Solar Control Glass for Rooflights

Chapter 1

What solar control glass is for

Solar control glass carries a microscopically thin coating that lets most of the visible light through while turning back a large share of the invisible infrared heat in sunshine. On a rooflight, where the glass faces the sky, that selectivity is what keeps a bright room from becoming a hot one.

Sunlight arriving at a roof in Cambridge is not one thing. Roughly half of its energy sits in the near infrared, which you feel as warmth on your skin but cannot see. A little over two fifths is visible light, the part that actually lights the room. The small remainder is ultraviolet. Ordinary clear glass treats all of this much the same way and lets the bulk of it through. A solar control coating is tuned to discriminate: it passes as much of the visible band as it can and reflects as much of the infrared as it can.

That is a different job from simply darkening the glass. Tinted glass from decades past cut heat by absorbing energy across the whole spectrum, so it cut the daylight almost as hard as the heat, and the absorbed energy warmed the pane itself. A modern coated unit reflects the unwanted part before it enters, and it can let through a good deal more light for each unit of heat it admits. The measure of how well it does this is the relationship between two figures on the datasheet: light transmission and g-value, which the next chapter unpacks.

A solar control glass rooflight is therefore not a gloomy rooflight. Chosen well, it gives a room most of the daylight of a clear unit with a fraction of the summer heat. Chosen badly, or fitted on a slope that never needed it, it gives up daylight for nothing. This guide covers how the coatings work, how to read their numbers, and, most usefully, which roof slopes in Cambridge homes genuinely call for one. For the wider picture on energy performance, start at the energy efficient rooflights hub.

Chapter 2

How the coating sits inside the unit

The coating is a stack of metal and metal oxide layers, often including silver, laid onto one face of the glass in a vacuum chamber. It is fragile in open air, so it always sits inside the sealed cavity of a double or triple glazed skylight, facing the gas fill.

Glass makers describe the faces of a sealed unit by number, counting from the outside. Face 1 is the outer surface that meets the rain, face 2 is the inner surface of the outer pane, face 3 is the first surface of the next pane in, and so on. A solar control coating almost always goes on face 2. There it meets the sun first, reflects the infrared back out through a single thickness of glass, and never has to deal with weather or cleaning.

The layers themselves are thinner than a wavelength of light. That is the point: a silver layer a few billionths of a metre thick is thin enough to pass visible light but continuous enough to reflect the longer infrared wavelengths. The more silver layers in the stack, the sharper that cut between light and heat can be. Single silver coatings are the everyday low emissivity products; double and triple silver stacks are the ones that earn the solar control name, because they add strong reflection of incoming solar infrared on top of keeping room heat in.

This is why solar control and low-E coatings and argon fill are closely related but not the same. A standard low-E coating is mainly aimed at the long-wave heat radiated by a warm room, and it helps keep that heat indoors in winter. A solar control coating adds control over the short-wave heat arriving from the sun. Many modern solar control coatings do both jobs in one stack, which means a rooflight can have a single coated face that improves its U-value and cuts its summer gain. Some specifications combine a solar control coating on face 2 with a separate low-E coating further in, particularly in triple glazing.

Because the coating is sealed inside, it cannot be added later to glass you already have. A sealed unit is made up as one assembly at the factory, and the coating goes on before the panes are joined.

Chapter 3

Reading the numbers on a datasheet

Two figures describe what a coating does to sunlight: light transmission, the share of visible light that gets through, and g-value, the share of total solar energy that ends up in the room. Divide the first by the second and you have the coating's selectivity.

Light transmission is usually written as LT or Tv and quoted as a percentage or a decimal. A figure of 0.70 means seventy per cent of the visible light striking the glass passes into the room. The g-value, sometimes called the solar factor, counts everything that ends up indoors: the sunlight that passes straight through, plus the share of energy absorbed by the glass that is re-radiated inwards. A g-value of 0.35 means about a third of the solar energy arriving at the glass ends up as heat in the room.

Selectivity is the light to solar gain ratio, LT divided by g. Clear double glazing with an ordinary low-E coating sits a little above 1, so it lets light and heat through in roughly equal proportion. A good solar control coating reaches 1.7 to 2 or so, which means it passes close to twice as much light as heat. There is a physical ceiling near 2 for daylight, because some of the solar energy is in the visible band and any glass that passes visible light also passes that energy. A product claiming far beyond that deserves a second look.

Treat those as rough bands that show the shape of the choice, not as figures for any particular product. The values for the exact glass build-up, including any laminated inner pane or third pane, are on the manufacturer's declaration, and those are what a light plan should be written against. Our separate guide to g-value and light transmission goes further into balancing the two for a given room.

One practical detail: check whether the figures quoted are for the glass alone or for the whole rooflight. The frame lets no light through, so a whole-unit light figure is always lower than the centre-pane one, and a small unit with a chunky frame loses proportionally more than a large one.

Chapter 4

Which slopes need it

Whether a solar control glass skylight is worth specifying depends above all on which way the glass faces and how steeply. In Cambridge, a flat rooflight and a south slope are the clear cases; a north slope rarely needs it at all.

Cambridge sits at about 52 degrees north. At midday on the summer solstice the sun climbs to roughly 61 degrees above the horizon; at midday in late December it manages only about 14 degrees. Glass gathers the most energy when the sun strikes it square on, and less as the angle becomes more glancing. So the question for any rooflight is how directly its glass faces the high summer sun, and for how many hours a day.

• Flat rooflights and lanterns. Horizontal glass faces straight up, so the high summer sun hits it close to square on for much of the day, from morning to evening, while the low winter sun barely grazes it. That is the worst possible balance: maximum gain when you least want it, very little when you would welcome it. A flat roof over a kitchen extension in Chesterton or a new home in Trumpington Meadows is the strongest case for solar control glass on this list.
• South facing pitched slopes. A roof window on a south slope pitched at 35 to 45 degrees faces the spring, summer and autumn midday sun almost directly. It also collects useful warmth on clear winter days, which is a real benefit, but in a loft bedroom the summer afternoons decide the matter. Solar control glass is normally the right call here.
• West facing slopes. West glass takes the sun from early afternoon until evening, when the house has already warmed through and the outside air is at its hottest. The heat adds to a room that is already warm, which is why west facing loft rooms can feel worse at bedtime than south facing ones. Solar control glass is usually worth it, alongside a blind.
• East facing slopes. East glass takes the morning sun on a cooler house. The gain is real but it arrives when there is room to absorb it, and it has faded by lunchtime. Solar control glass is optional and depends on the room: a bedroom where early light wakes people may want it; a kitchen used for breakfast may be better with clear.
• North facing slopes. North glass sees little direct sun for most of the year and only glancing early and late sun in high summer. It gives the soft, even light that artists' studios have always prized. Solar control glass here mostly costs daylight and saves little heat, so clear low-E glass is usually the better choice, with the heat loss side handled by the U-value.

Pitch matters as well as direction. A shallow south slope behaves more like a flat rooflight than a steep one does, gathering more summer sun and less winter sun. A very steep slope, such as the lower face of a mansard, behaves more like a wall window, with less summer gain than its orientation might suggest. When we write a light plan we note both the bearing of the slope and its pitch, because together they decide how hard the glass will be working in July.

Chapter 5

The room underneath changes the answer

The same slope can call for different glass depending on what the room is for, how high the ceiling is and how the heat can escape. A glass choice made from orientation alone misses half the picture.

Loft bedrooms are the most sensitive rooms in a house to roof glass. Heat rises and collects under the slope, the room is small, and it is used in the evening and overnight when the day's heat has had hours to build. A south or west facing roof window over a bed in a converted loft on a Romsey or Mill Road terrace is a textbook case for solar control glass, ideally with an opening sash for night cooling and a blind for the hottest afternoons. Our guide to rooflight overheating in summer covers the other design moves that work with the glass.

Kitchen and living extensions are more forgiving because they are larger, often have doors that open to the garden, and are used through the day. Even so, a big lantern or a run of flat rooflights over a dining table can turn a July lunch uncomfortable, and the glass is the first line of defence. The larger the area of horizontal glass relative to the floor, the stronger the case.

Home offices raise a different concern. People working at screens care about glare first and heat second. A solar control coating lowers the brightness of direct sun a little, but a sharp sun patch still crosses the desk and the screen still washes out. In a study, the position of the opening relative to the desk matters more than the glass, and a north light or a blind may do more than any coating.

Stairwells, landings and hallways sit in between. They are passing spaces, so a little extra warmth is rarely a problem, and they are often the darkest parts of a house. Clear glass usually makes more sense there unless the rooflight is large and south facing.

Chapter 6

What the coating will not do

Solar control glass cuts the heat that sunlight brings in, but it does not stop direct sun from entering, it does not make a room cooler than the air outside, and it gives up some winter warmth along with the summer heat.

A skylight with solar control glass still lets direct sunlight through. The beam is a little less intense and a little cooler on the skin, but the bright shape on the floor still moves across the room through the day, and it will still catch a sofa, a worktop or a screen. For the eye, the difference between clear and solar control glass is modest. Whether that matters is a question of placement and shading, and our answer on whether solar control glass makes a room darker looks at how the reduced transmission reads in practice.

The coating also works in both directions over the year. The same layers that reflect summer infrared reflect winter infrared too. On a clear day in February, a clear south facing rooflight would have added some free warmth to the room; a solar control unit adds less. On a north slope that trade is irrelevant, and on a flat roof the winter sun is so low that little is lost. On a steep south slope it is a real cost, and one reason we do not treat solar control glass as an automatic upgrade.

Glass cannot compete with shading that stops the sun outside. Even a strong coating admits a quarter or more of the solar energy that reaches it. An external blind or awning intercepts the sun before it touches the glass, so it can cut far more heat, though it also cuts the view and the light while it is in use. For the hottest slopes the best result tends to come from both: solar control glass for everyday comfort, external shading for the few weeks a year when the sun is relentless.

Fading is a separate property. Much of the ultraviolet in sunlight is already stopped by the laminated inner pane that overhead glazing needs for safety, rather than by the solar control coating. Visible light and heat also contribute to fading, so the coating helps, but it is the laminate doing the heavy lifting on UV. The answer on whether rooflights fade furniture sets this out.

Chapter 7

How it looks from inside and out

Solar control coatings reflect more than clear glass and can carry a faint colour, so they change both what the roof looks like from the street and the tone of the light in the room. On a conservation rooflight that can be as important as the heat figures.

Seen from outside, any coated unit is more mirror-like than clear glass, especially at a low viewing angle. On a flat extension roof hidden behind a parapet that rarely matters. On a front slope in a conservation area, such as the Victorian streets of De Freville or a village cottage in Grantchester, a bright reflective panel or a noticeably blue unit can draw the eye in a way that a planning officer may question. Neutral coatings with low external reflection are the usual answer there, and they sit more comfortably with slim conservation frames.

Inside, the question is colour. Neutral solar control glass keeps the daylight close to its natural tone. Blue or other tinted glass gives the light a cool cast that some people like in a hot south facing room and others find flat or clinical, particularly on warm wood and white walls. The choice between the two is personal and deserves its own discussion, which you will find in the answer on whether blue or neutral solar control glass is better. The practical advice is to see a sample against the sky, not under showroom lighting, before committing.

Low iron outer panes, which remove the faint green edge of standard glass, pair well with neutral coatings when the aim is the clearest possible view of the sky with the heat still under control.

Chapter 8

Specifying it with the rest of the rooflight

Solar control glass is one decision among several in a sealed unit, and it has to sit alongside the U-value, the safety laminate and the frame. Getting the whole build-up right on the day the order goes in is far easier than trying to improve it afterwards.

The unit still has to meet Part L of the Building Regulations for heat loss, which for a rooflight in an existing home means a limiting U-value of 2.2 W/m²K assessed in the horizontal position. Solar control coatings often improve the U-value because they include low-E function, so they rarely make compliance harder. For a new dwelling, Part O sets overheating limits across the whole home, and roof glass with a low g-value can be part of how a designer meets them. Extensions to existing homes fall outside Part O, but the overheating risk is just as real, which is why the glass still deserves the same thought. Where the installation is notifiable, we handle the Building Control notification as part of the job.

A typical specification for a south facing flat rooflight over a kitchen might read: toughened outer pane with a neutral solar control coating on face 2, argon filled cavity with a warm edge spacer, and a laminated inner pane for overhead safety. A triple glazed version adds a third pane and a second cavity, which lowers the U-value further and trims light transmission a little more. For a north slope in the same house, the same build without the solar control coating, using a standard low-E instead, keeps the daylight up where there is less sun to fight.

If you are replacing an older rooflight, this is the moment to change the glass. Applied films on an existing sealed unit can trap heat in the outer pane and stress the glass or the edge seal, and many manufacturers will not stand behind a unit that has been filmed. The answer on whether window film works on rooflights explains the risks. A replacement unit with the right coating built in avoids them.

Every written quote we give comes with a light plan that names the glass for each opening and gives the reason, slope by slope. The method behind it is set out on how we plan the light, and if you would like it applied to your own roof, book a light survey.

Chapter 9

Questions people ask

Is solar control glass the same as tinted glass?

Not quite. Older tinted glass cut heat by absorbing energy across the spectrum, so it cut light almost as much as heat. Solar control glass uses a thin coating that reflects infrared selectively, passing far more light for the same heat reduction. Some solar control products are also tinted, but the coating does most of the work.

Do I need solar control glass on every rooflight in the house?

No. It earns its place on flat rooflights, lanterns and south or west facing slopes over rooms that get warm. On north slopes, and often on east slopes, clear low-E glass gives more daylight for very little extra heat. A house can sensibly have different glass on different openings.

What g-value should I look for?

It depends on the orientation, the size of the glass and the room. For a south facing slope or flat roof over a living space, a g-value somewhere in the 0.30s is a common target. The answer on a good g value for a south facing rooflight works through the choice in more detail.