Solar control glazing for south-facing roofs

Strip a solar-control unit back and it is a pane of glass carrying an ultra-thin metallic coating, usually built around one or more layers of silver a few atoms thick, sandwiched between layers of metal oxide. You cannot see it. It is measured in nanometres and it sits on the surface of the glass facing into the sealed cavity, where nothing can touch it.

The coating works by being fussy about which part of sunlight it lets through. Daylight is not a single thing. Roughly 3 per cent of the energy in sunlight is ultraviolet, about 44 per cent is the visible light your eye actually uses, and the remaining 53 per cent or so is near-infrared, the invisible band that you feel as radiant heat but never see as brightness. Ordinary clear glass treats all three much the same and passes most of the lot. A solar-control coating is engineered to reflect a large share of that near-infrared band back outwards while still letting the visible light stream through. That is the whole trick. You are separating the light you want from the heat you do not, using a difference in wavelength.

This is why solar control is a completely different animal from tinted glass. Tint darkens the visible band, so it dims the room and only dents the heat. A proper coating leaves the room bright and takes the heat out of the invisible part of the spectrum you were never enjoying anyway. Ask to see the two numbers side by side, the light figure and the heat figure, and a good coating shows a wide gap between them.

Heat does not build in a south-facing room at a steady rate through the day. It ramps. A roof plane facing south and open to the sky starts collecting soon after sunrise, climbs through the morning, and holds a long high plateau from late morning until the middle of the afternoon before it eases off. By the time a family gets home and wants to use the kitchen or the loft bedroom, the fabric of the room has been soaking that up for hours and is releasing it back.

A roof gets the worst of it for reasons a wall never faces. It looks at the whole sky, not half of it, so it collects diffuse light off a bright overcast sky as well as direct beam. In this corner of Essex there is a second helping on top: the estuary lies directly to the south, a broad flat sheet of water throwing reflected light up onto every south-facing slope along the coast. And the heat that does get through arrives at the ceiling, which is exactly where warm air already wants to gather, so it sits in a layer over your head instead of drifting off.

The upshot is that the same pane of clear glass behaves far more aggressively in a roof than it would in a wall. A south-facing rooflight is, in plain terms, a heat collector aimed at the sun for the length of an English summer. That is not a reason to fit fewer skylights. It is the reason to fit the right glass in the ones you do.

Several products get sold under the same heading, and they are not equal. It helps to know what you are being offered before a price is attached to it.

Pyrolytic coatings, the hard coat. Applied to the glass while it is still hot on the float line, so the coating fuses into the surface and is tough enough to be handled, cut and toughened as a single sheet. Durable and cheaper to produce, but less selective, so it takes out less heat for the daylight it costs you. Fine on planes that only need a modest hand.

Magnetron-sputtered coatings, the soft coat. Laid down in a vacuum chamber onto already-cut glass, layer by layer, which is how the silver goes on. Far more selective, and the coating has to live inside a sealed unit because it would not survive being exposed. This is the glass that gives you a low heat figure without darkening the room, and it is what belongs over a south-facing extension. The number of silver layers is the shorthand for how hard it works: single, double or triple silver, each generation blocking more heat while holding onto the daylight.

Body-tinted glass. Colour run right through the glass so it absorbs solar energy. The trouble is that absorbed energy heats the pane, and a hot pane radiates a good part of that heat straight back into the room. It dims the daylight and only half-solves the heat, which is why it is rarely the right answer on its own.

Applied film. A polyester film stuck to the surface of an existing pane, carrying a metallic or ceramic layer. It exists mainly to treat glass that is already in place. On a rooflight it is a compromise: films sit on the warm inner face where they are less effective than a coating inside the cavity, they can stress an old sealed unit by trapping heat, and the good ones still darken the view. If the glass is wrong, the honest fix is the right unit, not a film over the wrong one.

These are typical published ranges for the common coating families in a double-glazed rooflight. Exact figures move with the manufacturer and the precise coating, and whether a value is quoted for the centre of the pane or the whole finished unit, so read them as the shape of the market rather than a fixed price list. The pattern is what counts.

Two things jump out of that table. First, the daylight figure barely moves as you climb the soft-coat rows, from around 0.70 down to about 0.60, while the heat figure falls by well over a third. You are buying a large drop in heat for a small give in brightness. Second, look at the body-tinted row: its heat figure is no better than a middling coating, but its daylight has collapsed to match. That is the difference between a room that stays cool and bright and a room that just goes gloomy. The value to divide is daylight over heat: the further above 2.0 that ratio climbs, the harder the glass is working for you.

Glass is not the only way to deal with sun on a roof, and it is worth being straight about where it sits among the alternatives.

Internal blinds are the weakest option for heat, because by the time the sun has passed through the glass to reach a blind on the room side, the energy is already inside. A blind then re-radiates much of it into the room. Blinds are for privacy and for softening glare on a screen, not for keeping a room cool, and treating them as heat control is where a lot of overheated loft rooms go wrong.

External shading, a louvre or an awning over the glass, is genuinely effective because it stops the sun before it reaches the pane at all. On a roof, though, external shading is awkward to fix, exposed to estuary wind, and often unwanted because it blocks the very view and sky the skylight was fitted for. It has its place on some flat-roof lanterns but it is not the default.

Solar-control glazing sits between the two and, for a roof, usually wins. It works at the glass line, where external shading works, but with nothing to fix, nothing to maintain and nothing to catch the wind. It does not clear a room of heat the way an opening vent does, which is why glass and ventilation are a pair rather than rivals: the coating controls what comes in, and an opening rooflight at the top of the room clears the warm air that gathers there. On tall ceilings that pairs naturally with an electric opening unit, so nobody is wrestling a pole twice a day. Get the glass right first, then give the hot air a way out.

The reason a single glazing spec across a whole roof is a mistake is that the roof rarely faces one way. Most houses present at least two roof planes, and the right coating for one is the wrong coating for the other.

Take the stock this patch is built from. The 1930s semi belt that runs right across south Essex almost always ends up with a rear extension over a knocked-through kitchen, and where the garden faces south that flat or shallow-pitched roof is prime territory for a roof lantern or a line of flat-roof skylights. Flat glass facing straight up at a south sky is the hardest case there is, and it wants a double or triple-silver coating without argument. The Victorian and Edwardian villas and terraces along the cliff tops in Leigh and Westcliff tend to gain their light through loft conversions, with bedroom windows set into a pitched slope. A south-facing pitched slope is a shade gentler than dead-flat glass but still needs a real coating; the north-facing slope of the same conversion wants clear glass and every scrap of daylight it can pull in.

West is the plane people underestimate. A west-facing roof takes the low, strong late-afternoon sun at a favourable angle just as the walls and floors of the house have finished absorbing heat all day, so a west plane often deserves the same coating as a south one. East is easier: the morning sun is strong but the room has the rest of the day to shed it, and it usually arrives when the house is empty. Match the coating to the compass, not to the price list. That plane-by-plane thinking is the backbone of our energy-efficient skylight installation, and it starts with working out which way each roof plane looks before any glass is ordered.

You can pin this down in a single conversation with whoever is quoting. A few questions sort the careful specifier from the box-shifter.

1. Which coating family is this, and how many silver layers? A straight answer means the person knows the glass. Vagueness here tells you the rest.
2. What is the G-value, and is it centre-pane or whole-unit? Centre-pane figures ignore the frame and the spacer and always flatter the glass. Ask for the whole-unit number, because that is the honest one and the one Building Control assesses.
3. What is the light transmittance alongside it? You want a low heat figure that has not been bought by darkening the glass. A collapsed daylight number is a body tint dressed up as solar control.
4. Can I see the manufacturer’s data sheet? Every real coating has one, with values worked out to the British and European test standards. It costs nothing to ask and it tells you plenty.
5. Is the coating on the right surface for the roof? On a solar-control unit the coating belongs on the inner face of the outer pane, looking into the cavity. It is a detail, but it is the sort of detail a careful survey gets right.

If the glass already in your roof is the wrong specification, the answer is not a film over the top of it. It is a new unit with the correct coating, and swapping a failed or ill-chosen sealed unit for the right one is a straightforward replacement. New and enlarged rooflights are notifiable under the Building Regulations, with Part L governing the thermal performance and Part K the safety glazing overhead, and we make that notification to Southend-on-Sea City Council, or to the relevant local authority for your address, as part of the work. Solar-control units still carry laminated inner panes for overhead safety, and the laminate interlayer blocks essentially all the ultraviolet, so a coated south-facing rooflight also spares your floor and furniture the fading that clear glass allows. That is a bonus rather than the reason to fit it.

One regulatory gap is worth knowing, because it explains why so many roofs end up wrong. The thermal regulations set a limit on U-value but say nothing about the G-value of a rooflight in an existing house. The rule that does address summer overheating applies to new-build homes, not to an extension on the house you already own. So for most domestic roof work, nothing on paper forces anyone to think about solar gain, which is exactly why plenty of installers do not, and why a roof full of perfectly compliant glass can still leave you with a room nobody wants to sit in on a bright afternoon.

Skylights On Sea has been installing skylights and rooflights across south Essex for over fifteen years, every installation backed by a ten-year workmanship guarantee, and the orientation survey is where each job begins. If you want the coating chosen for the way your own roof faces, plane by plane, ask us for a specification with the G-value and the light figure written down for each one, and compare it against the typical costs before you commit to anything.