Why a window in the slope gains more heat than one in a wall
The angle of the glass does most of the damage: a pitched pane faces the high summer sun, while a wall window turns its shoulder to it.
Cambridge sits at about 52 degrees north. At midday in late June the sun stands roughly 61 degrees above the horizon, and a typical 40 degree slope facing south points its glass at 50 degrees above the horizon. The two are nearly aligned, so the sunlight arrives close to square on and very little bounces off the pane. A vertical window in the gable wall below meets the same sun at a steep glancing angle and reflects a good share of it away. Hour for hour, a roof window on that slope takes in considerably more solar energy than a wall window of the same area.
The loft itself adds to it. The room sits directly under the tiles, warm air rises into it from the floors below, and the ceiling is often only a little above head height, so the warm layer that gathers at the top of any room is sitting right beside you. Put those together with a clear pane in full sun and the room can climb several degrees above the rest of the house by late afternoon. None of that means a roof window is a poor choice. It means the heat has to be planned for, the same way the light is.
Which slopes overheat and which stay cool
South and west slopes carry most of the risk, east slopes give a warm morning that fades, and a north slope rarely overheats at all.
| Slope faces | Direct sun in summer | Heat risk |
|---|---|---|
| North | Early and late only | Low |
| East | Sunrise to late morning | Moderate, early |
| South | Late morning to mid afternoon | High |
| West | Mid afternoon to sunset | High, late |
West is the one that catches people out. The sun that reaches a west slope arrives after the roof and the room have been warming all day, and it keeps coming until eight or nine in the evening in June. A loft bedroom under a west-facing window can be at its warmest exactly when someone wants to sleep. South delivers the most energy in total, but the hottest part of it lands in the middle of the day, when a home office or a playroom may be the only rooms in use.
A north slope gives steady, soft light with barely any direct sun, which is why it suits studies and studios; our answer on roof windows on a north facing roof goes into that. On the Victorian terraces around Mill Road and Romsey, where the rear slope faces the opposite way from the front, choosing the cooler slope for the bedroom and the warmer one for a landing or bathroom is sometimes the simplest answer of all.
- North slope. Soft, steady light all day with no direct sun: even, glare-free and good for studios, kitchens and home offices.
- East slope. Bright morning sun that has moved off by lunchtime: bedrooms and breakfast rooms.
- South slope. The strongest light, peaking at midday. Worth pairing with solar-control glass or a blind to keep summer heat down.
- West slope. Warm, low evening sun: living rooms and kitchens used after work.
- The sun’s path. It rises in the east, is due south at noon and sets in the west, higher in summer and lower in winter.
What the glass can do before any blind goes up
The glass sets how much of the sun's energy gets through, and on a south or west slope it is worth paying attention to the g-value as well as the U-value.
The U-value measures how well the pane holds heat in during winter; current Building Regulations set a limit for roof windows and every new unit has to meet it. The g-value is the other half: the share of solar energy striking the glass that ends up inside the room. A standard low-E double glazed pane lets through roughly half. A solar control coating can bring that down markedly while keeping the view clear and most of the daylight, which is the balance a loft room needs. Triple glazing tends to lower the g-value a little as well, though its main job is keeping heat in.
Tinted and heavily coated panes cut more heat but also cut the light you put the window in for, and they can turn the view a little grey or blue. On a north slope they are rarely worth it. On a south slope over a bedroom they often are. Our guide to roof window glazing options sets each pane type against the room it suits, and the lower pane should be laminated in any case, since it sits over people.
Why the blind belongs on the outside
An external awning blind stops the sun before it reaches the glass, which is why it keeps a loft far cooler than any blind hung inside.
Once sunlight passes through the pane and hits an internal blind, the blind absorbs it and warms up, and that heat is already in the room. An internal blackout blind is still useful for sleep, but on its own it does little for temperature. An awning fitted on the outside of the frame is a fine mesh that shades the whole pane, keeps a filtered view out and lets air move when the sash is open. Plenty of loft rooms end up with both: the awning for the daytime heat, the blackout inside for dark at night.
Ventilation finishes the job. Warm air collects at the highest point of the room, and a roof window is usually close to that point, so opening it lets the hottest air out. A window high in the slope paired with a second lower one, or a roof window with an opening wall window below, sets up a flow that clears a stuffy loft quickly. Our guide to roof window ventilation covers the settings.
Sizing and placing the window with the heat in mind
How big the window is and where it sits in the slope matter as much as the glass, because both decide where the sun patch falls and for how long.
A window set low in the slope throws its sun patch across the floor and the bed; the same window set higher lights the ceiling and upper walls first and spreads the light more evenly. Our guide to positioning a roof window in a room works through sill and head heights. Size matters too, and two modest windows spread across the rafters usually give a more even, cooler room than one large pane. Oversizing is its own problem, set out in can a roof window be too big for a room.
This is the work our light planning does before anything is ordered. At the survey we check which way each slope faces, where the sun will fall on the room in June and in December, and what the room will be used for, then write down the position, size, glass and shading with the reason for each. For the wider picture on fitting, see our page on roof window installation in Cambridge, or book a light survey.
Questions people also ask
Will a roof window make my loft cold in winter?
Not if it is a current unit correctly fitted. A new roof window has to meet the thermal limits in Building Regulations, and the insulation collar and lining round the frame close the gap to the roof. A south slope will even give some welcome winter sun, since the low December sun still reaches it for a few hours around midday.
Is an east-facing roof window too hot for a bedroom?
Usually not for heat, since the sun has moved off by late morning and the room cools before night. The early light is the bigger question in a bedroom, so a blackout blind matters more than solar control glass there. Our guide to roof windows for loft bedrooms covers the choice of slope.
Can I add an external blind to an existing roof window?
In many cases, yes. Awning blinds are made to clip to the outside of the common roof window frames and are fitted from inside through the open sash. Very old units may need a replacement window to take one, and that is often the moment to upgrade the glass too.
Does a pitched roof skylight get hotter than a flat rooflight?
It depends on the slope. A flat rooflight faces straight up, so it takes the high summer sun from every direction through the middle of the day. A pitched skylight on a north slope gains far less than that, while one on a south slope can gain slightly more at midday. The slope you choose makes the difference.
