What the light transmission figures actually say
Every sealed glass unit has a published light transmission figure, and comparing that number is the fastest way to see how much daylight a solar control skylight gives up.
Light transmission, often written as LT or Tv, is the share of visible light that passes through the whole unit. The g-value sits beside it and tells you the share of total solar energy that gets through, heat included. Solar control coatings are thin metal oxide layers designed to reflect much of the invisible infrared part of sunlight while passing as much of the visible part as they can. The better the coating, the wider the gap between the two numbers. The ranges below are typical of current double and triple glazed units from mainstream manufacturers, as a guide only; the data sheet for the exact unit is what counts.
| Glass | Light in | Heat in (g) |
|---|---|---|
| Clear low-E double | 70 to 80% | 0.55 to 0.65 |
| Neutral solar control double | 55 to 70% | 0.28 to 0.40 |
| Blue tint solar control | 40 to 55% | 0.25 to 0.35 |
| Triple with solar control | 45 to 60% | 0.22 to 0.35 |
Read the table across rather than down. A neutral unit that drops light from 75% to 65% while cutting heat gain from 0.60 to 0.35 has given up about an eighth of its daylight to remove over two fifths of its solar heat. That trade is the whole point of the product, and our guide to g-value and light transmission shows how to weigh one against the other for a particular room.
Why the room rarely feels as dark as the numbers suggest
Your eyes do not measure light on a straight scale, and overhead glass starts with far more light to spare than a window in a wall.
Human vision works roughly logarithmically: a ten percent drop in daylight reads as a small change, and the eye adjusts to it within seconds of walking into the room. What people notice far more is contrast, the difference between the brightest patch and the darkest corner. A clear south facing rooflight in July throws a hard, hot sun patch onto the floor and leaves the rest of the room looking dim by comparison. Solar control glass softens that patch, so the room can look more evenly lit even though the total amount of light has fallen.
A rooflight also sees the brightest part of the sky. A horizontal or shallow pitched pane faces the zenith, which on an overcast day is several times brighter than the sky near the horizon that a wall window looks at. In flat, open Cambridge, with no hills and few tall buildings to cut off that view, a skylight in a Romsey side return or an Eddington loft has a lot of daylight to work with. Taking a slice off it with a coating rarely leaves the room short.
When you will notice the loss
The difference becomes visible when several light reducing choices stack up, or when the rooflight is already the only source of light in a deep room.
Each layer in a unit takes its own share. A third pane of glass, a strong tint, a laminated inner pane with an obscure interlayer and a self-cleaning outer coating each remove a few percent, and together they can bring a unit down towards 40%. On its own none of these is a problem; stacked on a small opening they can be. The situations where we check the numbers carefully are:
- a north facing slope, where there is no direct sun to control and every percent of light counts;
- a small rooflight lighting a long, deep room, such as the back of a terrace kitchen off Mill Road;
- a deep roof build-up or tall upstand, which already narrows the view of the sky before the glass does;
- blue, bronze or mirror finish glass, which cuts light harder and also shifts the colour of the room;
- short winter days, when grey afternoons leave little to spare.
Our answer on whether energy efficient rooflights let in less light covers the cost of extra panes and low-E coatings in more detail, and blue or neutral solar control glass explains why tint matters for colour as well as brightness.
Matching the glass to the slope
Solar control glass earns its place where the sun hits the glass directly for hours; elsewhere clear, high transmission glass usually suits the room better.
On a south or west facing slope, or on a flat roof that sees the sun all day, the heat is the bigger risk and a neutral solar control unit is the sensible default. The room stays bright, the afternoon sun patch is gentler, and the space is far less likely to overheat on a July evening; see rooflight overheating in summer for why flat glass gathers so much heat. For a south slope our answer on a good g-value for a south facing rooflight gives target figures. An east slope over a bedroom can benefit too, because early morning sun arrives low and bright.
On a north slope the case reverses. The light arrives as steady, soft skylight with little direct sun, so a coating costs daylight without much heat to remove. Clear low-E glass, or low iron glass for the clearest possible view, gives more back. Where heat on a sunny slope is extreme, external shading can let you keep a lighter glass and still stay cool. The solar control glass guide sets out which coatings suit which slope.
How we check it before the roof is cut
At a light survey we look at the slope, the room depth and how the space is used, then choose the glass by its published figures rather than by name.
The written light plan that comes with every quote states the glass, its light transmission and g-value, and why we picked it for that roof. If a solar control unit would leave a deep room short of light, the plan says so and sets out the alternatives: a slightly larger opening, a second rooflight nearer the back of the room, a lighter coating with a blind, or clear glass with external shading. You can read more about the approach on how we plan the light and across our energy efficient rooflights service. To see the numbers for your own room, book a light survey or message us on WhatsApp.
Questions people also ask
Can you see out clearly through solar control glass?
Yes. Neutral solar control glass looks close to clear from inside, with perhaps a faint grey or green cast when you look up at the sky. Blue and mirror finish units change the view more, and the sky can look noticeably tinted through them.
Does solar control glass change the colour of the room?
Neutral coatings have very little effect on how colours read indoors. Stronger tints alter the light itself, so white walls under a blue unit can look cooler and timber can look greyer. If colour matters, ask for the colour rendering figure on the data sheet and view a sample against the sky.
Is solar control glass worth having on a north facing rooflight?
Rarely. A north slope gets little direct sun, so there is not much heat for the coating to stop, and the light it removes is light the room needs. Clear low-E glass is usually the better choice there.
Does self-cleaning glass make a skylight darker as well?
A self-cleaning outer coating reduces light transmission by a small amount, typically a few percent. Because it helps rain wash dirt off a shallow pane, a self-cleaning rooflight can stay brighter over time than a plain one left dirty between cleans.
