Solar control glass for rooflights

A rooflight is the most exposed piece of glazing in the house. A wall window sees the sun at an angle and is often shaded by eaves, trees or the house next door. A rooflight faces the sky, and in the middle of the day in summer the sun is high and strong. Sunlight passes through the glass, lands on the floor, the furniture and the people, and is absorbed as heat. That heat then has nowhere to go, because the glass that let the energy in is poor at letting it back out.

The effect is strongest where the rooflight is large and the room is small, which describes a lot of loft conversions. A roof window in a sloping ceiling over a bed catches the sun for hours, and the room under a slate or tile roof is already warmed from above. Kitchen extensions with a big lantern or a flat rooflight have the same trouble on a larger scale. The house may be perfectly comfortable in March and hard to sit in by July.

Heat gain is a separate problem from heat loss. Choosing between double and triple glazing, covered in double or triple glazed rooflights, is mostly about keeping warmth in during winter. Solar control is about keeping the sun's heat out in summer, and the two can pull against each other if you are not careful about which room needs what.

A manufacturer's data sheet for a rooflight glass carries three numbers that matter here, and they are worth asking for by name.

g-value

Also called total solar energy transmittance, this is the share of the sun's energy that gets through the glass as heat, shown as a number between 0 and 1. A lower g-value means less solar gain. Clear double glazing has a high g-value. Solar control glass brings it down.

Light transmission

The share of visible light that passes through, again as a percentage or a fraction. This is what decides how bright the room feels. A coating that cuts heat well but also cuts light heavily will leave you with a dimmer room and the lights on earlier in the afternoon.

U-value

How well the glass holds heat in. A solar control coating can be combined with a low-emissivity layer so the unit still insulates well. Our guide to rooflight U-values explained covers how to read it.

The aim is a glass with a low g-value and a light transmission you are happy with. The ratio between the two is sometimes given as a selectivity figure: the higher it is, the more daylight you keep for each unit of heat you turn away.

There are several ways to make glass reject heat, and each gives a different look and a different balance.

Coated glass

A very thin layer of metal oxides is deposited on one pane of the sealed unit, usually on a surface inside the cavity where it is protected. The coating reflects part of the sun's energy while letting most visible light through. This is the most common choice for a house because the glass still looks close to clear, with at most a faint cast of blue, green or grey depending on the coating.

Tinted glass

The glass itself is coloured through its body, in grey, bronze or green, and absorbs part of the sun's energy. It is a simple, durable option, but it works by absorbing heat, which warms the glass itself, and it changes the colour of the daylight in the room. Grey and bronze are the usual tones. It is less selective than a coating, so it takes light away at much the same rate as it takes heat.

Reflective glass

A heavier metallic coating gives a mirrored appearance from outside. It rejects a lot of heat, but it is rarely a good fit for a house, because the glass looks conspicuous on a roof and can show up in a streetscape or a conservation area.

Laminated solar control

The inner pane is two sheets bonded with an interlayer, and the solar control sits in the coating or in the interlayer. Laminated inner panes overhead are good practice, because if the glass is ever broken the pieces stay held in place, and this type gives you both safety and solar control in one pane. It is common in lanterns and flat rooflights over seating.

Self-cleaning and other coatings

Some units also carry an outer coating that helps rain wash dirt away. It can sit alongside a solar control layer, and our answer on self-cleaning glass on a rooflight explains when it is worth having.

Orientation is the first test. A rooflight on a south-facing slope collects sun for most of the day. East and west faces catch it in the morning or the late afternoon, when the sun is low and the heat is strong. A north slope rarely sees direct sun at all. In Exeter and the villages around it, where plenty of terraced and semi-detached houses run roughly east to west along a street, one slope or the other is often a good candidate.

The room matters next. A loft bedroom under a modern tiled roof, a Victorian villa's top-floor study in Torquay or a bright kitchen extension in a 1930s semi in Heavitree all collect heat from the roof as well as the glass. A cold north-facing stairwell does not. Thatch, thick cob walls and deep roof builds, such as those in cottages across the Culm and Exe valleys, hold temperatures steadier, so a room under them may need less help than a thin-roofed loft in a newer house.

Finally, ask how much of the roof is glass. A single small roof window over a landing may not justify anything beyond a good standard unit. A large flat rooflight or a lantern over a kitchen island is a very different amount of sky. In that case solar control is a part of the design rather than an extra, because blinds alone have to fight a great deal of heat coming through a large area.

An internal blind stops light and glare, but the heat is already inside by the time it works, because the glass has absorbed and passed the energy. An external shade does better, since it stops the sun at the outside, though it must cope with wind and weather. Solar control glass is always on, which is its strength and its cost: it also reduces the useful winter sun.

Every solar control glass cuts some daylight, and a modest cut in a bright room is easy to live with. In a dim room it can feel like a loss. The way to keep the light is to ask for a glass with good selectivity, and to look at a sample in the room if you can, because a glass that looks fine on a data sheet can read differently against your own ceiling and walls.

Colour is part of it. A neutral coating keeps the glass looking clear. A grey or bronze tint changes the colour of the light, which shows up most on white ceilings and pale plaster. If the room has a lime-washed or warm-painted finish, a strong tint can make it look dull. Ask which coating is used and what it does to the colour of the sky seen through it, and look at the glass against a white sheet of paper before you commit.

The lining below the glass can help the light you do keep. A splayed reveal spreads what comes through, so the room looks brighter than the transmission figure would suggest. That is one reason we form the lining to suit the room instead of leaving a plain box, a point developed in how a rooflight is finished inside.

Ventilation helps as well. Hot air collects at the highest point of a room, so an opening rooflight lets it escape in a way a fixed one cannot. Pairing a solar control unit with an opening or electric rooflight gives the room two defences, one that reduces the heat arriving and one that lets out what does arrive. Our electric opening rooflights page covers the opening side.

A new rooflight, or the replacement of a whole unit, has to meet Part L of the Building Regulations, and Approved Document L Volume 1 (2021 edition) sets the limiting U-values. Solar control glass can be specified to meet them, since the coating can be combined with a low-emissivity layer. The whole-unit replacement is notifiable, and we handle the Building Control notification where the work is notifiable.

The same document sets the framework for limiting solar gain in new work, and the guidance on overheating applies to new dwellings. It is not something to apply to an existing home as a requirement, though the thinking behind it is useful: a room that is comfortable in July is better than one that needs a fan. You can read the current documents on gov.uk.

On safety, a laminated inner pane overhead is guidance and good practice rather than a Part K requirement, and it combines well with solar control. For rooflights over a staircase, a bed or a seating area, it is a sensible choice. For walk-on glass the specification is a different matter and follows structural loading guidance.

Quotes often say only "solar control" or "low-E", which hides the real differences between products. Ask for these in writing:

• the g-value and the light transmission of the unit as a whole, not of a single pane;
• the U-value of the unit, and whether it is for the glass alone or the whole rooflight;
• where the coating sits and whether the inner pane is laminated;
• the colour of the glass as seen from outside and from inside;
• whether the glass is the same on every unit if you are fitting several.

Matching glass across a run of units matters more than people expect. Two rooflights side by side with slightly different coatings can look mismatched from the garden, and that is cheap to avoid when it is decided at the start.

A rooflight is chosen on the roof and judged in the room, so the glass decision belongs with the finish decision. Because solar control reduces the light slightly, a bright, crisp lining and a well-formed splay do more work than they would under clear glass. Because the inner pane may be thicker or laminated, the frame depth and the lining's meeting edge need checking. And because heat and condensation behave differently in a room with strong sun, the insulation behind the lining deserves attention.

All of that goes into the finish schedule that comes with every Brightview quote, alongside the ceiling build and how the lining will be formed and decorated. For the wider energy picture, see our energy efficient rooflights page. If a room of yours runs hot, tell us which way the roof faces through the quote form, and send a photo of the room and the roof if you can.