Do roof lanterns make a kitchen too hot?

Why overhead glass is the worst case

In summer the sun is high and shines almost straight down. A vertical window catches that obliquely; a lantern catches it nearly square on. Overhead glass receives more solar energy than any other glass in the building.

A kitchen makes it worse, because it already produces heat from cooking and appliances.

The number that matters

The g-value, the proportion of solar energy that gets through. Clear double glazing is around 0.7. Solar control glass is around 0.4 or lower.

The difference between those two on a south facing kitchen lantern is the difference between a room you use on a July afternoon and one you avoid.

The three things that fix it

Solar control glass. Stops the energy at the outer pane before it enters. Not optional on south or west.

An opening ridge vent. Lets accumulated warm air leave at the highest point of the room. Extremely effective, and it works on a still day.

External shading on a very large area. Rare domestically, but it is the only thing that beats solar control glass, because it stops the energy before it reaches the glass at all.

What does not fix it

Internal blinds. The energy has already passed the glass; the blind absorbs it and re-radiates it into the room as heat. Blinds are for glare, privacy and blackout, not for temperature.

North facing

No solar control needed at all. North light is even, cool and glare free, and specifying solar control there is money spent making a good room slightly darker.

They can, and it is the commonest complaint

A lantern over a kitchen extension is a large area of glazing, much of it close to horizontal, collecting the midday sun directly. Add a room that already generates heat from cooking, and a south or west facing aspect, and it will be uncomfortable in July unless it was specified for it.

The good news is that everything that prevents it is decided at order stage and none of it is expensive relative to the lantern.

Solar control glass

Close to essential on any lantern facing south or west, and worth considering on any lantern at all, because the sloping faces collect from several directions.

It reflects a large proportion of the infrared energy carrying heat while transmitting most of the visible light, so the room stays bright and stops being an oven. It costs a modest premium at the point of order and a great deal to change afterwards.

An opening vent

The second half of the answer, and the one that deals with heat already in the room.

Warm air rises and collects at the apex of a lantern, which is exactly where a vent should be. Opening it lets that heat leave and draws cooler air in through anything open lower down.

The useful version is night purge: open it late, let the accumulated heat out overnight, close it in the morning. On a rain sensing motorised vent that happens without anybody thinking about it.

Sensible glazed area

Around 15 to 20 percent of floor area gives a bright room without turning it into a greenhouse. The common mistake is specifying the largest lantern the extension will take.

Doubling the glazed area doubles the solar gain, and on a kitchen that is already the warmest room in the house that matters.

Measures that will not solve it

Internal blinds, mostly. By the time the blind stops the sun the energy has already entered the room. They control glare and privacy and they are not a solution to heat.

External awnings do work and they are the retrofit answer where the glazing is already in, at considerably more cost than getting the glass right first.

If you already have one that overheats

External shading, and adding an opening vent if the lantern does not have one. Replacing the glazing is possible and it is the expensive route, because the labour and access are most of the cost.

Improving the extract ventilation in the kitchen helps more than people expect, since a cooker hood removes heat as well as steam.

They can, and it is the most common regret

A lantern faces straight up, so it receives the full summer sun through the middle of the day. Put one over a kitchen on a south or west facing rear extension, add bifold doors below it, and the room can become genuinely unpleasant on a hot afternoon.

A kitchen makes it worse than most rooms, because ovens and hobs are adding heat of their own.

What prevents it

Solar control glazing. The g value is the number that matters, and bringing it down costs a modest amount in light transmission. On a south facing lantern this is close to always worth it.

An opening vent in the lantern. Hot air collects at the highest point in the room, which is inside the lantern. Venting there, combined with an opening at low level, produces real stack ventilation rather than an open window.

Restraint on size. More glass does not deliver proportionally more perceived brightness, because the eye adapts. It does deliver proportionally more solar gain. A smaller lantern with better glass frequently produces a room that is both bright and usable.

What to avoid

Deciding to deal with it later. Internal blinds on a lantern are expensive, awkward and far less effective than getting the glass right, because they stop the heat after it has already entered the room.

The regulatory backdrop

Overheating provisions now apply to new dwellings, and while an extension to an existing house is treated differently, building control will have a view on a large area of unshaded horizontal glazing. The physics does not change with the paperwork.