Misting between the panes of a VELUX window means the sealed unit has failed. The perimeter seal that keeps the two panes of glass bonded together with dry argon between them has broken down, moist air is now moving in and out with the temperature, and the haze you see is that moisture condensing on the inner faces of the glass where no cloth can reach it. On a roof window old enough to have blown, the honest route is a full replacement, not a re-glaze, because the sealed unit is usually no longer made, and the frame and flashing around it have aged at the same rate. This page explains why.
Apex Skylights fits and fully replaces roof windows across Ipswich, Suffolk and north Essex. Our installers have 25 years fitting VELUX, Keylite and Roto units, and we do not offer glass swaps or resealing on roof windows. What follows is the reasoning, with the numbers, so you can judge that position rather than take it on faith.
What a sealed unit actually is
A modern roof window is not glazed with a single sheet of glass. It carries a sealed unit: two panes held apart by a spacer bar around the edge, with the gap filled with argon and the whole perimeter bonded with a butyl and polysulphide seal. Inside the hollow spacer bar sits a desiccant, a drying agent that mops up the small amount of moisture left in the cavity when the unit was made. As long as that seal holds and the desiccant has spare capacity, the cavity stays dry and clear.
Three things happen as the unit ages. The perimeter seal slowly loses its grip, so the argon fill leaks out and ordinary damp air works its way in. The desiccant fills up and can no longer absorb what arrives. And the low-emissivity coating on the inner face, the microscopically thin metallic layer that reflects heat back into the room, starts to degrade once it is no longer in a sealed, dry environment. The visible misting is the last stage. By the time you can see it on a cool morning, the thermal performance has already gone.
This is a different fault from condensation on the room-side face of the glass, which is about ventilation and warm damp air, not a failed unit. If your haze wipes off with a cloth, the unit has not blown. We cover that distinction in full on our VELUX window misting page, because getting it wrong sends people replacing perfectly good glass or, worse, ignoring a genuine failure.
Why roof windows blow earlier than the ones in your walls
A sealed unit in a vertical window in a sheltered wall might run twenty five years before it goes. The same unit set into a pitched roof often fails in fifteen to twenty. The reason is thermal cycling, and a roof window gets a brutal dose of it.
Think about a south-facing slope over an Ipswich loft on a clear June day. At dawn the glass surface can sit below freezing. By early afternoon, in direct sun with the loft heat pushing up from beneath, that same glass surface can pass 50°C. That swing happens every clear day, and every swing expands and contracts the glass, the spacer bar and the perimeter seal at slightly different rates. The seal is the weak link. It is being flexed thousands of times a year, far harder than any wall-mounted window ever sees.
Pitch makes it worse in a second way. A shallow roof, and Suffolk terraces are often laid at 25 to 30 degrees, holds the glass closer to the horizontal, so it collects more direct midday sun and drains rainwater more slowly off the surface. The lower the pitch and the more southerly the aspect, the harder the sealed unit works and the sooner it goes. This is why we can often guess which window in a house has blown before we are up the ladder: it is the one facing the sun on the shallowest slope.
The tell that it has genuinely blown. Room-side condensation clears when you wipe it and comes back with cooking or a shower. A blown unit hazes from inside the glass, comes and goes with temperature not humidity, and over a year or two develops permanent milky staining and fine mineral tide marks around the edges as the trapped moisture leaves deposits. Those tide marks cannot be cleaned off, because they are between the panes.
What a blown unit costs you while it sits there
A misted window is not only an eyesore. A sealed unit from around 2005 started life at roughly 2.8 to 3.2 W/m²K for the glass. Once the argon has escaped and the low-E coating has degraded, that figure is worse than the day it was made. A current replacement roof window runs around 1.1 to 1.3 W/m²K. In plain terms, you are heating the loft straight out through the glass, and the gap between a blown 2005 unit and a modern one is roughly a doubling of heat loss at that window.
For a replacement rooflight in an existing home, Part L of the Building Regulations sets a whole-unit U-value of 2.2 W/m²K or better. A blown unit does not meet that. A new one comfortably beats it. So the moment you replace, you are not just clearing the haze, you are bringing that opening back inside the current thermal standard.
Re-glaze the old unit, or replace the whole window?
This is the real question, and it deserves a straight answer with the economics laid out rather than a slogan. In principle a sealed unit can be swapped without changing the frame. In practice, on a roof window old enough to have misted, three things usually stand in the way.
- The unit is often no longer made. Roof window generations change roughly every ten to fifteen years, and each change moves the glazing bead detail and the glass thickness. A sealed unit for a pre-2000 or early-2000s sash frequently cannot be sourced as a like-for-like part any more.
- The frame has aged with the glass. The seal did not fail in isolation. The same fifteen to twenty years of weather and thermal cycling have worked the timber, the gaskets and the hinge friction pads. Putting a new sealed unit into a tired sash is spending money on the one part that was not going to fail next.
- The flashing beneath it is at the end of its life too. A window that old sits in a flashing kit and a set of collars that are due for renewal. Disturbing the glazing does nothing for the detail that actually keeps water out.
We do not offer sealed-unit swaps or resealing on roof windows, and the reason is not that it is impossible in every case. It is that on a window old enough to be misting, the honest cost comparison almost always lands on replacement. Here is that comparison worked through.
A worked example
Take a single MK06 centre pivot fitted in 2005, now misted, in a Chantry loft. Suppose a glass-only unit could still be sourced and fitted from a roof, with access, at somewhere around £450 to £650 for the one pane swap. Set that against a full like-for-like replacement of the whole window at £1,150 to £1,650, which includes a new unit meeting current U-values, a fresh flashing kit matched to your covering, a new insulation collar, a new underfelt collar, all fixings and the covering reinstated, with a 10-year workmanship guarantee behind it.
| Route | Up-front | What it fixes | What it leaves | Likely ten-year spend |
|---|---|---|---|---|
| Source and fit a replacement sealed unit (if available) | £450 to £650 | The haze, for now | Aged sash, tired gaskets, old flashing, old collars, poor U-value in the frame line | Often a second intervention within a few years, then replacement anyway: £1,600 to £2,300 all in |
| Replace the whole window, like for like | £1,150 to £1,650 | Glass, frame, flashing, collars, U-value, in one visit | Nothing outstanding; guaranteed for ten years | £1,150 to £1,650, then nothing |
Figures are supply-and-fit ranges for Ipswich and Suffolk, sound trimmers and reasonable access assumed. Your survey confirms the exact figure and it is then fixed.
The re-glaze looks cheaper on the day and often is not cheaper over the decade, because it treats the one symptom you can see and leaves the four you cannot. The moment the sealed unit is unavailable, which is the common case on anything pre-2005, the re-glaze route closes entirely and you are replacing regardless. We break the money side down further on our glass versus whole-window cost page, and the wider case for renewal over patching sits on why an old VELUX is replace, not repair.
Misting rarely arrives alone
The window that has blown is telling you its age, and age shows up in more than the glass. When we survey a misted unit we look straight at the timber, because the same years that killed the seal have been working the frame. Press the bottom rail of the sash with a thumb from inside. If it gives, the timber has gone, and a re-glaze into a rotten sash would be money poured away. That failure has its own page: rot in timber VELUX frames, and on exposed coastal slopes it comes years earlier, covered on coastal VELUX frame rot.
So the sequence we see, again and again, on loft windows fitted across Ipswich between 2000 and 2010, is misting first, then a stiff or leaky sash, then a damp patch, all inside a couple of years, because they are all the same window reaching the end of the same service life at once. Replacing the whole unit resets the clock on every one of those. Re-glazing resets it on none of them except the glass.
When we would replace, and what you get
A misted roof window is a replacement job, and on a like-for-like swap it is a job measured in hours, never overnight. The old unit comes out, a new frame goes in on its brackets, packed and squared so the sash pulls evenly onto the gasket, then the insulation collar, the underfelt collar and the correct flashing kit for your covering, and the slates or tiles re-hung with the lap maintained. The roof is watertight the same day. You can read the full sequence on the VELUX window replacement page.
Common questions about misted roof windows
Can you just replace the misted glass and keep my frame?
No. We do not swap sealed units or reglaze roof windows. On a unit old enough to have blown, the matching glass is often no longer made, and the frame, gaskets and flashing around it are the same age as the failed seal. Replacing the whole window in one visit clears the haze, restores the U-value and renews the detail that keeps water out, with a 10-year guarantee. A glass-only swap treats one symptom and leaves the rest.
Is misting between the panes the same as condensation on the glass?
No, and the difference matters. Condensation on the room-side face wipes off with a cloth and returns after a shower or cooking. It is a ventilation issue, not a broken window. Misting trapped between the two panes cannot be wiped, comes and goes with temperature rather than humidity, and means the sealed unit has failed. If you are not sure which you have, our VELUX window misting page walks through both.
My window only mists on some mornings, is it really failing?
Yes. An intermittent haze that appears on cool, humid mornings and clears by afternoon is early sealed-unit failure. The seal is letting moisture cycle in and out, and the temperature swing across a roof slope drives it. It will not recover. Over a year or two it becomes permanent milky staining with mineral tide marks around the edges. Catching it early does not save the unit, but it does let you plan the replacement rather than react to it.
Why do my roof windows all seem to be misting at once?
Because they were almost certainly fitted at the same time, most often during a loft conversion between 2000 and 2010, so they reach the end of their sealed-unit life together. Roof windows also blow earlier than wall windows because of the temperature swing across a pitched, sun-facing slope. When several go within a couple of years, replacing them together on the same visit is usually the more economical route, and we price multiple units on one slope accordingly.
Get a fixed price for a misted roof window
Send a photograph of the data plate on the top edge of the sash if you can reach it, one of the window from inside showing the haze, and one of the roof from the garden showing the covering. That is usually enough for us to say whether a like-for-like replacement fits and roughly what it costs before anyone comes out. Then we survey, confirm the opening and the flashing kit, and give you one written price with the U-value stated on it.
That price is fixed, and every installation carries a 10-year workmanship guarantee. Request a fixed quote for a misted VELUX in Ipswich, Suffolk or north Essex, or look at our published prices across every service first. More on the parent topic is on our VELUX window installation hub.
Thinking about this job? We survey across Ipswich, Suffolk and north Essex and give you a fixed price before any work starts, with a 10-year workmanship guarantee.