What glass stops solar gain?

The coating that does the work

A solar control coating is a stack of extremely thin layers, silver, metal oxides and protective films, deposited on float glass in a vacuum chamber. The silver layers are a few atoms thick, which is why the glass still looks clear to the eye while behaving like a mirror to wavelengths just beyond what the eye can see.

Sunlight arriving at a roof is roughly 3 per cent ultraviolet, 44 per cent visible, and 53 per cent near infrared. The infrared carries heat and no light at all, so a coating that reflects it and passes the visible band removes over half the energy while barely darkening the room. That gap between what heats and what illuminates is the entire basis of the product.

Two families exist. Soft-coat, applied offline in a vacuum, gives the best performance and must live inside a sealed cavity where it cannot be touched. Hard-coat, applied online while the glass is still hot, is more durable and can face the weather, but performs less well. Rooflight units use soft coats.

Which surface it sits on

Surfaces in a sealed unit are numbered from the outside inwards. In a double unit, surface 1 faces the weather, 2 and 3 face the cavity, 4 faces the room.

Solar control coatings go on surface 2. That puts them on the cold side of the cavity, so the energy they reject is thrown back out before it has entered the insulated part of the unit. Low-emissivity coatings, which control heat loss rather than solar gain, go on surface 3, on the warm side, where they reflect long-wave heat back into the room. A unit doing both jobs properly has two different coatings on two different surfaces.

Ask which surface the solar coating is on. A solar coating placed on surface 3 is working in the wrong direction and will absorb into the cavity instead of rejecting outwards.

Tinted glass, and the older approach

Body-tinted glass has colouring oxides mixed into the melt, giving bronze, grey, green or blue float. It reduces the energy passing through by absorbing it, and absorbed energy heats the pane.

A hot outer pane loses a fair share of that heat back to the outside air, so tinting does reduce gain. But a meaningful part comes inwards, arriving as a warm surface radiating at the room rather than as a beam of sunlight. Tinted glass also cuts visible light in the same proportion as heat, so you get a darker room for the same reduction that a coating achieves without dimming anything. It has largely been superseded for rooflights, and where you see it now it is usually for appearance.

Laminated interlayers

Laminated glass bonds two panes with a plastic interlayer, most often PVB. Standard clear PVB blocks essentially all ultraviolet, which stops fading of fabrics and floors, and it makes almost no difference to heat.

Solar control interlayers exist and do reduce gain, and they can be tinted or carry dispersed infrared-absorbing particles. They are worth knowing about because laminated glass is a safety requirement in overhead glazing anyway, so the inner pane of a rooflight is usually laminated regardless. If you are having laminate for safety, a solar interlayer is a small upgrade rather than a new component. It will not match a surface 2 coating on its own.

Switchable and electrochromic glass

Electrochromic glass darkens under a low voltage, taking a unit from roughly 0.45 down to under 0.10 in a few minutes, then clearing again. It solves the fundamental problem with fixed solar control, which is that a coating cannot tell July from January.

It is genuinely good and genuinely expensive, several times the cost of a conventional unit, and it needs wiring to the rooflight. It also tints blue and takes ten to twenty minutes to switch. On a large fixed flat-roof unit over a room that cannot be shaded externally, it can be the right answer. On a standard pitched-roof unit it almost never is, and an external awning does more for a tenth of the money.

What the figures look like side by side

Selectivity is light transmittance divided by g-value, and it is the honest measure of whether a product is controlling heat or just dimming the room. Anything at 1.7 or above is doing real optical work. Anything under 1.0 is a tint wearing a technical name. The bottom row shows why we recommend external shading first wherever it can be fitted, as set out on the stopping a rooflight overheating page.

The light you give up

Every step down in g-value costs some daylight. Going from 0.60 to 0.28 typically costs 15 points of light transmittance, from around 0.75 to around 0.60. In a room lit only from the roof that is noticeable on a dull February afternoon, though rarely objectionable, because a rooflight starts with two to three times the daylight of an equivalent window.

Heavy solar control units in the 0.15 to 0.20 range do make rooms feel dim and often carry a visible tint. We would specify them only on a large horizontal unit facing a genuine overheating problem with no shading option, and we would tell you what it will look like in winter before you order it.

What no glass can do

Glass cannot remove heat that is already in the room, cannot adapt to the season unless it is electrochromic, and cannot rescue a rooflight that is simply too large for the space beneath it. It also does nothing for glare, which is a separate complaint caused by brightness contrast rather than by energy, and which is usually solved with a diffusing internal blind.

Where a room is overheating badly, glass alone is rarely the whole answer. Ventilation at high level moves the heat that has arrived, and that combination beats either measure by itself.

Specifying it for a roof rather than a wall

The g-value is declared with the sun striking the pane square on, and a roof slope in midsummer comes closer to that condition than a wall ever does. A specification that would be excessive in a window is often correct in a rooflight on the same elevation.

We choose the figure slope by slope. North slopes get clear high-transmission glass and no solar control at all. East gets the standard specification. South gets a mild control at around 0.40. West and near-horizontal units are where the strong specifications belong. Tell us the pitch, the compass direction and what the room is for, and we will give you a range by email and a fixed price after a survey. Fifteen years in the trade, Oxford and twenty five miles by road, installations and replacements only, ten-year workmanship guarantee.