The short answer
Any glass can collect condensation, and a conservation rooflight is no exception when a cottage is damp. The risk is lower than with old single glazing. A sealed double glazed unit, a thermally broken frame and an insulated, sealed reveal keep the inner surface warm, and ordinary ventilation clears the moisture that the house itself makes.
Where the water in the room goes
Condensation is not a fault in the glass. It is warm, damp air meeting a surface cold enough to turn some of that moisture back into water.
Every household makes moisture. Breathing, cooking, showers and drying washing all put water into the air, and an older cottage with solid walls and little ventilation holds more of it than a modern house. That air rises. A rooflight sits at the top of the room, so it is the first cold surface the damp air reaches.
The glass itself is only half of the picture. The temperature of the inner pane, and of the frame around it, decides whether that moisture settles. The colder the surface, the sooner it reaches the point where air can no longer hold its water. So the job is to keep every inner surface of the rooflight warm, and to give the damp air somewhere else to go.
A single glazed roof light, such as the cast iron type found on Victorian outbuildings, fails on both counts. The glass is thin, the iron frame carries cold straight through, and water runs down the bars in winter. That is why people often expect a conservation rooflight to behave the same way. It does not, because it is built differently.
What stops the glass running wet
Four parts of the build keep the inner surface above the temperature where water forms. They work together, and dropping one weakens the rest.
- A sealed double glazed unit. Two panes with a gas-filled cavity keep the inner pane much closer to room temperature than a single sheet. Our page on are conservation rooflights double glazed covers the unit itself.
- A warm edge. The spacer bar that holds the two panes apart is where a cold line can form. A low-conductivity spacer keeps the edge of the glass from becoming the coldest strip in the room.
- A thermally broken frame. Steel and aluminium both carry cold quickly. A break in the section, made of a material that does not, stops the outer cold reaching the inner face. The materials are compared on steel or aluminium conservation rooflights.
- An insulated, airtight reveal. The plaster or lining between the frame and the ceiling is often the real culprit. If cold air can get behind it, the reveal itself feels damp. We insulate that zone and seal it at the roof line so the warm room air stays away from cold metal.
The same seals that keep the room airtight also help with sound. Gaskets and a laminated inner pane are part of the written quiet spec that comes with every quote, so you are paying once for a build that does both jobs.
Three kinds of wet glass, and what each one means
Not all moisture on a rooflight is the same problem. Where the water sits tells you the cause.
| Where you see it | Likely cause | What to do |
|---|---|---|
| Inside face, in the morning | Damp room air meeting cool glass | Ventilate, warm the reveal |
| Outside face, on still clear nights | Well insulated glass shedding heat to the sky | Nothing: it clears as the sun comes up |
| Between the two panes | The sealed unit has failed | Replace the unit |
The middle row surprises people. Good insulation means less heat reaches the outer pane, so on a clear, calm night that pane can fall below the dew point of the outside air. A film of dew appears outside, and it is a sign the glass is doing its job. It is not something to fix.
The bottom row is different. Moisture between the panes means the seal has gone and the cavity is open to the air. That unit needs replacing, and our page on misted and failed rooflight replacement covers how we go about it.
Letting the damp air out of the loft room
A warm pane cannot do everything. If the house keeps producing more moisture than it loses, it will find a cold spot somewhere.
The Building Regulations deal with this in Part F, which sets ventilation for kitchens, bathrooms and habitable rooms. Where we fit a conservation rooflight into a loft room or a cottage bedroom, the question to ask is how that room breathes. A trickle vent in the frame, or an opening unit that you can crack in the morning, gives the damp air an exit. Do conservation rooflights open explains which units have that option.
Extract fans in the kitchen and bathroom below do more for a rooflight than any coating on the glass. Keeping the door of a wet room shut, and venting it, stops moisture climbing the stairwell to the loft.
Old cottages, honest expectations
A solid-walled cottage that is cold, damp and closed up will show moisture on any surface. A new rooflight will not stop that by itself, though it will make the glass one of the last places it forms.
We match the build to the room. A loft bedroom slept in every night needs a unit and a reveal that hold their temperature. A storeroom under a slate roof needs less. Either way, the insulation and airtightness of the reveal are written into the quote, and the heat you keep is covered in double glazed conservation rooflights and heat loss. The wider picture of efficient glazing sits on energy efficient rooflights.
Questions from damp cottage owners
Will a new conservation rooflight stop my loft room feeling damp?
It removes one cold surface, which helps. Whether the room feels damp depends on the air in it and how it is ventilated, so we judge the room as well as the glass at the survey.
Is a little condensation on the glass normal?
Yes, on a cold morning after a night of breathing and a shower. It should clear within the hour. If it stays or pools on the frame, the reveal or the ventilation needs attention.
Does the thermal break change how the frame looks?
The break sits inside the section, so from the room and the street the frame keeps its slim, flat appearance. The look comes from the outline and colour, not from the hidden layers.
Can I ask for this in the quote?
Yes. Mention the room and any damp you already see through the quote form or on WhatsApp. Our main page on conservation rooflight installation in Crawley shows the range we fit.