A south-facing extension is finished in spring and everyone is delighted with the light. By July the family has stopped sitting at the end of the table beneath the glass, the blinds are down at noon and the thermometer on the wall is reading more than the one in the hallway. Nothing is wrong with the build. The roof is simply letting a lot of sun in.
Solar control glass is the specification choice that deals with this at the source, by reducing the heat that passes through the pane before it reaches the room. It is not suited to every roof, and it changes the look of the light, so it is worth deciding on purpose. The sections below cover what it does, how orientation decides whether you need it, the difference between tints and clear coatings, what it does to light and colour indoors, how it combines with other coatings and what to put in the order. The wider subject of heat, glass and comfort is on the energy efficient rooflights page.
What solar control glass does
Sunlight carries both light and heat. Ordinary clear glass lets most of both through. Solar control glass is made to let in a smaller share of the heat while still passing a good share of the light. It does this either with a very thin coating on one of the glass surfaces, or with a tint through the body of the glass, or in some products with both.
The measure for this is the g-value, sometimes called the solar factor or total solar energy transmittance. It runs from 0 to 1 and says what proportion of the sun's energy reaches the room. A lower g-value means less solar gain. A separate figure, light transmission, says how much visible light gets through. A good solar control product keeps the g-value low while holding the light transmission up, and the gap between the two is what you are paying for.
It is a different job from insulation. The U-value, explained in the guide to rooflight U-values, measures how fast heat leaves through the glass in winter. The g-value measures how much heat arrives in summer. A unit can be strong on one and weak on the other, and a unit with a very low g-value can lose some of the free winter warmth that the sun would otherwise give.
That trade-off is the reason solar control glass is not a default. In a room that is cold in winter and mild in summer, plain clear glazing may suit better. In a room that is bright, large and faces the sun, the coating can make the difference between a room used all summer and one avoided between noon and four.
Orientation and the roof
Where the rooflight faces matters more than any other factor. The sun is high and strong in summer, and a rooflight looks straight up at it. A vertical window sees the sun at a lower angle and is partly shaded by the eaves, but a roof window or flat rooflight takes the sun directly for most of the day. That is why a roof glazed room can overheat faster than a room with the same area of wall glazing.
| Roof faces | What happens | Solar control glass |
|---|---|---|
| South | Strong sun for most of the day | Usually worth considering |
| West | Low, hot afternoon and evening sun | Often worth considering |
| East | Morning sun, cooler afternoons | Sometimes, if the room is large |
| North | Little direct sun | Rarely needed |
| Flat roof | Sun from every direction as it moves | Depends on shade and glass area |
A flat roof does not have an orientation in the same way, but the sun still crosses it. What decides the case for a flat rooflight is how much glass there is and what shades it. A large lantern in an unshaded garden on a south-west plot is a stronger candidate than a small unit under a mature tree.
Room use counts as well. A home office, a kitchen where people sit all day or a bedroom under a roof window will notice overheating most. A utility room or a landing is a much lower priority. Shading from nearby trees, a neighbour's building or a deep overhang can change the picture, and we take a note of what shades the roof when we survey. If the extension is part of a bigger scheme, the architect's overheating calculation may already point to a target g-value, and that figure should reach us with the drawings.
Orientation is also a reason to look at the condensation question in the same breath. The condensation guide explains how the glass specification and the room work together, and the two topics often come up on the same job.
Tints and clear coatings
There are two broad families of solar control glass, and they look and behave differently. It is a good idea to see a sample of any glass, in daylight, against the room's finishes, before the order goes in.
Tinted glass
A tint is colour added to the body of the glass, usually grey, bronze or green. The glass absorbs part of the sun's energy, which warms the pane itself, and some of that heat passes on to the room. Tints are simple and long established, and they cut glare noticeably. They also darken the view and the room and can give the daylight a coloured cast.
Coated glass
A solar control coating is a metal-oxide layer, invisible at normal viewing angles, that reflects part of the sun's energy back out instead of absorbing it. The glass can look almost as clear as ordinary glass, with a faint tone in some lights. Because it reflects instead of absorbing, the pane runs cooler than a tinted one at the same level of control. Coated glass is the usual choice where people want the room to stay bright and the sky to look natural.
Some makers offer a range of coatings at different strengths. A lighter coating suits a room that needs bright light and moderate control, a stronger one suits a hot, exposed aspect. The maker's data sheet gives the g-value and the light transmission for each, and the quote lists the one you are buying. Where the rooflight is large, the glass may also need to be toughened or laminated for strength, and a tinted pane can be more stressed by uneven heating, so the maker's advice on the combination should be followed.
Light and colour in the room
Every solar control product costs a little light. How much, and in what colour, differs from one product to another. A rooflight is often chosen to flood a room with daylight, so a glass that takes a lot of the light away works against the purpose. A good coating keeps the loss modest, and a poor choice makes a bright kitchen look dull.
Colour is the second consideration. Tints cast their tone onto the room: grey can make the light a little cooler and flatter, bronze warms it and green gives a cooler tone. A coating may give a faint blue, green or neutral reflection when viewed from outside, which is how you can spot it on the roof. Neither is wrong, but it should be a choice, not a surprise.
From the garden, a reflective coating can make the glass look slightly mirrored in bright light, particularly at certain angles. If the roof is overlooked or sits within a conservation area, that look is something to talk through with the local planning authority, because appearance is part of what they weigh. The planning position for rooflights is set out in the planning permission guide, and the position at your address is something we check and report back on, with the council making the decision.
Glare matters too. A room used for screens, such as a home office, can be tiring under a bright, unshaded roof. A tint or coating that cuts glare helps. For a dining space where the sky and the garden are part of the appeal, a clearer coating is a better fit.
- Look at a full-size sample against the floor and wall colours.
- View it in daylight, at different times, not under shop lighting.
- Hold it at the angle it will sit on the roof.
- Ask for the g-value and the light transmission in writing.
Combining with other coatings
A rooflight pane can carry more than one coating, and the sensible approach is to treat the glass as a system. A double-glazed unit has four glass faces. The low-e coating that helps winter insulation sits on one, a solar control coating on another, and a self-cleaning coating on the outer face of the unit. The maker arranges them in the unit to work together.
Some products combine solar control and low-e in a single coating stack, so the unit gives good insulation and reduced solar gain without a second layer. This is common in modern rooflights, and it is simpler to specify. Always read the data sheet to see the Ug, the g-value and the light transmission for the combined unit, not for the separate parts.
Self-cleaning glass carries its coating on the outer face that breaks down organic dirt in daylight and lets rain wash it away. It does not alter solar gain in a meaningful way, and it is a separate decision, which the self-cleaning glass answer covers. On a hard-to-reach flat roof it can reduce how often the glass needs washing.
A triple-glazed unit adds another pane and may change the balance again, since more panes cut light and gain together. The triple glazing answer looks at when it earns its place. Heat loss in winter, which these choices change, is discussed in do skylights lose a lot of heat.
| Layer | Job | Season it helps |
|---|---|---|
| Low-e coating | Reflects room heat back inside | Winter |
| Solar control coating | Reflects sun energy outside | Summer |
| Self-cleaning coating | Breaks down dirt on the outer face | Year round |
| Tint | Absorbs part of the sun's energy | Summer |
Ordering the right glass
By the time of the order, the decision should be written down. Glass is made to size for each rooflight, so a change once we have ordered means a new order and a new wait. The lead time is given by the maker, named in the quote and confirmed when we order, and special coatings can sit differently on the maker's list from standard glass.
We recommend settling these points before you sign the quote: which way the roof faces and what shades it, the room's use, the g-value and the light transmission you are aiming for, the U-value required by Part L, whether the glass needs toughening or laminating, and whether there is a preference for the look of the coating from outside. If an architect has produced an overheating calculation, send it with your request.
A free survey lets us look at the roof, the shading and the room on the day, and the written quote then names the unit and its glazing. The builder gets the opening and kerb sizes for that exact unit before the roof structure is made, and the order is timed to the build programme so the roof is not left waiting for glass. Solar control glass is no slower to arrange if it is decided early, and no faster if it is left until the glass is due.
Glass is not the only way to manage summer heat. External blinds, internal blinds, shading from a pergola or a tree and opening the rooflight on a warm evening all help, and an opening unit lets the hottest air leave through the top of the room. If you are weighing how to open it, the manual or electric comparison explains the choice. A well-chosen glass reduces how much work those measures have to do.
Before you choose
- Does solar control glass make a room dark?
- It lowers the light slightly, by an amount that depends on the product. A good coated glass stays bright. Ask for the light transmission figure and compare it with clear glazing.
- Do I need it on a north-facing rooflight?
- Usually not, since there is little direct sun. A large north-facing unit with an exposed, unshaded outlook might still benefit, but the case is weaker than on a south or west roof.
