Solar control glazing rejects a large share of the sun’s heat before it enters the room, using a microscopically thin metal-oxide coating on the glass rather than a tint you can see. It cuts the g-value from the 0.60 to 0.72 of clear glass down to roughly 0.28 to 0.38, while keeping light transmission above 60 per cent, so the room stays bright but no longer overheats. The trade-off is that every step of heat rejection costs a little daylight, so you specify it by orientation and room use: solar control on south and west-facing and all flat rooflights, moderate coating east, and usually clear glass on north-facing pitches where you want the light and there is little heat to reject.
A rooflight is the worst case for solar gain in the building, because it faces the open sky and collects sun all day. Getting the g-value right in the glass is the single most effective thing you can do about summer overheating, and it is the one fix you cannot retrofit without taking the unit out.
How the coating works
A solar control coating is a stack of extremely thin layers, often based on silver and metal oxides, deposited on one face of the sealed unit. It is engineered to let visible light through while reflecting and absorbing a large part of the near-infrared, which is where most of the sun’s heat energy sits. Because it works at the outer pane, it rejects the energy before it gets into the room. That is the crucial difference from an internal blind or a film, which only act after the heat is already inside. We set that comparison out on solar control glass or rooflight blinds.
Modern coatings are described as soft coat or hard coat. Soft coat performs better and is the norm in a sealed unit, protected inside the cavity. The key point for a homeowner is that these coatings are neutral to the eye. The heavily tinted blue and bronze glass people remember from the 1980s is largely historic. A good solar control rooflight looks like clear glass from the room.
The light versus heat trade-off
You cannot reject heat without giving up some daylight, because both travel through the glass together and the coating cannot perfectly separate them. Every step down in g-value costs a little light transmission. The art is finding the point where the room no longer overheats but still reads bright.
| Glazing | g-value | Light transmission | Character in the room |
|---|---|---|---|
| Clear double | 0.60 to 0.72 | 78 to 82% | Bright, but overheats on sunny aspects |
| Light solar control | 0.40 to 0.50 | 68 to 74% | Bright, moderate heat control, good for east |
| Standard solar control | 0.28 to 0.38 | 60 to 68% | Bright and neutral, strong heat control |
| High-performance solar control | 0.22 to 0.28 | 50 to 60% | Noticeably calmer light, strongest heat control |
Ranges vary by manufacturer and unit build-up. Your survey confirms the exact figures for the unit specified.
Below roughly 60 per cent light transmission the room starts to feel a little flatter, which is fine over a west-facing kitchen that bakes but wrong over a north-facing sitting room where you wanted the light. That is why the same product is not right everywhere, and why we specify from the compass.
Specifying by orientation
- North-facing pitch. Heat loss dominates and solar gain is mostly diffuse. Prioritise the U-value and keep light transmission high. A solar control coating here costs you brightness on a dull Suffolk day and buys almost nothing back in August. Clear or lightly coated is usually right.
- East-facing. Morning gain, which most people want in a kitchen. A moderate coating around 0.40 to 0.50 takes the edge off without dimming the morning light.
- South-facing. The default problem case. Standard solar control at 0.28 to 0.38, keeping light transmission above 60 per cent so the room reads bright rather than tinted.
- West-facing. The worst aspect, because peak gain arrives late in the day when the building has already absorbed a day of heat. Same coating as south, and we push harder for an opening vent so the hot air has a route out.
- Flat roof, any aspect. A flat or near-flat rooflight sees the whole sky dome whichever way the house faces. Treat it as south-facing and specify solar control regardless of orientation.
Room use changes the answer too. A home office wants glare control and neutral light, which pushes toward solar control even on a moderate aspect. A bathroom wants privacy more than daylight, so obscure or diffusing glazing matters more than the last few points of g-value. A bedroom under a rooflight wants to stay cool for sleep, so solar control plus a blackout blind. We match the glazing to what the room is for, not just which way it faces. Glare specifically is a different problem from heat, covered on what glazing reduces glare from a rooflight.
Solar control and U-value together
Specifying solar control does not mean giving up on heat loss. A solar control double unit with argon and a warm-edge spacer sits at roughly 1.2 to 1.4 W/m²K whole unit, which is as good as or better than the clear equivalent, because the same low-emissivity technology that rejects summer heat also slows winter heat loss. So on a south or west-facing rooflight you generally get both benefits from one specification. Where the design needs an even lower U-value, the solar control coating carries across to triple glazing, at 0.7 to 0.9 W/m²K for slim krypton-filled triple. We cover the full build-up options on specifying U-values and g-values for rooflights.
What it means locally
Most solar control work we do in Suffolk is on flat or shallow-pitched rooflights over single-storey kitchen extensions, which overheat regardless of aspect because they see the whole sky. On the newer estates at Kesgrave, Martlesham and Sproughton, where extensions tend to be large and glazed generously, getting the g-value right at order stage is what keeps the room usable through a hot spell without air conditioning. If your extension already overheats, the measures you can take now are on how to stop a room overheating under a rooflight.
Will solar control glazing make my rooflight room dark?
No. Standard solar control keeps light transmission above 60 per cent while cutting solar heat gain to roughly a third, so the room reads bright and neutral. Modern coatings are close to invisible, unlike the tinted glass people remember. Only high-performance coatings below about 60 per cent light transmission start to feel a little calmer, and we reserve those for aspects that genuinely bake. On north-facing pitches we usually leave the coating off so you keep the winter light.
Can I add solar control to an existing rooflight?
Not properly. A solar control coating is inside a sealed glazing unit, so adding it means a new unit, which on most rooflights means a new sealed unit or a full replacement. Retrofit film applied to the room side works on the wrong side of the glass, behaves more like an internal blind, and can cause thermal stress that cracks a pane, so we do not fit it. If the room overheats and the unit is otherwise sound, the honest options are an opening vent, external shading, or replacing the glass.
Get the glazing specified from the compass
We take orientation at survey and set the g-value and light transmission to suit the aspect and the room, then write both on the quote alongside the whole-unit U-value. One fixed price, a 10-year workmanship guarantee, and installers with 25 years on rooflights.
Request a fixed quote, read controlling solar gain through rooflights, or start from 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.