What does light transmittance mean on a rooflight?

It counts only the light you can see. Heat and ultraviolet are measured separately, which is why one pane can have a good transmittance and a poor heat rating, or the other way round.

The technical name is visible light transmittance, often shortened to Lt or Tvis, and some makers write it as VLT. You will see it on a data sheet for a sealed unit, next to the U-value and the g-value. A skylight visible light transmission figure means exactly the same thing, just in the wording used on American and some European sheets.

Think of it as a fraction of the daylight that reaches the outer face of the glass. If the sky delivers a given amount of light to the roof, the transmittance tells you what fraction survives the journey through every pane, coating and interlayer in the unit. Whatever is lost stays outside, or is absorbed in the glass as a little heat.

The figure belongs to the glass build, not to the rooflight as a whole. A frame blocks light too, and so do bars and a deep upstand, but those are matters of unit size and shape. Transmittance describes the glazed part only, measured at the centre of the pane.

Every layer the light crosses takes a small share. Clear float glass lets through most of what arrives, and every addition lowers that a little.

Clear glass is never perfectly clear. Each sheet reflects a small amount at both faces and absorbs a little inside, and the iron content gives thick clear glass its faint green edge. Stack two sheets and you lose a bit more. Stack three and you lose more again, which is one of the real costs of a third pane, covered in our page on triple glazed rooflights.

Then come the coatings. A low-E layer is a microscopically thin metal oxide film that reflects heat back into the room. It is designed to be nearly invisible, and modern versions are very good, but it still turns back a fraction of visible light along with the heat. A solar control layer is tuned harder, to turn back more of the sun's energy, and it has a bigger effect on transmittance. The pages on low-E coatings and solar control glass show what each one is for.

Laminated glass adds its own layer. The interlayer is clear, but two sheets and a film are a little less transparent than one sheet. That is a fair trade overhead, because laminated glass holds together if it breaks and it calms the sound of rain. We simply note it in the specification so the brightness and the quiet are chosen together.

The table shows direction, not exact numbers, because the figures vary between coating makers and change from one data sheet to the next. Always read the sheet for the actual unit on your quote.

Three figures describe a glazed unit, and they pull against each other. Comparing one without the others gives a false picture.

• U-value: how fast heat leaves. Lower is warmer. Our explainer on rooflight U-values covers it.
• g-value: how much of the sun's heat comes in. The g-value answer explains the trade.
• Light transmittance: how much daylight comes in. Higher is brighter.

A north-facing loft window gets little direct sun, so heat gain hardly matters and you can favour brightness. A south-facing kitchen lantern may need a coating that holds back summer heat, and in that room a slightly lower transmittance is a sensible price. The sensible test is whether the room will still feel light at the end of a dull November afternoon, because that is when transmittance is most visible.

Transmittance tells you about the glass. How bright the room feels depends on the glass, the size of the opening and the room around it.

Area usually matters more than a small difference in the coating. A rooflight a fifth larger will outdo any small gain from swapping one clear coating for another. Overhead glazing also sees the brightest part of the sky, so even a modest opening does a great deal compared with a wall window of the same size. If you are weighing size against glazing, our page on the area limit under Part L sets out how the energy rules treat it.

Finish counts as well. A pale ceiling and pale reveals carry light further into the room than dark ones, and a deep, square upstand shades the glass at low winter sun angles, where a splayed one does not. Dirt on the outer face cuts more light than you might expect, which is why the self-cleaning coatings exist. Those coatings are a separate matter from warmth and have only a small effect on how much light gets in.

Where the figure does matter is at the choice between two glass builds that are otherwise close. If one comes with a clearly lower transmittance, ask what it is giving in return. If the answer is summer heat control in a room that faces the sun, it may be worth it. If the room faces north and the answer is vague, it probably is not. The fuller question of dimming is answered in whether solar control glass makes a room darker.

We write the figure down next to the U-value, so you can see the trade instead of taking our word for it.

At the survey we look at which way the roof faces, how much of the sky the opening will see, what the room is for and what is around the roof. A loft bedroom with a neighbour's gable close to the south is a different case from a kitchen extension in an open garden. From that we choose the glazing build, and the quote lists its transmittance and its U-value in the same paragraph as the panes, the coating and the spacer.

That description sits in the quiet spec that comes with every quote, because the same build decides the light and the sound. A laminated acoustic inner pane, for example, is chosen for the noise the room hears, and the spec records what it does to brightness at the same time. Nothing is left for you to find out after the glass arrives.

If a particular rooflight is already fitted and you want to know whether a new unit would brighten or dim it, bring the data sheet or the maker's name and we will compare. The wider range of builds is on our energy efficient rooflights page.

Is a higher light transmittance always better?

For brightness, yes. For comfort, not always. A very high figure on a south-facing roof can bring more summer heat with it, so the right number depends on orientation and on what the room is used for.

Will a low-E coating make my room noticeably darker?

A good modern coating costs a small amount of light, and the difference is hard to see by eye. You would see it only if you held the old and new units side by side. The heat it keeps in is usually the larger gain.

Does a dirty rooflight change the figure?

The rating is for clean glass, so grime on the outer face lowers the light you actually get. Regular cleaning keeps the room closer to its rated brightness, and it is easiest on a unit with a pitch rather than a dead flat one.

Where do I find the figure for my glass?

It is on the data sheet for the sealed unit, and a good supplier will give it on request. If you are replacing a unit, get a quote and we will write the light transmittance into the specification beside the U-value.