Solar Gain, Overheating and Shading

The geometry that causes it

The sun in Oxford reaches roughly 62 degrees above the horizon at midday in late June, and about 15 degrees at midday in late December. That difference is the whole story.

A vertical window in summer meets that high sun at a glancing angle, and much of the energy is reflected away rather than transmitted. The window’s own head and reveal cast a shadow down the glass. In winter, when the sun is low, the same window takes it almost square on, which is exactly when the free heat is welcome.

A rooflight does the opposite, and in the least helpful way. A slope pitched at 30 degrees faces a summer sun almost perpendicularly, so it transmits nearly everything. A flat rooflight or a shallow lantern is worse still. In winter, when you would like the gain, that same slope meets the low sun at a glancing angle and receives comparatively little.

So roof glazing collects energy in inverse proportion to when you want it. It is not a defect in any product; it is where the sun is.

How much energy is involved

On a clear summer day, solar radiation reaching a surface aimed at the sun is on the order of a kilowatt per square metre.

A three square metre lantern with ordinary clear double glazing, with a g-value around 0.6, therefore admits something like 1.8 kilowatts at peak. That is a substantial electric heater running in the room, all afternoon, with no thermostat and no off switch.

Put that number next to the room and the complaint stops being mysterious. It also explains why fans and open doors make so little difference: they are moving air around a room that is being actively heated faster than ventilation can remove it.

Orientation, and why it should be specified slope by slope

The direction a slope faces changes the answer completely, and the worst habit in the trade is ordering identical units for a whole roof so they match.

A north-facing slope receives no direct sun in this country and is a daylight source rather than a heat source. Specify it for maximum light and best insulation: high transmittance, high g-value, triple glazing if the room is occupied.

An east-facing slope takes the morning sun. It warms a room early, which is pleasant in a kitchen and unwelcome in a bedroom somebody is trying to sleep in past six in June.

A south-facing slope takes the most energy overall, across the middle of the day, in every season.

A west-facing slope is the difficult one and the most often underestimated. It takes the afternoon sun at a low angle, arriving at the end of a day when the building has already absorbed heat, into the hours when people are actually in the room. West-facing roof glazing over a kitchen or a living space is the single most common source of the July complaint.

Solar control glass, and the trade it makes

The most effective single decision, and it costs a modest amount if made at order and cannot be made afterwards.

Solar control glazing carries a coating that reflects a large part of the infrared while passing most of the visible light. It is described by its g-value: clear double glazing sits around 0.6, and a good solar control unit reaches the low 0.3s. Halving the g-value halves the heat admitted.

The trade is that light transmittance falls too, and there is usually a faint tint, sometimes slightly blue or bronze depending on the coating. Nobody notices it after a fortnight, and the room remains dramatically brighter than it was before the rooflight existed.

Where a room is genuinely dark and north-facing, the trade is not worth making. On a south or west slope over a room people occupy in the afternoon, it is not really a decision.

External shading beats internal shading, and it is not close

This is the point most often got wrong, and the physics is simple.

An internal blind stops light after it has already passed through the glass and entered the room. The energy is now inside. The blind absorbs it, heats up, and radiates that heat into the room. A dark internal blind can be hotter than the glass. The room is dimmer and barely cooler.

An external blind, awning or shutter stops the energy before it reaches the glass, and what it absorbs is carried away by outside air. The difference in effectiveness is severalfold rather than marginal.

External rooflight awnings are made for most standard roof windows, are usually a mesh that preserves the view outward while cutting most of the gain, and can be motorised with a sun sensor so they deploy without anybody being at home. They are far cheaper fitted at installation than retrofitted, because the fixings and the electrical supply are already there.

Internal blinds still have their place, for privacy, for blackout in a bedroom, and for glare on a screen. They are simply not a heat solution, and it is worth knowing that before buying them as one.

Ventilation, and the stack effect

Getting hot air out of a room is a matter of geometry rather than fan power.

Hot air rises and collects at the highest point. An opening at that highest point lets it leave, and as it leaves it draws cooler air in through any low opening. That is the stack effect, and a rooflight is the ideal high opening because it is the highest thing in the room.

An opening rooflight plus an open door or window at low level moves a considerable volume of air with no energy input at all. Two rooflights on opposite slopes are better still, giving cross flow as well as stack.

The practical caveat is height. A vent four metres up needs an electric actuator to be usable, and the supply for it should be run at first fix. A rain sensor that closes it automatically is worth having on anything you cannot see from where you sit, and on anything you might leave open when you go out.

Night purging is the other half of this. Opening up in the evening once the outside air has dropped below the inside temperature dumps the day’s accumulated heat and starts the next day cooler. In a well-insulated room this makes a larger difference than anything done during the day.

Thermal mass, and why lofts suffer most

Heavy materials such as masonry, concrete and plaster absorb heat slowly and release it slowly, which flattens the peak of a hot afternoon. Light materials do not.

A converted loft is the lightest room in the house: timber structure, plasterboard, insulation, and very little mass. It is also at the top of the building with the largest roof exposure. So it heats fastest, peaks highest, and holds that heat into the evening.

Nothing in a finished loft can add much mass, which means the levers there are the ones above: the right glass on the right slope, external shading, and vigorous purge ventilation at night. It is why we push harder on solar control specification in loft conversions than anywhere else.

Overheating in the regulations

Since 2022 the Building Regulations have addressed overheating directly, in Part O, for new dwellings and for some conversions. It sets limits on glazed area by orientation and requires a means of removing excess heat, and it can be satisfied either by a simplified method with fixed limits or by dynamic modelling.

It does not apply to fitting a rooflight into an existing house, which is where most of this work happens. But it is a useful check on whether a design is sensible: if the glazing you are proposing would fail Part O in a new build, it will make an existing room uncomfortable in exactly the same way.

Getting the size right in the first place

Daylight in a room saturates. Beyond a certain glazed area the room does not read as brighter, while the heat gain keeps rising in direct proportion.

Two well-placed modest rooflights on different slopes will light a room better than one very large one, distribute the light more evenly, offer stack and cross ventilation, and admit less total heat. The instinct to buy the largest unit the roof will take is the origin of a great many summer problems.

Glare, which is a separate complaint

Heat and glare arrive together and are not the same problem, and confusing them leads people to buy the wrong remedy.

Glare is a contrast problem. A very bright area of sky in an otherwise ordinary room forces the eye to adapt, and screens become unreadable while the rest of the room looks dark by comparison. It is worst when the bright source is in the field of view, which for a rooflight means when you are looking up or sitting under it at a desk.

Solar control glass reduces glare somewhat, because it reduces transmittance, but it is not primarily a glare product. What works is diffusion and position: splayed reveals that spread the light across the surrounding plaster rather than delivering it as a hard-edged beam, a light-coloured reveal that becomes a secondary soft source, and placing a desk out of the direct pool of light rather than directly beneath it.

For a home office under a rooflight, a translucent internal blind that diffuses rather than blocks is often the right answer, and here an internal blind genuinely is the correct tool. Just do not expect it to help with temperature.

What overheating does besides feeling unpleasant

Worth knowing, because it changes how urgent the problem is.

Sustained high temperature in a bedroom affects sleep more than most people credit, and a loft bedroom that sits above 26 degrees overnight is a health matter rather than a comfort preference, particularly for the very young and the elderly.

Timber furniture, floors and fabrics fade and dry under sustained direct sun through unfiltered glass, which is a slow cost people attribute to age rather than to the rooflight.

And the cooling that people reach for has a running cost. A portable air conditioner in a loft bedroom every summer night, fighting solar gain that could have been halved by a coating chosen at order, will cost far more over a decade than the coating would have.

What to do if you already have the problem

In order of effect for the money.

Fit external shading. It is retrofittable on most roof windows and it is the largest single improvement available after the fact.

Add night purge ventilation, motorising an existing opening unit if it is out of reach, and use it every evening in summer.

Check the extract and airflow through the room, since a room with only one opening cannot flow.

Consider replacing the glazing. On some units the sealed unit can be changed for a solar control equivalent without replacing the frame, which is a fraction of the cost of a new rooflight and delivers most of the benefit.

Internal blinds last, for glare and privacy, with realistic expectations of what they do to temperature.

And if you are at the specification stage rather than the regret stage: the g-value on a south or west slope is the decision. It costs little, it is invisible, and it is the difference between a room you gained and a room you gained for eight months of the year.