measuring to someone with safe access.
On a survey we check the frame, the seals and the flashing as well as the pane, and we keep pictures of the findings for the quote. The price arrives as one written total, and where the smaller job will do, that is the one we recommend. Windows of many ages and makes are covered in the wider skylights and roof windows section, and conservation-area owners should also read about heritage-style rooflights before choosing a replacement.
Failed glazing in a roof window: two questions
Can misted glass be cleared without replacing the unit?
Treatments exist that drill and vent the gap, but the seal stays broken, and the glass does not regain its insulating performance. A sealed unit that has failed is generally replaced. If the unit is not leaking and the haze is slight, it is a cosmetic fault you can live with for a while.
Does misted glass mean my roof window is about to leak?
Not necessarily. The glazing seal and the weatherproofing round the frame are separate things. Still, a window old enough for its seal to fail may also have tired gaskets and ageing flashing, so it is sensible to have both checked on the same visit.
Tell us about your roof
Describe what is happening and we will call you back to arrange a visit.
- Photographs of what we find
- A written quote with one total price
- Or call 01903 299063
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