Air for the room under the glass
Rooms need a steady, small supply of fresh air to carry away moisture from breathing, cooking and washing. A roof window helps in two ways. The sash opens, so you can air the room quickly, and many windows carry a vent in the top of the frame, which is a narrow slot that lets a trickle of air in even when the sash is shut. Trickle vents give background ventilation, and they matter most where a loft room has been fitted out and sealed up.
Without background air, damp builds up on cold surfaces. The glass is usually the coldest part of a loft room, so it is where you see the first signs: a wet pane in the morning, then dark spotting on the lining. Our page on roof window condensation sets out the causes and separates condensation from a leak.
If you are creating a new bedroom, ask your council's building control team what ventilation the room needs. That is a Building Regulations question, and the answer depends on the room and on the way the loft has been converted.
The roof around the window
Ventilation inside the room is one thing. The roof structure around the window is another, and it has its own rules. BS 5250:2021 is the British Standard on moisture that is in force today, and roofers work from it when they decide how a roof should breathe.
Where the roof is a cold roof, meaning the insulation sits at ceiling level and the space above it is vented, Approved Document C treats a ventilated cold deck as one way of satisfying it. Marley's guidance for a cold roof under non-breathable underlay gives an eaves gap of 25 mm for pitches of 15 degrees or less, and 10 mm for steeper ones.
A roof window interrupts that airflow. The opening cuts across rafters, and the insulation and lining around the frame form a short stretch of warm roof. The air path between the eaves and the ridge has to be kept clear or carried round the opening, which is a detail for the survey and not a guess. Cutting a hole without allowing for it is how moisture ends up in the rafters beside a new window.
The same care applies when a window is swapped for another, if the lining or the insulation is disturbed.
The energy figure for the window
Ventilation and energy go together, because a cold pane is where the condensation described above forms. Approved Document L sets the standard when a roof window goes into an existing home. In Table 4.2, the line for a window, a term that takes in roof windows, sets 1.4 as the U-value, with Band B accepted as the other way to qualify. A rooflight, meaning a fixed or flat-roof unit, has its own figure of 2.2.
Those are different numbers for different products, and the difference is explained on our page about the skylight and roof window distinction. The edition that applies today is AD L 2021 with its 2023 amendments. A new edition, AD L 2026, takes effect on 24 March 2027.
- A roof window: 1.4 W/m²K, or Band B.
- A rooflight: 2.2 W/m²K.
- Compare the U-value on the data sheet, not the brand name.
The U-value tells you about heat loss. It does not tell you about airflow, so you need both facts when you choose, and a supplier should be able to give the vent details alongside the thermal figure.
Planning ventilation before the order
Before ordering, work through this list with whoever is quoting:
- How does the sash open, and is there a vent in the frame for background air?
- Does the roof under and around the window keep a clear route for air from eaves to ridge?
- Is the insulation lapped and sealed round the frame so warm, damp air cannot reach cold timber?
- If the room will be lived in, has building control been asked about ventilation and approval?
Those questions apply whether the window is new or a replacement. A swap in the same opening is simpler, but the checks stay the same.
We look at the roof structure around the opening before anything is cut, and the quote is one written total after that survey. For the larger subject, see roof windows and skylights. The right answer for a bedroom, a bathroom or a stairwell is not the same, so say which it is when you ask, and mention any damp you have already seen, such as spotting on the lining or a wet sill.
