How the coating works
Sunlight carries both visible light and heat. Ordinary clear glass passes most of both. Solar control glass has a microscopically thin metal or metal oxide coating on one of the inner faces of the sealed unit, and the coating is tuned to reflect or absorb a larger share of the heat part of the sun's energy while still passing most of the visible light.
The measure for this is the g-value, also called the total solar energy transmittance. It is a number between 0 and 1 that says how much of the sun's energy gets through the whole glazing. A lower g-value means less solar heat enters. Clear double glazing sits towards the upper end of the range, and solar control units are specified with a noticeably lower figure.
The coating sits inside the sealed unit, so it is protected from cleaning and weather. You cannot see it as a separate layer. What you notice is a very slight tint, often a pale blue or green cast when you look at the glass from an angle.
Why a rooflight needs it more than a wall window
A rooflight faces the sky. At midday in midsummer the sun stands a little over 60 degrees above the horizon in the south of England, so it strikes a roof opening far more squarely than a wall. A wall window in the same room gets a lower angle of sun, and the eaves, a porch or a neighbouring building often shade it.
A sloping or flat roof has none of that shade, so the glass takes the full strength of the sun for hours. Heat gathers in the loft room beneath, which is already close to the underside of the roof covering. Lofts and top-floor extensions are where overheating tends to show first.
Part O of the Building Regulations deals with overheating, but it applies to new dwellings. For an existing house there is no requirement to fit solar control glass, so the choice is yours, made on the comfort of the room.
What you give up
Any coating that cuts heat also changes light and warmth to some degree, and it helps to know the three changes before you order.
- A little less daylight. Visible light is reduced slightly. In a well lit room this is hard to notice, though a dark landing or a north slope with little sun to begin with may feel a touch dimmer.
- Less free winter warmth. The coating also turns back some of the low winter sun, so a room gains slightly less solar heat on a cold, bright day. On a roof facing north there is little sun to lose.
- A tint. The faint colour is visible against clear glass fitted alongside, which matters if two rooflights sit side by side. Fit the same glass in both.
The thermal insulation of the unit, its U-value, does not depend on the solar control coating alone. It comes from the glazing build-up, the gas fill and the spacer, as set out in our guide to U-values.
Which roofs gain most
| Roof or room | Solar control glass | Reason |
|---|---|---|
| South or west slope, loft bedroom | Worth considering | Strong afternoon sun, little shade |
| Flat roof over a kitchen extension | Worth considering | Open to the sky all day |
| North slope, landing | Seldom needed | Little direct sun to cut |
| Roof lantern over a living space | Check the make | Large glass area, heat gathers |
The orientation of the roof, the size of the opening and what is in the room all feed into the decision. At the survey we note which way the slope faces and what the room is used for, and both go on the measured sheet we leave with you.
A larger glass area collects more heat, so a big flat rooflight or lantern gains more from the coating than a small roof window. The options for flat units are laid out in our flat rooflight glazing answer.
Other ways to cut the heat
Glass is one of several tools, and it can be combined with the others.
- A blind on the outside stops sunlight before it reaches the glass, so less heat is absorbed. A blind on the inside shades the room but lets the glass warm up first.
- An opening unit lets hot air leave at the top of the room. Electric openers can be linked to a timer or a sensor, covered in our answer on electric and manual units.
- Tinted or reflective film can be added to existing glass, but it does not match a coating built into the unit, and it may affect the glass maker's guarantee.
Compare any glazing specification on its g-value, its light transmission and its U-value together. A unit that scores well on one and poorly on another will not suit every room.
Asking for it in a quote
Solar control is a specification line, and the quote should name it. Ask for the g-value and the light transmission figure of the glass in each rooflight, along with the U-value. Makers publish these figures for their glazing build-ups.
If you already have rooflights in the house, say which rooms get too hot and which stay comfortable. We note this when we are on the roof, alongside the pitch and the opening size, so the glass for each unit is chosen for its own slope rather than copied across the whole roof.
Where one roof carries several units facing different ways, it is normal to specify different glass in different units, as long as units that sit next to each other and are seen together match.
faq
solar control questions
Does solar control glass look dark?
No. Looking up through it you see a slight tint, but it does not black out the room the way a dark film would.
Can it be added to a rooflight I already have?
The coating is applied inside a sealed unit, so it is chosen when the unit is made. For an existing rooflight that means a new sealed unit or a new rooflight.
Do I need it on a north-facing roof?
Seldom. A north slope gets little direct sun, so the coating has little heat to turn back.