What is solar control glass?

The coating is a stack of layers, some of them silver, applied to the glass under vacuum and measured in nanometres. You cannot feel it and, in a good product, you can barely see it. What it does is behave like a one-way filter for radiant energy: visible daylight passes through with only a modest loss, while a big portion of the near-infrared, the part of sunlight you feel as warmth, is reflected straight back outside.

Heat reaches a room by two paths, and the coating interferes with both. Some solar energy is reflected off the outer surface before it ever enters the glass. The remainder that does get absorbed warms the panes, and the coating helps that heat shed back outdoors rather than radiate inwards. That second effect is why a coated unit keeps working on a windless afternoon, when clear glass would sit there soaking up warmth and passing it into the room below.

There are two broad families. Solar control can be an offline sputtered coating, the higher-performing kind used inside a sealed double-glazed unit, or an online pyrolytic coating fired onto the glass as it is made, which is tougher but blocks less. For a rooflight over a living space the sealed sputtered unit is almost always the right specification, because the coating sits protected on an inner face where it will not scratch or weather.

Any solar control unit worth buying comes with a specification sheet, and three figures on it matter. Read them together, never one on its own.

Clear double glazing with no solar coating typically sits around a G-value of 0.6, which lets about three-fifths of the sun’s heat through. A solar control double-glazed unit brings that down to roughly 0.3, halving the heat load for the same opening. The catch is that no coating is perfectly selective, so pulling the G-value down usually pulls the daylight figure down with it, from perhaps 75 per cent on a clear unit towards the high fifties on a strong solar control one.

The U-value is a separate matter entirely. It governs winter heat loss, and Building Regulations set a ceiling on it under Part L for any new or replacement rooflight. What the regulations do not cap is the G-value, so a unit can be fully compliant on winter warmth and still cook the room every July. That gap is precisely why the glass specification, not just the frame or the opening, is worth getting right at the point of order.

Solar control glass matters most on glazing that catches direct sun for hours at a stretch, and that means south and west elevations and every flat roof. A flat rooflight or lantern points at the sky rather than at a compass bearing, so it takes the high summer sun almost head-on and gains very little protection from which way the house happens to sit.

Southend-on-Sea faces south across the Thames Estuary, which is unusual for an English seaside city and is the reason this comes up so often here. The southern horizon over the water is open, with no opposite terrace or tree line to throw the roof into shade, so a south-facing rooflight collects sun from mid-morning until it swings round to the west. Bright water adds reflected light on top of that, which is why estuary-facing rooms can feel glary even with the sun off the glass. Drop clear rooflights into a south-facing rear extension in that setting and you have built a solar collector over the kitchen. Coated glass is the fix that lets the daylight in and leaves most of the heat outside, and it sits at the centre of our energy-efficient skylight installation work.

The coating is not free, in money or in daylight, and it helps to know the trade-offs before you order.

• A little less light. Because the coating cannot separate heat from brightness perfectly, a strong unit trims the daylight coming through. Ask for the light transmittance figure and picture the room at it, not just the G-value.
• A faint tint. Most coatings carry a slight neutral, blue or green cast. From inside your eye adjusts and stops noticing it. From the garden, looking up at a lantern with several panes in view, it can read more clearly.
• Less free winter warmth. The same glass that blocks August sun blocks February sun, so on a north-facing roof, where there is no overheating to solve, a coating is a cost with little return.
• A higher unit price. Solar control glazing adds to the cost of a plain equivalent, modest on a single roof window and more noticeable across a large lantern.

Glass is also only one lever. External shading stops heat before it touches the pane and outperforms any coating, while internal blinds help with glare but do little once the heat is already inside. On a deep room a sensible plan often pairs a coated fixed pane with one opening unit set high, so hot air can escape at the top rather than pool under the glass.

Start with orientation. Stand in the room, note which way the roof faces and whether it is pitched or flat, and if it points anywhere from east through south to west, or if it is flat, solar control glass is worth specifying rather than clear. Then ask any supplier for the glass specification sheet, and check the G-value and light transmittance figures are quoted centre-pane against centre-pane so you are comparing like with like.

If you would rather hand that part over, tell us the orientation and the rough opening size and we will come back with a glass specification and a price that suits the room. Ring 01702 898232 or request a quote, and we will look at the whole roof rather than just the hole in it. Our skylight installation costs page shows where the glazing choice lands on the final figure.