What an ordinary VELUX pane does with sunshine
The standard pane on a current VELUX window is built to hold heat in during winter, not to keep the sun out in summer, so on a sunny slope it still lets a lot of solar heat through.
Current VELUX windows are double or triple glazed with an argon-filled gap and a low-emissivity coating on one of the inner faces. That coating is very good at stopping warmth from the room radiating back out through the glass on a cold night, which is why the U-value of a modern roof window is so much better than one from the 1990s. Sunlight is a different kind of energy. Most of it arrives as visible light and short-wave infrared, and a standard low-E pane is designed to let plenty of that pass, because in winter free solar warmth is welcome.
The figure that describes this is the g-value: the share of the sun's energy striking the glass that ends up inside the room. A clear double glazed unit with an ordinary low-E coating sits roughly in the range of 0.5 to 0.6, meaning around half or more of the solar energy gets through. Published overheating guidance puts it in household terms: a one metre square of clear double glazing at g 0.60, facing a clear June sky, can admit heat on the scale of a 540 watt heater. A VELUX window on a pitched roof catches the sun at a less direct angle than a flat rooflight, so the gain is usually lower, but on a steep south slope in midsummer it is still very noticeable in a small loft room.
What solar control glazing changes
A solar control pane carries an extra coating that reflects a larger part of the sun's infrared energy, lowering the g-value while letting most of the visible light through.
VELUX offers glazing with solar protection properties for many of its windows, alongside the standard double and triple panes. The coating works on the outer pane of the sealed unit, so much of the heat is turned back before it reaches the room rather than being absorbed and released inside. A lower g-value means less heat on the bed, the desk and the floor under the window, and less need to open the sash on a hot afternoon.
There is a trade to understand. Any coating that cuts solar energy also trims some visible light, and some solar control glass has a faint neutral or cool tint when you look up at it. On a north slope that loss buys you nothing, because the heat was never the problem. On a south or west slope over a bedroom it is usually a good exchange. The exact g-value and light transmission for a given window and pane are on the VELUX product data, and we check them for your size before we specify anything. Our answer on whether VELUX windows are double or triple glazed covers the thermal side, and whether VELUX glass is laminated covers the safety pane underneath.
Glass against an external awning blind
Glass alone only reduces the heat that reaches the room; an external awning stops a large part of the sun before it touches the glass at all, which is why it outperforms any coating on a hot slope.
Once sunlight has passed through the pane, anything inside the room, including an internal blind, is already holding that heat. A blackout or pleated blind reflects some of it back, but much of the warmth stays in the air between blind and glass and drifts into the room. The VELUX awning blind is a dark mesh stretched on the outside of the window. It intercepts the sun on the roof, so the glass, the lining and the room behind stay cooler, while you can still see the sky through it.
| Measure | Where heat stops | Daylight | Suits |
|---|---|---|---|
| Standard low-E pane | Partly, at the glass | Full | North and east slopes |
| Solar control pane | More, at the glass | Slightly less | South and west slopes |
| Internal blind | Inside the room | Reduced | Glare, privacy, sleep |
| External awning | Outside the glass | Dimmed, view kept | Hot south and west rooms |
The glass works every hour of every day without anyone touching it. The awning works only when it is down, and it can be rolled up in winter when you want the solar warmth. That is why the two complement each other rather than compete. The VELUX blinds guide sets out the awning, shutter and blackout options by room.
Which Cambridge slopes need what
The flat land around Cambridge means very few roofs are shaded by hills, so orientation decides almost everything about how hot a roof window runs.
A north slope sees sky light all day and direct sun only at the ends of a summer day, so standard glazing is enough. An east slope brings bright early sun into a bedroom, which is more a glare and sleep question than a heat one. South slopes take the high midday sun from spring to autumn, and west slopes catch the low, hot afternoon sun that arrives just as a room has already warmed up. A loft conversion in a gault brick terrace off Mill Road, with a small room and a low ceiling right under the slates, feels this far more than a tall stairwell in a 1930s semi. New homes at Eddington and Trumpington Meadows, with generous roof glazing and high airtightness, can hold summer heat for longer than an older, draughtier house.
Ventilation is the third part. Warm air collects at the top of a room, which is exactly where a roof window sits, so an open sash vents the hottest air first. An electric window with a rain sensor, described in our guide to INTEGRA electric and solar windows, or sensor control through VELUX ACTIVE, can open it while you are out.
How we decide it for your room
We work out the slope, the sun path and the use of the room first, then choose the pane, the blinds and the opening method to match.
Every quote comes with a written light plan that shows where the sun patch falls through the day and the year, and which glass and shading that slope needs. The same method is explained in how we plan the light. If an older window is already overheating a room, replacing an old VELUX window is the moment to move to solar control glazing. The full service is on our VELUX window installation page, or you can book a light survey.
Questions people also ask
Does triple glazing keep a loft room cooler in summer?
Not by much on its own. Triple glazing improves the U-value, so it keeps heat in during winter, and its extra pane lowers the g-value a little. A solar control coating or an external awning has a far bigger effect on summer heat.
Can solar control glass be added to an existing VELUX window?
The coating sits inside the sealed glazing unit, so it cannot be applied to the pane you have. It comes with a new window or, for some models and generations, a replacement sash. An external awning can usually be fitted to an existing window instead.
Does solar control glass make the room darker?
Slightly. It lets through most of the visible light but not all of it, and some panes carry a faint tint. On a sunny slope the difference is hard to notice against the brightness, while the drop in heat is easy to feel.
Is an internal blackout blind enough to stop the heat?
It helps with light and sleep, but the heat has already passed the glass by the time it meets the blind. For a hot room, pair the blackout with an external awning or roller shutter.
