What glass stops solar gain?

What to ask for by name

Ask for a solar control soft coat sealed unit, stating the g-value you want, the light transmittance you will accept, and which surface the coating is on. The three well-known coating families come from the major float glass producers and appear on datasheets under their trade names, but a supplier does not need the brand: a written specification of g-value, light transmittance and surface position defines the product unambiguously.

The build-up for a pitched roof window is typically a 6mm toughened outer pane carrying the solar control coating, a 16mm argon cavity, and a 6.8mm laminated inner pane carrying a standard low-emissivity coating. On a flat rooflight over habitable space the inner pane is laminated as a matter of course, and the outer is toughened and heat soaked.

How the coating works

Sunlight arriving at a roof is roughly 44 per cent visible light, 53 per cent near-infrared and 3 per cent ultraviolet. Over half of the energy is therefore heat you cannot see, and it is possible to turn that half away while keeping the half you can.

A soft coat is deposited by magnetron sputtering: alternating layers of silver a few atoms thick, separated by metal oxide layers, built up to a total thickness measured in tens of nanometres. Silver is highly reflective to infrared and near-transparent to visible light at that thickness. Two or three silver layers, tuned to slightly different wavelengths, give a sharper cut-off between what passes and what reflects, and that is what separates a high-performance solar control product from a basic one.

Crucially the energy is reflected rather than absorbed. That matters, because absorbed energy warms the glass and roughly half of it then re-radiates into the room. The full account of coatings and how they are made sits on low-emissivity coatings and argon fills.

Where the coating has to sit in the build-up

Glass surfaces are numbered from the outside in. On a double unit, surface one is the weather face of the outer pane, surface two is its inner face, surface three faces the cavity from the inner pane, and surface four is the room face.

The solar control coating goes on surface two. It has to intercept the energy before it enters the cavity, and it has to be inside the sealed unit because a soft coat will not survive on an exposed face. The thermal low-emissivity coating goes on surface three, where it reflects long-wave heat from the room back inward. Get those the wrong way round and the unit will not perform as declared. Always specify the surface number in writing; asking for the coating to be “on the outer pane” is ambiguous and has produced units glazed the wrong way up before now.

Tinted glass, reflective glass and applied film

Body-tinted glass has metal oxides melted into the glass itself, colouring it grey, bronze, green or blue right through. It works by absorption, so the pane heats up, and a hot pane sitting above your head re-radiates a good share of what it absorbed straight down into the room. It also cuts daylight more than it cuts heat, which is the wrong way round.

Heavily reflective mirror glass does control gain, and it looks like an office block from the garden. Applied film retrofitted to the room face of existing glass is the one worth treating carefully: film on surface four absorbs energy that has already crossed the cavity, and on a toughened pane it is usually harmless while on an annealed or partly shaded pane it raises the risk of thermal stress cracking. It also voids many sealed unit warranties. Where the frame and the size allow it, a new sealed unit with the coating in the right place is the better answer, discussed on can you retrofit better glass into a rooflight.

What it costs you in daylight

The honest figure is that a standard solar control unit transmits around 66 per cent of visible light against 79 per cent for a plain low-emissivity unit, and a high-performance one transmits about 55 per cent.

In a room lit from above that is less dramatic than it sounds, because overhead glazing starts with far more available light than a wall window does. A loft bedroom with two solar control roof windows is still a bright room. Where it does bite is a north-facing slope with diffuse light only, a small unit in a large room, or a deep unsplayed reveal that is already throwing light away. There is also a colour question: the better coatings are close to neutral, while cheaper ones can leave a faint green or blue cast that shows against white paint. Ask for a sample, and see do solar control rooflights make a room dark.

Combining it with low emissivity and a third pane

Solar control and thermal performance are separate jobs done by separate coatings on separate surfaces, and a well-built unit does both. Surface two turns the summer sun away, surface three keeps the winter heat in, and the argon fill and warm edge spacer serve both.

A third pane pushes the g-value down further as a side effect, reaching 0.24 or below with a solar control layer included, and it takes the light transmittance down towards 0.48. That is a dark unit, and it is only the right answer where the pitch is shallow, the aspect is west or south-west, and the room has light from elsewhere. The trade is discussed on is triple glazing worth it in a rooflight.

The winter side of the bargain

A coating that reflects the sun in July reflects it in January as well, and in January the house wanted it. On a south-facing pitch at Gloucester the winter sun sits at 14.7 degrees at midday and shines down a shallow slope for several hours, and a standard unit collects a useful amount of free heat that a solar control unit largely turns away.

That is why the specification changes with aspect rather than being applied everywhere. West and south-west: solar control, near 0.27, because the afternoon problem outweighs the winter credit. South on a steep pitch: 0.35 to 0.45, keeping some of it. East: 0.40 to 0.50. North: none at all, and specify for light transmittance instead. The aspect-by-aspect detail is on specifying glazing by which way the slope faces.

Heat in the pane itself, and the toughening that follows

A coated outer pane absorbs more energy than a clear one, so it runs hotter, and hot glass expands. Where part of a pane is in full sun and part is shaded, by a chimney, an adjacent gable, the edge of a dormer, or a blind pressed against the inside face, the temperature difference across the sheet sets up tensile stress at the cool edges. Annealed glass fails at around 40 degrees of differential. Toughened glass tolerates roughly five times that.

Solar control glass on a roof is therefore toughened as a matter of course, and toughened glass for overhead use is heat soaked to weed out the rare nickel sulphide inclusion that causes spontaneous breakage years later. Safety glazing rules apply on top: a toughened outer and a laminated inner wherever there is habitable space, a stairwell or a bath beneath. The lantern version of this is on roof lantern glazing and solar control.

Specifying it for a Gloucestershire roof

West-facing rear slopes on the interwar semis at Longlevens, Hucclecote, Warden Hill and Charlton Kings are the standing case for a high-performance coating, because the pitch is around 32 degrees and the afternoon sun arrives nearly square-on. Roof lanterns over Severn Vale kitchen extensions want a moderate coating combined with an opening at the ridge, since the room makes heat of its own.

Near-horizontal flat rooflights, on rear additions through Gloucester and Tewkesbury and behind Cheltenham’s parapets, see every aspect across the day and generally want 0.30 to 0.35. Conservation rooflights on Cotswold stone slate through Painswick, Bibury and Northleach are the constrained case: the frame is slim steel, the unit is often a slim double, and the coating options are narrower, so shading and ventilation carry more of the load there. Severn takes the compass bearing, the pitch, the shading and the room use at survey, and the glass is specified from those. The full balance against heat loss is on the energy efficient rooflights hub.