Do solar control rooflights make a room dark?
Slightly, and far less than most people expect. A solar control rooflight typically passes around half to two thirds of the visible light that an equivalent clear unit passes, and because a rooflight faces the whole dome of the sky rather than half of it, the room below still receives more daylight than a vertical window of the same area would deliver with clear glass in it.
The fear is understandable. The words sound like sunglasses fitted to a ceiling. What actually happens is more selective than that, because the coating is tuned to reject the part of the solar spectrum you cannot see while passing most of the part you can. The wider thermal argument sits on energy efficient rooflights, and the specific coatings are covered on what glass stops solar gain.
Light transmittance, and what the percentage counts
The figure is written as LT or sometimes as Tv, expressed as a percentage of visible light that arrives at the outer face and reaches the room. A clear double glazed unit with a low emissivity soft coat sits around 78 to 80 per cent. Take that up to triple glazing and it falls to the high sixties simply because there is another pane in the way.
A moderate solar control unit lands around 60 to 70 per cent. A strong one, the kind specified for a large south facing lantern, drops to the low fifties or the forties. That is the whole range you are choosing within, and the difference between the two ends of it is smaller in a room than it is on paper.
Two figures that move together, and by different amounts
Every solar control product publishes both its light transmittance and its g-value, and the useful measure is the ratio between them. That ratio is called selectivity, and it is the honest test of whether a coating is clever or merely dark.
| Glass type | Light through | Solar energy through | Selectivity |
|---|---|---|---|
| Clear double with low-e | About 79 per cent | About 0.62 | 1.27 |
| Light solar control | About 70 per cent | About 0.41 | 1.71 |
| Standard solar control | About 62 per cent | About 0.33 | 1.88 |
| Strong solar control | About 50 per cent | About 0.25 | 2.00 |
| Body tinted grey glass | About 40 per cent | About 0.50 | 0.80 |
Read the bottom row against the others. A tinted glass loses more light than it rejects heat, which is why body tints have largely left domestic glazing. A modern coated unit does the reverse: it gives up a fifth of the daylight to reject two thirds of the heat.
Why a twenty per cent drop is not visible as a twenty per cent drop
Human vision responds to light logarithmically, not linearly. The eye adapts across a range of roughly one hundred thousand to one between a bright afternoon outdoors and a dim room, and it does that adaptation continuously without announcing it.
The practical consequence is that a reduction from 79 per cent to 62 per cent, which is a fifth of the light gone, reads as a very slight change in a room and is usually undetectable once you have been in the space for a minute. The change people do notice is glare, and solar control reduces that in a way they welcome. A room lit through a coated unit reads as calmer rather than darker.
The colour a coating leaves behind
This is the real specification risk, and it is not about brightness at all. Older solar control coatings had a pronounced tint, usually blue, sometimes bronze or a faint green, and on a large area of overhead glass a colour cast is very obvious because you are looking at sky through it all day.
Current soft coat products are close to neutral. Manufacturers publish a colour rendering index for the transmitted light, and anything at 95 or above is effectively invisible in use. Below about 90 you will see it, particularly on white walls and on skin tone, which is why an ordered sample held up against the sky is worth the delay before a large lantern goes into production.
What the room actually receives, in numbers
Take a four by five metre rear extension in Chelmsford with three square metres of flat rooflight. On an average overcast day in this part of Essex, outdoor horizontal illuminance sits around ten thousand lux.
Through clear glazing at 79 per cent that opening delivers a working daylight factor around three per cent across the room, which is a bright, comfortable space. Through a standard solar control unit at 62 per cent it delivers around two and a third per cent, which is still above the two per cent threshold used to define a well daylit habitable room. In July, when the same opening would be handling a kilowatt of solar energy per square metre, the coating is doing exactly what it was fitted for.
The pane the coating sits on
Position matters as much as product. A solar control coating belongs on surface two, meaning the inner face of the outer pane, because it has to reject the energy before it enters the cavity. A low emissivity coating belongs on surface three, the outer face of the inner pane, where it reflects long wave heat back into the room.
Some units combine both functions in a single coating on surface two. Others run two separate coatings. Either arrangement works, but a coating put on the wrong surface loses most of its effect, which is one of several reasons the glass make-up is worth writing into the order rather than leaving to the fabricator.
Where the coating is worth it, and where clear wins
A north facing slope in a Victorian terrace off Moulsham Street receives steady, cool, shadowless light and almost no direct gain. Coating that opening spends daylight to solve a problem it does not have, and clear glazing with a good U-value is the right call.
A flat roof over a south facing kitchen at Beaulieu Park is the opposite case. In July the sun sits at sixty degrees at midday and its energy arrives almost perpendicular to the glass, which is the worst geometry there is. There, solar control is not a preference. The room-side consequences of getting that wrong are covered on will a rooflight make my room too hot.
Reveals, finishes and the light you give back
If a specific unit has to be coated and the room is already on the dark side, the recovery is done inside rather than in the glass. A splayed reveal, opening outwards from the glass towards the room, spreads light across the ceiling instead of dropping a hard rectangle on the floor, and it roughly doubles the apparent size of the opening.
Finishes then do the rest. A matt white ceiling reflects around 85 per cent of what hits it, a mid grey wall around 40, and a dark timber floor under ten. Moving the ceiling and the upper walls to a high reflectance finish recovers considerably more perceived brightness than the coating cost you in the first place. It is the cheapest correction available on the whole job.
Getting the daylight written into the quotation
Four lines settle it. The product name of the coating, the light transmittance percentage, the g-value, and the colour rendering index of the transmitted light. Add the surface the coating sits on and the make-up is fully described.
Sunspire puts those figures on the fixed price after survey, alongside the whole unit U-value and the plane it was declared in. Coated glass cannot be changed later without replacing the unit, so the twenty minutes spent agreeing the specification before the order goes in is the whole of the decision.
Tell us about your roof
We start with the building, the covering and the slope. Then we tell you what will suit it, and what it will cost, as a fixed written number.
- Surveyed before it is priced
- 10-year workmanship guarantee
- Building Control notification handled
- New installations and replacements