Will a rooflight make my room too hot?

Four things decide it, and the glass is only one of them

Aspect comes first. A south-facing slope takes the most energy over the year but takes it when the sun is high, so a steep pitch turns much of it away. A west-facing slope takes less over the year and takes it at four in the afternoon with the sun low and square-on to the glass, into a house that has already been warming since morning. West is the hard case in this county and it is not close.

Pitch is second, and it interacts with aspect rather than standing on its own. Glass area relative to floor area is third: a single 780 by 1180mm roof window in a four metre by four metre bedroom is a different proposition from three of them, or from a two metre lantern over a kitchen island. And fourth, the room itself. A loft conversion with a lightweight timber floor, plasterboard on all sides and no way to cross-ventilate has nothing to absorb heat and no way to get rid of it.

Why overhead glass gains more than a window in a wall

Sunlight arriving square-on to a pane delivers its full intensity. Arriving at an angle, it spreads the same energy over a larger area of glass, and the reduction follows the cosine of the angle between the beam and a line perpendicular to the surface.

At Gloucester’s latitude the midsummer sun reaches about 61.6 degrees above the horizon at noon. A vertical south-facing window is then presented at a steep angle and takes only part of what is available. A roof plane at 30 or 35 degrees is far closer to square-on, so it takes considerably more. That reversal is the whole reason a loft room heats up faster than the bedroom below it with a bigger window in the wall. In December, with the sun at 14.7 degrees, the arrangement flips and the vertical window collects more, which is why the same rooflight that is a liability in July is worth having in January.

How much heat is actually coming through

Peak solar irradiance on a well-oriented surface in a Gloucestershire summer is around 700 to 850 watts per square metre. A standard double-glazed unit with a low-emissivity coating has a g-value near 0.5 to 0.6, meaning roughly that fraction of the arriving energy ends up in the room. Two square metres of glass at 800 watts per square metre and a g-value of 0.55 is around 880 watts standing in the room.

That is comparable to a small fan heater running continuously, in a room that also contains people, a television and possibly a computer. A solar control unit at a g-value of 0.27 brings the same two square metres down to roughly 430 watts. Neither number is the whole story, because how quickly the room sheds it matters as much, but it explains why the effect is not subtle.

The rooms where it genuinely happens

Loft conversions on a west or south-west facing rear slope, with the pitch between 25 and 40 degrees, used as a bedroom or an office. These are the calls that come in during the second week of a heatwave, and across Gloucester, Cheltenham and the vale there are a great many of them because the rear slope is where a conversion window can go without a planning conversation.

Roof lanterns over kitchens are the other one. The room already makes heat from an oven and a hob, the glass area is large, and the plan is usually open to the rest of the ground floor so the heat spreads. That has its own page at do roof lanterns make a kitchen too hot. Flat rooflights over a single-storey rear addition are a third: near-horizontal glass sees the sun from every aspect across the day and there is no pitch to turn anything away.

The rooms where it does not

North-facing slopes take diffuse sky light and almost no direct beam between the equinoxes, so the gain is steady, low and welcome. A rooflight on a north slope can be sized up without a penalty, which is covered on do rooflights work on a north facing slope.

Steeply pitched roofs help. A Cotswold stone slate roof at 50 degrees presents its glass at a much less favourable angle to a high summer sun than a 30 degree concrete tile roof in Hucclecote does, so an identical unit gains noticeably less. Small units in large rooms rarely cause trouble, and neither do halls, landings and bathrooms, where nobody sits for three hours in the afternoon. A ground-floor room under a rear addition often has a neighbouring gable, a mature tree or the main roof of the house shading it for part of the day, and that shade is worth checking before assuming the worst.

Summer peak against the rest of the year

The uncomfortable period in a Gloucestershire loft room is genuinely short: perhaps fifteen to twenty-five afternoons across June, July and August, concentrated in two or three settled spells. For the other eleven months the same glass is delivering daylight and, from October to March, useful free heat that reduces what the heating has to supply.

That is worth keeping in proportion, because the instinctive answer to overheating is to specify the darkest glass available, and doing so costs daylight in December when there is very little of it to spare. The balance point sits differently for a bedroom, an office and a landing, and it is a decision worth making deliberately.

Heat that has nothing to do with the rooflight

Before blaming the glass it is worth counting what else is in the room. An adult at rest gives off around 100 watts. A desktop computer and two monitors is 200 to 350. A television is 60 to 150. Downlighters in an older loft conversion can be another 300. A room with two people working in it can be generating 700 watts before any sun arrives.

The roof itself is a second source. A dark concrete tile roof in full sun reaches 60 to 70 degrees at the surface, and where the rafter depth allows only 100mm of insulation, that heat arrives in the room through the ceiling several hours later. Plenty of loft conversions across the interwar estates at Longlevens and Warden Hill are hot in the evening for that reason rather than because of the window.

Gloucestershire slopes, and the ones that cause the calls

Interwar semis with hipped concrete tile roofs at 30 to 35 degrees across Longlevens, Hucclecote, Tuffley and Charlton Kings put the largest usable slope at the rear, and on a great many of those houses the rear faces somewhere between south and west. That combination of pitch and aspect is close to the worst case for a July afternoon.

The trussed estates at Quedgeley, Abbeydale, Bishops Cleeve and Ashchurch are similar in aspect but the units are narrower, so the glass area is smaller and the problem is milder. Cotswold roofs are steeper and the villages sit in landscape that often shades a slope in the late afternoon. Stroud valley houses can have a hillside cutting off the sun by five o’clock. Severn Vale rear additions are the flat-roof case, where the glass is near horizontal and aspect stops mattering.

Deciding before you order, rather than after the first July

The survey records the aspect off a compass, the pitch off a digital level, the glass area against the floor area, what shades the slope and at what time of day, and what the room is going to be used for. Those five things decide the glass specification, whether shading is designed in from the start, and whether the unit needs to open. Adding solar control glass at the point of order costs a fraction of what changing the glazing afterwards does, which is set out on can you retrofit better glass into a rooflight. The energy efficient rooflights hub covers how gain and heat loss get balanced against each other.