Will a rooflight make my room too hot?
It can, and whether it does is decided before anything is ordered. Three things settle it: how much glass goes in relative to the floor area, which way the roof plane faces and how steeply it sits, and what the glass is specified to let through. Get those three right and a rooflight adds daylight without adding a July problem. Get the first one wrong and no amount of glazing technology rescues it.
This page is about predicting the answer for your room. The measures that bring a room back under control once the glass is already in are covered on how do you stop a rooflight overheating, and the full specification sits on energy efficient rooflights.
The sun sits almost overhead in June
At the latitude of Chelmsford the sun reaches about 61 degrees above the horizon at midday in late June, and it stays above 50 degrees for roughly six hours either side of noon. At that altitude a vertical window is being struck at a glancing angle and a good deal of the energy reflects off the pane. A horizontal rooflight is being struck almost square on.
The consequence is that a flat rooflight collects its peak solar load in high summer, precisely when the room needs it least, while a south facing wall window collects its peak in spring and autumn. That inversion is the whole of the overheating question in one sentence.
Glazed area against floor area, the ratio that tips it
The daylight rule of thumb is glazing at fifteen to twenty per cent of the floor area below. For overheating the useful threshold is lower and depends on the plane.
| Roof plane | Comfortable without solar control | Above this, specify it |
|---|---|---|
| North facing pitch | Up to about 20% of floor area | Rarely an issue at any size |
| East facing pitch | Up to about 12% | Early morning bedrooms first |
| West facing pitch | Up to about 8% | Late afternoon is the worst hour |
| South facing pitch | Up to about 8% | Almost always worth it |
| Flat roof, horizontal | Up to about 7% | Assume solar control glass |
These are starting points, not regulations. A room with a tiled floor, a decent volume and an opening high in the roof tolerates more. A small room with a plasterboard ceiling and one sealed unit tolerates less.
Which rooms actually run hot
Not every room with a rooflight has a problem, and the pattern is fairly consistent across the housing in this county.
- Rear kitchen extensions with a flat roof. Large glazed area, a west or south west aspect off the back of a Victorian terrace, and appliances adding their own load. The most common case by a distance.
- Loft bedrooms under a west facing slope. Small volume, low thermal mass, and the heat arrives at the end of the day when the room is about to be used for sleeping.
- Loft bedrooms under an east facing slope. Comfortable in the evening, uncomfortably bright at four in the morning in June.
- Single storey side returns with glass at both roof and wall. The combined area is often well past twenty per cent of the floor.
Rooms that rarely trouble anyone: north facing slopes, sun tunnels of any size, hallways and landings, and any room where the rooflight is under two per cent of the floor area.
What happens to the heat once it is inside
Solar energy passes through glass as short wave radiation, lands on the floor and the furniture, and is re-radiated as long wave heat that the same glass will not let back out. That is why a room heats faster than it cools.
Where it goes next depends on what the surfaces are made of. A screed floor, a tiled finish or exposed masonry absorbs the energy and releases it slowly, which flattens the afternoon peak by two or three degrees and pushes it later into the evening. A timber floor over joists with a plasterboard ceiling has almost nothing to absorb into, so the air temperature follows the sun within the hour.
The glass specification, and what it is worth
The number that governs solar heat is the g-value, the fraction of solar energy that passes through the unit. A clear double glazed rooflight sits around 0.6 to 0.7. A solar control unit sits around 0.25 to 0.40.
Dropping from 0.65 to 0.35 removes roughly half the incoming solar energy, which on four square metres of flat glass at midday is several hundred watts of heat that never enters the room. It is the single most effective decision available and it costs nothing to implement, provided it is made before the unit is manufactured. The trade is a small reduction in daylight, which is covered on do solar control rooflights make a room dark.
Ventilation changes the ceiling on the problem
Heat collects at the highest point in a room. An opening at roof level gives it somewhere to go, and cooler air is drawn in through windows lower down without a fan running. In a room with a two and a half metre ceiling the effect is modest. In an extension with a raised lantern it moves a genuinely useful volume of air.
This is why a fixed rooflight and an opening one of the same size can produce different summer experiences. If the room is south or west facing and the glass is over about six per cent of the floor, an opening unit earns its cost.
What the Regulations expect of a new room
Approved Document O applies to new dwellings and to some material changes of use, and it sets limits on glazed area by orientation together with a required amount of purge ventilation. It does not apply to a rooflight added to an existing house.
The method behind it is still worth borrowing. It works by limiting how much solar energy can enter, then requiring a route for heat to leave. Those are the same two levers on any project, notifiable under Part O or not.
Predicting it before you commit
You can settle this on paper. Take the floor area of the room, take the proposed glazed area, and divide. Note which way the roof plane faces and what its pitch is. Then look at the floor finish and whether the unit opens.
A four by five metre kitchen with three and a half square metres of flat glass is at seventeen and a half per cent, facing the whole sky. That room wants solar control glass, an opening unit and preferably a hard floor. The same three and a half square metres in a nine metre by five open plan space is at seven and a half per cent, and the conversation is much shorter. The survey does this arithmetic, but doing it yourself first means the quotation arrives with the right glass already in it.
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
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