The short answer
Yes. A misted rooflight loses more heat than it did when new, because the sealed cavity that does most of the insulating has failed. The gas fill escapes and humid air takes its place, so the unit behaves more like two panes of plain glass than a modern sealed unit. How much more heat it loses depends on the glazing it started with, and it is not a figure anyone can give from the mist alone.
How a sealed unit insulates
A modern rooflight unit is a sandwich: two or three panes, a warm-edge spacer round the rim, and a gap filled with argon or another gas that conducts heat poorly. Usually one pane carries a low-emissivity coating, a thin layer that reflects heat back into the room and lets daylight through. The U-value of the unit, the rate at which it lets heat through, comes from all of these working together.
The gap matters most. Gas that is still and dry between two panes slows the movement of heat across it. Take that away and the panes stay but the benefit shrinks.
What changes when the seal fails
When the edge seal breaks, the gas leaks out over time and moist air moves in. That does three things.
- The gas fill goes
- Air conducts heat a little better than argon, so the cavity insulates less than designed.
- The desiccant is spent
- Moisture now condenses on the cold inner faces of the glass, which is the mist you see. Water in the gap also carries heat across more readily than still dry gas.
- The coating can be affected
- A low-emissivity coating inside a cavity is made to sit in a sealed, dry space. Persistent damp can mark or degrade it in places, and the room loses part of that reflective benefit.
The loss is gradual and uneven, since a seal rarely fails all at once. A unit that has misted at one corner may still do most of its job for a while. A unit that is fully fogged has lost the most.
How you notice it
There is rarely a dramatic sign. The room under the glass may feel cooler on a winter morning, the inner pane may feel colder to the hand, and the glass may collect condensation on the room side more readily, as the inner surface is now colder than it was. In summer, a failed unit does not hold back solar heat as well either, although that mostly depends on any solar control coating.
Try a simple check: lay your hand against the inner pane on a cold day, then against a sound pane in the same room if you have one. The difference tends to be noticeable, though it is not a measurement.
Does it matter in a rooflight more than in a wall window
A little, yes. Glass in a roof faces the open sky, which is a colder surface than the ground on a clear night, and warm air from the room rises towards it. So a rooflight tends to be the coldest point in a ceiling, and a failed unit makes that worse. Over a kitchen extension with a lantern, or above a landing in a Georgian terrace, the effect is on a large, prominent area of the room.
If you want the background on how much heat rooflights lose when they are in good order, our answer to do rooflights lose a lot of heat covers it, and rooflight U-values explained shows what the numbers mean.
What the loss looks like in a real room
Picture a landing in a 1930s semi in Heavitree, with a roof window letting daylight down a stairwell. The window is the largest cold surface the heating has to deal with in that space. When the unit fails, warm air rising up the stairs meets a colder pane, and the radiator downstairs works a little harder to hold the temperature. Nothing about it looks dramatic, which is why people live with it for years.
In a kitchen extension with a lantern, the same fault is felt differently: the glass is large and the room is open plan, so a cold downdraught can be noticed at the island on a still winter evening. A misted lantern in that room is both a heat problem and a cloudy patch in the middle of the ceiling.
What a replacement does about it
New sealed glass restores a dry, gas-filled cavity and brings in a modern specification: a low-emissivity coating, a warm-edge spacer and, where the frame allows, a better glass build than the original. If the whole rooflight is replaced, the new one has to meet Part L of the Building Regulations, which sets limiting U-values for replacement rooflights, and that is notified to Building Control. Changing just the sealed unit within the existing frame does not need notifying, yet the specification you pick still decides how much you gain.
Whether a new unit in an old frame is enough, or a new frame is worth having, depends on the frame. An old frame with a poor thermal break will limit what better glass can do. Our guide on replacing just the glass in a rooflight helps you weigh the two.
Should you wait
If the misting is slight and the room is comfortable, there is no immediate cost beyond a little extra heat loss, and you can plan a replacement for a convenient time, ideally before the cold months. If the mist has spread across the whole pane and the room is noticeably colder, the argument for acting soon is stronger. In both cases, check for the other signs of a failing rooflight, such as damp at the frame or a cracked pane, which are covered in is a misted rooflight dangerous.
Wiping will not bring the performance back, as the answer on whether misted glass can be cleared explains.
Getting it priced
Send a few photos of the glass and the room, with the size if you can find it on the frame, through the quote form. We will survey the opening, say whether the glass alone will do, and put the glazing specification, the lining and ceiling finish and the Building Control position in writing. You can read more on our misted and failed rooflight replacement page.