Light Transmittance & Daylight Factor for Rooflights

Light transmittance tells you what proportion of visible daylight passes through the glass, while daylight factor describes how bright the room itself is. Rooflights score highly on both because roof glazing catches light from the open sky. A rooflight can deliver far more daylight than a wall window of the same size, which is why they transform dark rooms so effectively.

If you want a genuinely bright room rather than just a nice-looking skylight, these two ideas are worth understanding. This guide explains them in plain English and shows how to plan for the daylight you actually want.

Light transmittance: how much light gets through the glass

Light transmittance, sometimes written as LT or the visible light transmittance, is the percentage of visible daylight that passes through the glazing. A figure of 70% means seven-tenths of the daylight hitting the glass comes into the room.

• Clear double glazing: typically around 70–80% transmittance
• Solar control glazing: a little lower, often around 60–70%, as the coating trims some light along with the heat
• Triple glazing: a touch lower again, as each pane absorbs a small amount

Higher transmittance means a brighter room, but it is not the only thing that matters, because where the glass sits is just as important.

Daylight factor: how bright the room actually is

Daylight factor measures the brightness inside the room as a proportion of the daylight available outside. It accounts for the glazing, its position, the size of the opening and the room itself. A higher daylight factor means a brighter, more evenly lit space.

This is the number that reflects how the room feels. You can have high-transmittance glass and still get a gloomy result if the opening is small or badly placed, and you can get a bright, even room from well-positioned rooflights with sensible glass.

Daylight factor in numbers

It helps to put some figures to daylight factor, because they explain why rooflights are so transformative. Daylight factor is expressed as a percentage: it is the ratio of the light level at a point inside the room to the light level outdoors under an overcast sky, measured at the same moment. As a broad guide, an average daylight factor of around 2% gives a room that feels reasonably daylit, while 5% or more gives a room that feels genuinely bright and rarely needs the lights on during the day. A gloomy room languishing below 1% is one that always feels as though it needs artificial light, however large its windows look.

The reason rooflights move this number so effectively is that daylight factor has three components: the light coming directly from the sky, light reflected off external surfaces, and light bounced around internally off the room’s own walls and ceiling. A rooflight maximises the first and largest of these, the direct sky component, because it has an unobstructed view of the brightest part of the sky. A wall window, by contrast, often sees only a slice of low sky above whatever is opposite, which is why a modestly sized rooflight can lift a room’s daylight factor far more than a much larger window in the wall.

Why rooflights daylight so well

Roof glazing has a real advantage over wall windows, and it comes down to where the light comes from.

It sees the open sky

A rooflight faces the brightest part of the sky directly, unobstructed by neighbouring buildings, fences or trees that often shade a wall window. The sky overhead is the strongest, most consistent source of natural light.

It spreads light evenly

Light entering from above falls more evenly across a room than light from a single wall, reaching deep into the space and reducing dark corners.

It works where walls cannot

In a loft, an extension or an internal room with few external walls, a rooflight may be the only practical way to bring in good daylight at all. As a rule of thumb, roof glazing delivers substantially more daylight than the same area of vertical window.

What trims transmittance in a real unit

The figure quoted for a glass type is for the glass alone, but several things in a finished rooflight reduce the daylight that actually reaches the room, and it is worth knowing them so expectations are realistic. Each pane of glass absorbs and reflects a little light, so a triple-glazed unit transmits slightly less than a double-glazed one of the same glass; this is part of the trade-off when choosing triple glazing for a daylight-critical room. A low-emissivity or solar control coating trims a few more percent. The frame and any glazing bars cast their own shadow, so a unit broken up by bars admits less light than one large unobstructed pane of the same overall size.

Beyond the unit itself, the depth of the roof build-up matters enormously. A rooflight set at the top of a deep, narrow shaft through a thick insulated roof loses a great deal of light to the sides of that shaft, whereas the same rooflight over a splayed reveal that opens outward spills far more daylight into the room. This is why the way the opening is formed, not just the glass specification, is so often what determines whether a room feels bright. We consider the reveal and build-up as carefully as the glass when planning for daylight.

Balancing daylight against heat and glare

More light is not always purely a win. A large area of high-transmittance glass on a sunny roof can also bring heat and glare. The skill is in balancing daylight against the g-value, the measure of solar heat gain, so the room is bright but comfortable.

Solar control glazing trims transmittance slightly while cutting heat substantially, which is usually a good trade on a sunny roof. The light-to-solar-gain ratio is the number that captures this: a well-engineered solar control glass keeps a high light transmittance for the heat it rejects, so you lose little brightness for a lot of comfort. Our energy-efficient skylight installation page explains how to strike that balance, and our triple-glazed rooflight installation page covers how an extra pane affects both daylight and warmth.

Planning for a genuinely bright room

• Right-size the glazing. A modest rooflight in the right spot can outperform a larger one badly placed.
• Spread the light. Two rooflights can give brighter, more even daylight than one large pane in a corner.
• Mind the depth. A deep reveal or thick roof build-up can narrow the light; a splayed reveal opens it out.
• Match glass to orientation. A north-facing rooflight can use higher-transmittance glass; a south-facing one usually benefits from solar control.

Where rooflights make the biggest difference

• Loft conversions: roof windows bring light into a space with limited walls
• Kitchen extensions: a lantern or large rooflight floods an open-plan room
• Internal rooms: hallways, landings and en-suites with no external wall
• North-lit studios: soft, even daylight ideal for working

Getting the daylight right for your room

The brightest result comes from matching the glass, size and position to your specific room and roof, then balancing daylight against heat and glare. We assess all of that at a free survey, recommend a specification, and set it out in a fixed written quote. Every installation carries our written guarantee on our workmanship.

To plan a genuinely bright room, get a quote for a free survey, or see what we install on our services page and our roof lantern installation page.