The loft room that felt so bright in March can be too hot to sleep in by July. People blame the window and often they are right, but not always. Heat reaches a loft from three directions at once: sunshine straight through the glass, sun-warmed tiles above the ceiling, and warm air that has risen from the rest of the house and has nowhere to go. A fix that deals with only one of them can leave the room almost as bad as before.
This guide sorts the causes from the cures. It starts with why a roof window collects so much sun, then goes through the options in the order that usually pays off: stopping the sun before it reaches the glass, changing the glass itself, letting hot air out, and looking at what sits in the roof around the window. It also covers the sensible questions to ask about blinds and awnings fixed to the roof, and when a window that overheats is a sign of something else wrong. Nothing here needs a specialist survey to understand, though a roofer who has seen your roof can tell you which of the options will suit your slope, your window and your budget.
Why roof windows run hotter than wall windows
A window in a vertical wall sees the sun at an angle for much of the day and is often shaded by the eaves. A roof window is tilted toward the sky, so it faces the sun more directly, and nothing overhangs it.
How does the heat get in?
Sunlight passes through the glass and lands on the floor, the furniture and the people in the room. Those surfaces warm up and give out heat as warm air, which the glass then holds in, much as a greenhouse does. The bigger the glazed area and the more directly it faces south, south-east or south-west, the more energy arrives. A window in a north slope has far less to deal with.
What else warms a loft room?
The roof itself. Tiles and slates take up heat all day, and a loft ceiling that has little insulation behind it passes some of that heat down. Warm air from the floors below rises and collects under the slope. A room with a small window and a thin roof build-up can overheat on its own, and a large window then adds to it. So when you pick a cure, it is worth knowing which of these is driving the temperature. A thermometer on a sunny afternoon, readings with the blind closed and open, and a note of which slope the window sits on will show you where the heat is coming from.
If you are weighing up a new window for a conversion and want to see how the other types compare, our roof windows and skylights page gives the wider picture.
Blinds and awnings: stop the sun at the glass
The most effective measures work on the sun before it comes through the pane, because heat that never enters needs no removing.
Do outside blinds work better than inside ones?
As a rule, yes, and the reasoning is simple. An inner blind catches the sun after it has crossed the glass, so the heat is already inside the room and the blind only softens how it reaches you. An outer blind or roller shutter stops the light on the outside of the glass, so far less of it gets in. Outer blinds are made to match particular roof windows, and some run on a motor or a small solar panel so they close without anyone climbing up. Solar-powered controls need no mains wiring in the roof.
What about internal blinds?
They are cheaper, easy to fit and still worth having. A pleated or honeycomb blind with a reflective face toward the glass does better than a thin white roller. Blackout blinds help sleep but can trap heat between blind and pane, so leave a gap at the top for air where the design allows it, and open the sash when the room is empty.
Can you fit an awning or shade sail?
Fixing an awning to a tiled roof needs brackets that go through the covering into the rafters, which creates holes that have to be sealed. It should be done by someone who knows how to flash a fixing, not screwed on in an afternoon. Anything fixed to the roof of a listed building, or one within a conservation area, should be cleared with the planning authority first. A free-standing shade or a tree in the right place avoids the problem altogether.
Glazing: what the glass itself can do
Some of the heat gain is built into the glass. Changing the glazing can help a lot, or very little, depending on what is in there now.
What is solar control glass?
It is glass with a thin coating that reflects part of the sun's energy while letting daylight through. Roof windows are sold with different glazing options, some aimed at heat control and some at insulation or noise. The ratings on the data sheet are what to compare, so ask the supplier for them in writing rather than going on the sales description. A coating that cuts glare and heat does make the room slightly less bright or tints it, and some people notice that.
Can you change only the glass?
Sometimes a glass unit can be swapped in a sound frame, which costs less than a new window. Our guide to replacing a Velux window explains the order of work and what stays in the room. When a whole new window is going in anyway, choose the glazing at that point.
What do the Building Regulations say?
The thermal targets in Approved Document L are about heat lost in winter, not sun gained in summer, so a unit can meet them and still run hot. Pick glazing that passes the thermal test and also controls solar gain, then ask your building control body whether a replacement must be notified.
Letting hot air out: ventilation that works
Even with good shading, some warm air builds up under the slope. Ventilation takes it away, and it works best when it follows the way hot air moves.
How does cross-ventilation work?
Warm air rises. If there is an opening low in the room and another high up, cooler air comes in at the bottom and the warm air leaves at the top, with no fan needed. In a loft that often means opening the roof window at the top of the slope and a gable window or a lower window on the other side, or leaving the landing window open. The roof window is useful because it is high, but it works poorly if there is nowhere low for replacement air to enter.
When should you open the window?
Early morning and late evening, when the outside air is cooler than the room. During the hottest hours, an open sash lets in warm air as well as letting it out, so close the window and the blind and open up again after sunset. Rain sensors on motorised windows close the sash if a shower arrives, which helps if you leave the house with the window open.
What about the roof space itself?
Ventilation in the roof space itself matters in summer as well as winter, because stale hot air under the tiles warms the ceiling below it and the room feels it. BS 5250, the moisture code, sets out ventilation gaps at the eaves for cold roofs with non-breathable underlay, 25 mm at pitches up to 15 degrees and 10 mm steeper than that. Roll insulation pushed tight to the eaves can close those gaps off, so check it has been kept back. If your window is leaking or misting up, that is a separate fault, and our answer on condensation on a roof window covers it.
Insulation, fans and the roof around the window
When the glass has been shaded and the air is moving and the room is still too hot, look at the roof behind the plasterboard.
Does insulation keep a loft cool?
Insulation slows heat in both directions. A well-insulated sloping ceiling keeps the sun-warmed tiles from heating the room as quickly in summer, and keeps the warmth in during winter. The roof has to stay ventilated where the design needs it, and the insulation must not close the eaves gap or press on the underlay. This is a job for someone who has seen the construction, not a guess from the room side. Fix the cause of any leak or heat problem before covering the roof structure with anything.
Do fans and air conditioning help?
A portable fan moves air across skin and feels cooler, but it does not lower the room temperature, and it does nothing in an empty room. A fan placed to push hot air out of the open roof window while pulling cooler air in from a low window does help. Air conditioning is a last resort, and it removes the heat that the glass keeps adding, so the shading is better value.
When is the window the wrong place for glass?
If the window faces due south and the room gets too hot even after the measures above, a smaller or differently placed window might be better, or a window swapped for a type with a solar control option. A flat roof is a different case, since a big glass lantern gathers heat from several angles at once, which is worth reading about in our comparison of a flat rooflight and a roof lantern.
A sensible order of action
It helps to start with the lowest-cost steps and move up only if you need to. A realistic order for most homes runs like this.
What do you try first?
Start with behaviour and cheap shading. Close blinds before the sun reaches the window, open the sash and a low window in the cool of the morning and evening, and put a thermometer in the room to see what changes. If that is enough, stop there.
What is next?
Add an outer blind or a shade, then look at the glass. Ask a supplier for the solar control ratings of the glazing in your window and the alternatives. Compare quotes by their final figure, and confirm what is covered: VAT (20% at the standard rate), and any scaffold or roof access.
When should a roofer look?
When the room overheats despite shading and airflow, or when a hot, stuffy window also shows damp, stains or a musty smell. Those point to ventilation or insulation faults in the roof, and they need seeing in person. If you have any sign of bats in the loft, mention it before work begins, because their roosts have legal protection.
What helps us advise?
Send a photo of the window from inside and out, the direction it faces, the size code on its label and a note of when the room gets too hot. A roofer who can see the slope, the glazing and the loft behind it can tell you whether shading alone will do or the window itself should change, and the quote that follows comes as one written total.
Part of our Velux Roof Windows & Skylights in Crawley
Hot loft questions
Is an outside blind really better than one inside the room?
Usually, because it stops sunlight before it reaches the glass. An inside blind works after the sun has passed through, so the heat is already in the room. Outer blinds are made for particular window sizes, so send us the label code and a photo of the roof, and we can see how it would be fixed.
Will new glass stop my loft room overheating by itself?
It helps if you choose solar control glazing, but it rarely solves everything alone. Shading, airflow and the roof build-up all play a part. Ask the supplier for the solar ratings in writing, and compare them with your current glass before paying for a new unit.
Can I open a roof window all day to cool a loft?
In the heat of the day it can bring in warm air, so open it early and late and keep it shut with the blind drawn when the sun is high. A low window on the opposite side lets cooler air flow through. A rain sensor on a motorised sash is useful if you go out.
