Do solar control rooflights make a room dark?
No, not in any way people notice once they have lived with it for a week. A modern solar control coating cuts the heat entering a room by roughly half while taking around a fifth off the daylight, and the human eye responds to light logarithmically rather than in a straight line, so a fifth less light reads as very nearly the same room. The comparison that matters is not solar control glass against clear glass. It is solar control glass against the blind that gets pulled across clear glass every afternoon in July, and a closed blind removes far more daylight than any coating. The glass choice for heat is on what glass stops solar gain and the physics of gain on solar gain, overheating and shading. This page is about what the room looks like.
The short answer, in two numbers
Every glass specification carries a light transmittance figure, written as LT or as a percentage, and a solar factor, written as g. Light transmittance is the share of visible daylight that gets through. The g-value is the share of total solar energy that ends up in the room, including the part the glass absorbs and re-radiates inwards, and it is covered on what is a g-value.
A clear double glazed unit with a low-emissivity coating typically sits around 78 to 80 per cent light transmittance with a g-value near 0.60. A good selective solar control unit sits around 60 to 70 per cent transmittance with a g-value between 0.25 and 0.35. So the heat is roughly halved and the light drops by something between a tenth and a quarter. That is the whole trade, stated honestly.
Why a fifth less light barely registers
Human vision adapts. The pupil opens, the retina adjusts, and perceived brightness follows something closer to a cube root than a straight line, which is why a room lit at 800 lux does not look twice as bright as one at 400. In practice a change of about 20 per cent in illuminance is at the edge of what a person notices in a room they know well, and it is invisible in a room they have never seen with the other glass in it.
Set against that, daylight outdoors varies by a factor of ten between a bright June afternoon and a grey January one, and by a factor of a hundred between full sun and heavy overcast. The daily swing dwarfs the coating. What people actually report after solar control glass goes into a Gloucestershire extension is that the room is less glaring and the sofa is not hot, rather than that it is darker.
Selectivity: the ratio that separates good glass from tinted glass
The number worth asking for is the selectivity ratio, which is simply light transmittance divided by g-value. A clear low-emissivity unit lands around 1.3. A modern selective solar control coating lands around 1.9 to 2.2. That means it is passing roughly twice as much light per unit of heat.
The reason it can do that is spectral. Only about 44 per cent of the energy in sunlight is visible light. Slightly over half is near infrared, which carries heat and no vision at all. A selective coating is a stack of ultra-thin silver and metal oxide layers tuned to reflect the near infrared while passing the visible band, which is a completely different mechanism from making the glass darker. It is not a filter that dims everything equally, and that is exactly why it does not make the room dark.
What it looks like from inside and from the street
From inside, a good selective coating reads as a very slight cool cast, and the honest description is that a white ceiling under it looks a shade less warm than the same ceiling under clear glass. It is noticeable if you hold a sample of each side by side and effectively invisible otherwise. Poorer coatings push further towards green or blue.
From outside, the glass has a light reflective sheen and looks a little more mirror-like against a grey sky than clear glass does. On a flat rooflight that nobody sees from the ground it is irrelevant. On a roof lantern in a garden, or a conservation rooflight in a Cheltenham conservation area, the external appearance is a real consideration and worth looking at as a sample rather than a datasheet, because the finishes vary between manufacturers.
The products that genuinely do darken a room
Three do, and they get confused with modern coatings.
- Body tinted glass, coloured all the way through in grey, bronze or green. It cuts light and heat in roughly equal proportion, so the selectivity is around 1.0, and it absorbs energy into the pane which then radiates inward. Overhead it is the wrong tool.
- Reflective coated glass of the older commercial type, which achieves a low g-value by mirroring everything. Very effective, visibly dark, and it turns the rooflight into a mirror from the garden at dusk.
- Applied solar control film on existing glass, which varies enormously. Some products are close to a selective coating, many are closer to a tint. The retrofit case is on retrofitting better glass into a rooflight.
Ask for the transmittance and the g-value together for any of these, because either number on its own hides the trade.
Any blind removes more daylight than the coating ever will
This is the comparison that settles the question. A mid-tone fabric roller blind on a rooflight typically passes 5 to 15 per cent of the light. A blackout blind passes essentially none. An external awning or shutter, which is the most effective shading of all because it stops the energy before it reaches the glass, passes very little.
A room with clear glass and a blind that is down from noon until seven o’clock through July and August is dark for those hours. A room with solar control glass and no blind is at 65 per cent daylight all day and every day of the year. Over a summer that is far more usable light, not less. Where both are wanted the coating handles the ordinary days and the blind handles the extremes, which is the arrangement described on external shading against internal blinds.
North-facing slopes, where the trade is not worth making
Solar control earns its place where the sun actually lands. A north-facing pitched slope in Gloucester receives no direct sun for most of the year and the summer exposure is limited to early morning and late evening, so a solar control coating there gives up daylight in a room that has none to spare and prevents very little heat. Clear low-emissivity glass with the highest transmittance available is the right specification, and that case is on rooflights on a north-facing slope.
The opposite end is a west-facing loft slope or a near-horizontal rooflight over a kitchen, where a shallow plane sees the sky all day and takes the four o’clock sun square on. There the coating pays for itself in the first summer. Specification by aspect is set out on specifying glazing by which way the slope faces.
Rooflights start with more daylight than a window, which is the headroom
An opening in a roof sees a far larger area of sky than a vertical window of the same size, because it faces the whole dome rather than a strip of it interrupted by a fence, a hedge and the house opposite. The accepted rule of thumb is that a rooflight delivers roughly two to three times the daylight of the same area of wall glazing.
That is the reason the trade is comfortable. Taking a fifth off two to three times as much daylight still leaves far more than a window would have provided, and the glass is doing it without the glare that comes from low sun through a vertical opening. In a Gloucester rear-addition kitchen with one small window to the yard, that is not a marginal difference.
Where the choice gets made on a Gloucestershire roof
Flat rooflights over rear extensions across Gloucester, Quedgeley and Bishops Cleeve are the clearest case, because a near-horizontal plane takes the June sun at close to a right angle in the middle of the day and there is nothing above it casting shade. West-facing loft conversions in the interwar semis at Longlevens, Hucclecote and Warden Hill are the second, and they are the rooms people complain about most.
Against that, a Stroud valley cottage cut into a north-facing bank, or a Forest of Dean terrace in the shadow of the slope behind it, wants every photon it can get and the answer there is clear glass. The compass and the pitch are recorded at survey before any coating is priced, because the same glass is the right answer on one slope and the wrong one on the next. How the coating sits alongside the U-value and the rest of the specification is on the energy efficient rooflights hub.
Next on this subject.
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