Where the light goes when you add panes and coatings
Glass does not lose light in one place. A little is reflected at every surface the daylight passes through, a little is absorbed inside each pane, and a little more is taken by any metal oxide coating.
A single sheet of clear float glass has two surfaces, and each one reflects roughly 4% of the light that reaches it. A double glazed sealed unit has four surfaces and a triple unit has six, so the reflection losses stack up before any coating is involved. Ordinary float glass also carries a trace of iron, which gives it a faint green edge and absorbs a small amount of light inside the pane. Thicker or laminated panes, which overhead glazing needs for safety, absorb slightly more.
The coatings are the part people worry about. A low-E coating is a microscopically thin metal oxide layer that reflects long-wave heat back into the room. Modern soft-coat low-E is very clear, and costs only a few points of light. Solar control coatings are a different matter: they are designed to reflect part of the sun's energy before it enters, and the stronger ones also reflect a noticeable share of visible light. We explain the coatings themselves in low-E coatings and argon fill. Argon, the gas in the cavity, is invisible and has no effect on the light at all.
Typical light transmission by glass build-up
The figure to look for on a data sheet is light transmission, often written LT or Tv: the share of visible daylight that gets through the whole unit.
These are general ranges for common rooflight glass, as a guide only. The exact figure depends on the manufacturer, the coating and the pane thicknesses, and it should always be read from the product data sheet.
| Glass build-up | Typical light transmission |
|---|---|
| Single clear pane | about 88 to 90% |
| Double, low-E, argon | about 72 to 80% |
| Triple, two low-E coatings | about 62 to 72% |
| Double, solar control | about 45 to 70% |
So the step from an old single glazed skylight to a modern low-E double unit costs somewhere around ten points of light, and a triple unit costs a few more. Solar control glass has the widest spread, because some coatings are tuned to pass daylight and block heat while others darken the glass as well. The balance between daylight and solar heat is covered in our guide to g-value and light transmission.
Why a ten point drop rarely looks like one
The eye does not measure light the way a meter does. It adapts constantly and judges brightness by comparison, so a room receiving 70% of the daylight rather than 80% does not look an eighth darker.
A rooflight also starts from a strong position. Glass laid in the roof faces the open sky rather than the street, the house opposite or the garden fence, so the same area of glass brings in far more daylight than a wall window of the same size. In Cambridge, where the land is flat and there are few hills or tall ridges to cut off the sky, a flat rooflight on a single storey extension behind a Romsey terrace sees almost the whole dome of sky. Losing a few points of transmission from that is a small trim off a generous supply.
There is a practical gain on the other side too. An old single glazed skylight, or a double unit whose seal has failed, can put dirt, misting or scratched plastic between you and the sky. A clean new unit with a slightly lower rated transmission can make the room look brighter than the old one did on its best day. Our page on upgrading single-glazed rooflights sets out what changes when the old glass comes out.
When the loss becomes noticeable
The light cost of efficient glass shows when the room was already short of daylight: a small opening, a dull sky, a deep room, or several light-reducing choices stacked together.
- North facing slopes in winter. A north slope gets no direct sun in December, so the room lives on diffuse skylight alone. A triple unit with a strong solar control coating on that slope takes light from the season that can least spare it, and gives little back, since there is little summer sun to control.
- Small units in deep shafts. A compact roof window at the top of a deep lined shaft already loses light to the shaft sides. Any further loss at the glass is felt more at the bottom.
- Stacking every option. Triple glazing, solar control, a tinted outer pane and self-cleaning glass each take a little. Chosen together without a reason, they can bring a unit down towards half of the available daylight.
- Tasks close to the limit. A desk, a drawing board or a kitchen worktop under the opening will show a drop in light more than a hallway will.
Keeping the light while keeping the heat in
You rarely have to choose between a warm rooflight and a bright one. The better answer is to match the glass to the slope and then win back light with the opening itself.
Low-iron glass removes most of the green tint and lifts transmission by a point or two per pane, which adds up in a triple unit; see what is low iron glass in a rooflight? A neutral solar control coating on a south slope, rather than a heavy tinted pane, keeps the heat down without greying the room. On a north or east slope, a good low-E double unit is often enough, and our comparison of double vs triple glazed rooflights shows where the third pane earns its place.
The biggest gains come from around the glass. A lining that splays outwards towards the room spreads the light and makes the opening read larger from below. A slightly larger unit, or a second one, recovers far more light than any glass choice loses. Position matters as well: a rooflight over the part of the room you use lights the work, not the ceiling.
How we weigh it in a light plan
At the survey we look at which way the roof faces, what shades it and how the room is used, then choose the glass for the slope rather than from a default list.
Every quote comes with a written light plan that names the glass, its light transmission and the reason for it, alongside the position, the size and any shading. If an efficient unit would leave a north facing room dim, the plan says so and proposes the fix, whether that is a larger opening, a splayed lining or clearer glass. The approach behind this is set out on how we plan the light, and the full range of options sits on our energy efficient rooflights page. If you want to know what your own room would get, book a light survey or message us on WhatsApp.
Questions people also ask
Do energy efficient skylights let in less light than old ones?
By the numbers, yes: a low-E double or triple unit passes a lower share of daylight than a single clear pane. In practice a new unit can look brighter than the one it replaces, because old glass or plastic dulls, mists or scratches with age.
Does triple glazing make a rooflight noticeably darker?
The third pane and second coating typically take a few more points of light transmission than a good double unit. On a south slope or a flat roof you are unlikely to notice it. On a small north facing unit in winter, you might, so the choice depends on the slope.
Does argon gas reduce the light through a rooflight?
No. Argon is a colourless gas and has no measurable effect on daylight. It slows heat moving across the cavity, and that is all it does. The light losses in a sealed unit come from the glass surfaces and the coatings.
Will a rooflight with clearer glass still meet Building Regulations?
Usually, yes. Part L sets a limiting U-value of 2.2 W/m²K for rooflights, assessed in the horizontal plane, and a good low-E double unit with clearer glass can sit below that. The details are in our guide to Part L requirements for rooflights.
