The g-value is the proportion of the sun’s energy that passes through a glazing unit, expressed as a number between 0 and 1. A g-value of 0.6 admits sixty per cent of the solar energy falling on it. On a roof, where glass faces the sky directly, it decides whether a room is comfortable in summer.
Why it matters more on a roof
Because a roof sees the sun at a far better angle than a wall does.
A vertical window takes the summer sun obliquely, when it is high. A roof slope, and a flat roof unit in particular, takes it close to square on, which is when glass admits the most.
Which means the same glass in the same house does considerably more to a room through the roof than through a window.
What a typical number is
A clear double glazed unit sits somewhere around 0.6 to 0.7. A solar control unit brings that down substantially, frequently to around 0.3 or below.
Lower means less heat admitted. It also means less light and less useful gain in winter, which is the whole of the trade.
The relationship with light transmission
The number that sits alongside it and moves in the same direction.
Light transmission describes the proportion of visible light passing through. Cutting solar energy generally cuts light too, and the useful measure of a good solar control glass is how much energy it removes for how little light.
A coating that halves the heat and takes only a fifth of the light is doing something a tint cannot.
Against a tint
The distinction worth understanding, because they are frequently confused.
A body tinted glass absorbs energy across the spectrum, which darkens the room roughly in proportion to the heat it removes.
A modern solar control coating is selective. It turns back more of the infrared than of the visible light, which is why it can cut heat substantially without making a room gloomy.
The winter side of the trade
The part nobody mentions when selling solar control.
Solar energy admitted in December is useful. A low g-value cuts it in winter as well as summer, because the glass cannot tell which month it is.
Which is why solar control belongs on a south or west facing unit over a room that overheats, and not as a default.
Aspect decides it
More than anything else about the room.
A south facing roof takes the most, a west facing one takes it at the hottest part of the day, and a north facing one takes almost no direct sun at all.
On a north facing unit a low g-value removes useful light and gains nothing.
What a low g-value does not do
Worth stating, because it is oversold.
It does not stop a room overheating on its own. Ventilation, shading and the thermal mass of the room all matter, and glass alone will not fix a badly arranged space.
What overheating actually involves is on will a rooflight make my room too hot.
Against a blind
A different mechanism, and the position matters enormously.
Glass rejects energy before it enters. An internal blind stops light after the energy is already inside, and it then re-radiates much of it into the room.
An external blind or awning is a different matter and is far more effective. The comparison is on do blinds reduce heat through a rooflight.
Where the coating sits
On an internal surface of the sealed unit rather than an exposed face.
Which is why it cannot be scratched off in cleaning, and why a coating cannot be added to an existing unit without replacing the glass.
What can be retrofitted is on can you add solar control to an existing rooflight.
How it interacts with the U-value
Two properties of the same unit, and improving one can move the other.
A low emissivity coating improves the U-value and modestly reduces the g-value as a side effect. A dedicated solar control coating cuts the g-value much further.
Both figures should be quoted for the same product, because a unit is chosen on the pair rather than on either.
Whether regulations set one
Not as a limit in the way a U-value is limited.
Solar gain enters the regulations through overheating provisions for new dwellings rather than as a maximum g-value for a rooflight.
On a replacement in an existing house, the g-value is a comfort decision rather than a compliance one.
Rooms where it matters most
Three, consistently. A loft bedroom under a south or west facing slope. A kitchen extension with a large area of flat roof glass. And any small room with a large unit relative to its floor area.
Rooms where it barely matters
A hallway, a landing, a bathroom, and anything north facing.
Those are the units where specifying solar control costs light and money for a problem that does not exist.
What to ask a supplier
The g-value and the light transmission together, for the same product.
One without the other tells you nothing about whether a glass is selective or simply dark.
Where the number appears
On a manufacturer’s datasheet alongside the U-value and the light transmission, and rarely in a quote.
Which means it is generally a question that has to be asked rather than a figure that arrives with a price, and it is worth asking on any south or west facing unit.
Total solar energy transmittance
The full name of the same thing, and it is worth recognising on a datasheet.
It combines what passes straight through the glass with what the glass absorbs and then re-radiates inward, which is why a heavily absorbing tinted glass performs worse than its transmission alone suggests.
A coated glass reflects rather than absorbs, so more of what it rejects goes back outside rather than warming the room secondhand.
What we specify as standard on the glazing
We specify the g-value by aspect rather than by default, and we say where solar control would remove winter light for no summer benefit.
What the coating actually is is on what is solar control glass.
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