Do roof lanterns cause overheating?

So the honest answer is that a lantern does not cause overheating the way a leak causes damp. It concentrates sunlight, and whether that becomes a problem depends entirely on what you glaze it with and where it sits. A north-facing lantern with standard glass will be fine. A south-facing one over a kitchen, glazed with clear panes, will bake the room. The rest of this page is about telling those two situations apart before the glass goes in.

A lantern sits on top of the room and tilts its glass towards the sky. That geometry is the whole story. A vertical window only catches the sun when it is low, in the early morning or late afternoon. A lantern catches it when the sun is high and strongest, from late morning through the middle of the day, right when the estuary sun is at full strength in June and July.

There is also more glass than people expect. A lantern is not one flat pane. It has a ridge, sloping sides and often hipped ends, so the glazed area can be half as much again as the opening it caps. Every one of those panes is pointed at a bright part of the sky. On a clear summer day a well-oriented square metre of glazing can carry something in the region of 700 to 800 watts of solar energy, and clear double glazing lets most of that straight through as heat. Put four or five square metres of it over a room and the numbers stack up quickly.

The room underneath usually makes it worse. Lanterns tend to go over open-plan kitchen-diners, which already generate their own heat from the oven, the hob and the fridge working against the warmth. You are adding a solar collector to the one room that was already the warmest in the house.

Which way the room faces changes everything, and around here it changes it more than most places. Southend-on-Sea faces south across the Thames Estuary, so a great many rear extensions and the kitchens behind them look straight out over open water with nothing to shade them. Inland, a south-facing roof spends part of the day behind the house opposite or a line of trees. Along the estuary the southern sky is open, and the water throws reflected light back up at the glass on top of the direct sun.

The local housing stock lines up badly with that. The 1930s semi belt through Westcliff, Thorpe Bay and out towards Hadleigh was mostly built with gardens to the rear, and for a large share of those houses the rear elevation faces somewhere between south-east and south-west. Add a flat-roofed kitchen extension, drop a lantern into it for the daylight, and the roof is now aimed at the sun for most of the cooling season.

A flat-roofed lantern is the sharp case. Because the glazing points upwards rather than at a compass bearing, orientation barely rescues you: it sees the high summer sun almost square on whichever way the house is turned. That is why a lantern needs treating as a solar-gain problem first and a daylight feature second. It is the reasoning that sits underneath our roof lantern installation work, and it is why we ask which way the room faces before we talk about anything else.

The number that governs summer heat is the G-value, sometimes written as the solar factor. It is the fraction of the sun’s energy that gets through the glass and ends up warming your room, on a scale from 0 to 1. Clear double glazing sits around 0.6, meaning roughly sixty per cent comes through. Solar-control glazing brings that down to somewhere near 0.3, and the good units go lower.

Here is the difference in real terms. Take three square metres of lantern glazing on a bright day. At a G-value of 0.6 you are letting through enough solar energy to run a small fan heater under the ridge all afternoon. Swap the same panes for a solar-control unit at 0.28 and you roughly halve that load. The room still fills with daylight. It stops filling with heat.

The catch is that Building Regulations do not force the issue. Part L sets a ceiling on the U-value, which is about winter heat loss, but it puts no equivalent limit on the G-value. So a lantern can be fully compliant on paper, keep the warmth in beautifully on a January night, and still cook the room every July. That gap is exactly where overheating comes from, and closing it is a specification choice, not a regulation you can lean on. There is more on how we choose it in our guide to energy-efficient skylight installation.

Solar-control glass is not free of trade-offs, and it is worth knowing them before you ask for it. The coating cuts visible daylight as well as heat, so a strong unit transmits less light than a clear one, though a lantern has so much glazed area that the room rarely feels dark. The coating also carries a faint tint, usually a subtle blue or grey, more noticeable from the garden looking up than from inside. And it adds a little to the unit cost. On a lantern that is money well spent, because the alternative is a room you avoid on hot days.

Glass is the biggest lever, but it is not the only one. A comfortable lantern usually pulls two or three of these together:

• Ventilation at the top. Hot air rises to the highest point of the room, which in a lanterned kitchen is the ridge. An opening vent up there lets it escape instead of pooling. Pairing one opening unit with fixed solar-control panes often does more than pushing the glass specification alone.
• External shading. Anything that stops the sun before it reaches the glass beats anything fitted behind it. External blinds or a brise-soleil are the strongest options, because once the energy is through the pane, an internal blind can only manage the glare, not the heat.
• Sensible glazed area. A lantern sized to the room lets in plenty of light without turning the whole roof into glass. If the glazed area runs to more than about a sixth of the floor below, it is worth dropping the G-value a band lower to compensate.
• Getting the specification right at replacement. If a clear-glazed lantern is already overheating a room and coming out, the swap is the moment to change the glass. The opening and the kerb are there, so the only real variable is what you put back in.

If you are planning a lantern, work out the easy things yourself first. Note which way the room faces, whether the roof is flat or pitched, and roughly how much glass you are putting in against the size of the room. A south-facing or flat-roofed lantern over a kitchen wants solar-control glazing at 0.3 or below, ideally with a way to let hot air out at the top. A north-facing one can take standard glass and enjoy the winter warmth.

If you already have a lantern that turns the room into a hotbox every summer, the fix is a change of glass rather than anything a roofer would carry out, and replacing the unit with a solar-control one is exactly the kind of job we do. Either way, the sensible move is to price the glass properly rather than the opening. Our skylight installation costs page shows where that lands, and if you would rather we specified it, tell us the orientation and the rough size on 01702 898232 or request a quote and we will look at the whole roof, not just the hole in it.