Solar gain through a rooflight is driven by three things: the g-value of the glass, the orientation and pitch of the roof, and the area of glazing relative to the room. Get the g-value right and you fix most of it before anything is installed. A rooflight is the worst case for overheating in the whole building, because it faces the open sky and collects sun from dawn to dusk rather than for a few oblique hours like a wall window. The fixes, in order of what works, run from glazing specification, to an opening vent, to external shading, to internal blinds last.
That order matters, because most people plan it in reverse. They buy clear glass, discover the room is unusable in July, and try to solve it with a blind. This page explains why that is the expensive way round.
The three drivers of overheating
g-value: how much of the sun gets in
The g-value, or solar factor, is the proportion of solar energy hitting the glass that ends up as heat in the room. It runs from 0 to 1. Clear double glazing sits at roughly 0.60 to 0.72, so around two thirds of the sun’s energy comes straight through. A solar control unit brings that down to roughly 0.28 to 0.38 while keeping the room bright. We cover the number in full on what g-value means on a rooflight.
Orientation and pitch: how long the glass collects
A south or west-facing wall window sees the sun at an oblique angle for part of the day. A rooflight faces the sky, so it collects from sunrise to sunset, and a shallow-pitched or flat rooflight is close to square-on to a high summer sun for hours. That is why the same glass overheats a room far more in the ceiling than it would in the wall.
Glazed area: how much sky the room sees
Double the rooflight area and you roughly double the solar load, all else equal. A modest rooflight over a large room rarely overheats. A big lantern over a small snug does, because the glazed element dominates. This is the one people underestimate at design stage, when a bigger rooflight always looks like the better idea.
| Orientation | Summer solar load | Recommended glazing | Target g-value |
|---|---|---|---|
| North-facing pitch | Low, mostly diffuse | Clear, keep light high | 0.55 to 0.68 |
| East-facing pitch | Morning gain | Moderate coating | 0.40 to 0.50 |
| South-facing pitch | High, midday | Solar control | 0.28 to 0.38 |
| West-facing pitch | Highest felt, late day | Solar control plus vent | 0.28 to 0.36 |
| Flat, any aspect | Whole sky dome | Treat as south | 0.28 to 0.38 |
Ranking the fixes
Every measure below reduces overheating. They are not equal, and they are not equally expensive to add after the fact. Here they are in order of effectiveness, which happens to be roughly the order of how cheaply they can be built in at installation.
- Get the g-value right in the glass. Solar control glass rejects energy at the outer surface, before it enters the room, cutting solar heat to roughly a third while the room still reads bright. This is the single biggest lever and it costs nothing to run. It is also the one thing you cannot retrofit without taking the unit out. Our full write-up is on solar control glazing for rooflights.
- Add an opening vent. Hot air collects at ceiling level directly under the rooflight. An opening rooflight or a powered vent lets that hot air escape and pulls cooler air in low down. This purge ventilation is genuinely effective and it wants to be planned before the ceiling closes, because retrofitting a vent means removing the unit. Powered options are on our electric skylight installation page.
- External shading. An external blind, brise soleil or shade intercepts the energy outside the glass, so it works well in physics terms. On a domestic roof it is awkward, exposed to weather and needs cleaning, so it is rarely the right answer here.
- Internal blinds. An internal blind stops light after the energy has already passed through the glass, so the heat is already inside and the blind warms up and re-radiates it. It reduces glare well and cuts heat load only modestly. Useful for privacy and glare, weak as a heat fix on its own. We compare the two directly on solar control glass or rooflight blinds.
Why the order is the cheap order. Retrofitting a vent or better glass means taking the unit out, which costs more than the upgrade would have at installation. Doing it in the right order, glass first, vent second, blind last, costs less than doing it in reverse. That is not a sales line, it is the sequence in which the heat arrives and the sequence in which each fix has to be built.
The honest limits of retrofit film
Solar control film applied to the room side of existing glass is the tempting cheap fix, and it does something. But it is working on the wrong side of the glass, so it behaves more like an internal blind than like solar control glass. It also carries a real risk on a sealed unit: film absorbs heat, the glass heats unevenly, and thermal stress can crack a pane or stress the edge seal. On a rooflight, which already runs hot in the sun, that risk is higher than on a wall window. We do not fit film, and we would rather you knew the limit than found it out.
If the room already overheats and the unit is otherwise sound, the honest choices are an external shade, an opening vent added at the next opportunity, or glass replacement. Which one makes sense depends on the unit, the room and the budget, and that is a survey conversation.
What it means for a Suffolk extension
Most overheating we are called to look at is a flat or shallow-pitched rooflight over a single-storey kitchen extension, glazed clear because clear was cheaper, facing anywhere from south to west. The room is beautiful in spring and unusable by mid-afternoon in July. The fix on a replacement is a solar control unit at a g-value around 0.30, an opening vent for the hot air at the top, and a blind for glare and night privacy. Built in that order it is a comfortable room year round. Bolted on backwards it is a series of half-measures.
If your extension is already too hot, start with how to stop a room overheating under a rooflight for the measures you can take now, and whether the rooflight is to blame for the diagnosis.
Does orientation fix overheating on a flat rooflight?
No. A flat or near-flat rooflight faces the whole sky regardless of which way the house points, so it collects solar gain like a south-facing unit whatever the aspect. On a flat roof you have to control gain in the glass with a solar control g-value, not by pointing the room a different way. Orientation only helps on a pitched roof, where a north-facing slope genuinely sees far less sun.
Will solar control glass make the room gloomy?
No. A modern solar control unit keeps light transmission above 60 per cent while cutting solar heat gain to roughly a third. The room reads bright and neutral, not tinted. The heavily tinted blue and bronze glass people remember is largely historic. On a north-facing roof we usually leave the coating off, because you lose useful winter brightness and gain very little in August.
Get the glazing specified from the compass
We take orientation with a compass at survey and set the g-value from it, before anything is ordered. You get one fixed price with the g-value, light transmission and whole-unit U-value written on it. The installers have 25 years fitting rooflights and every job carries a 10-year workmanship guarantee.
Request a fixed quote for a rooflight specified to control overheating, or read the wider picture on energy efficient skylight installation.
Thinking about this job? We survey across Ipswich, Suffolk and north Essex and give you a fixed price before any work starts, with a 10-year workmanship guarantee.