Overheating in a south-facing kitchen extension

Every complaint about a stuffy extension comes down to a mismatch: a lot of glass, pointed at the sky, over a room with very little to soak up the heat. Put those three together and you have built a solar collector with a dining table under it.

Start with the glazing. A rear extension exists because someone wanted light. That usually means a roof lantern, a run of flat rooflights, or a large pitched slope full of glass, and it means far more glazed area per square metre of floor than any other room in the house has. A living room might have one wall of windows. An extension often has a whole ceiling of glass.

Then the roof faces up. A wall window meets the summer sun at a steep glancing angle and sheds a good deal of it. A flat or shallow rooflight meets the same high midsummer sun almost square on, so it takes in the beam at close to full strength. The roof plane is the part of the building that collects the most solar energy per square metre in July, and the extension puts your glass exactly there.

Last, a single-storey extension is light. The floor is often screed or tiles over insulation, the walls are plasterboard on a timber or block inner leaf, and there is no upstairs pressing down with its own cool mass. A heavy Victorian room with thick masonry walls absorbs a heat peak and releases it slowly overnight. A modern extension has nowhere to put the heat, so the air temperature simply climbs with the sun and falls with it, an hour or two behind. The peak lands in the late afternoon, which is exactly when people come home and want to use the room.

It helps to be honest about proportions, because it changes what you spend money on. In a typical south-facing extension on a clear summer day, the heat load breaks down roughly like this.

The number that catches people out is the first one. On a clear day the sun delivers something in the region of 700 to 950 watts to every square metre of a roof surface pointed at it. A modest lantern of two square metres of glass is intercepting well over a kilowatt of raw solar energy at the peak. How much of that reaches the room is set by one property of the glass: its G-value, the fraction of solar energy the pane lets through as heat. Clear double glazing sits around 0.6, so it passes roughly six hundred watts of every thousand that hit it. A proper solar-control unit sits around 0.3, so it passes half as much. That single swap takes more heat out of the room than any amount of extra wall insulation ever could, because the wall was never the problem.

This is the wedge the whole business turns on. On a north-facing roof in a cool part of the country you can ignore G-value and nobody suffers for it. On a south-facing extension along the Essex estuary you cannot, and a quote that lists a U-value and no G-value has skipped the number that decides whether your summers are bearable.

South Essex has an unusually large stock of the exact house this problem lives in. The 1930s semi belt runs for miles across the patch, and the standard move on those houses over the last twenty years has been to knock the back kitchen and the old dining room into one open space and throw a glazed extension across the rear. The garden on a 1930s semi very often faces south or west, because the estate was laid out to catch the light. That means the new glass roof faces the sun.

Then there is the coast itself. Southend-on-Sea faces south across the Thames Estuary, which is rare for an English seaside settlement, most of which look north or east out to sea. Here the open water sits to the south, there is little tall development between a rear roof and the sun’s arc, and the estuary is a wide flat surface bouncing extra diffuse light up onto every south-facing roof plane along the shore. The sunshine totals for this corner of Essex are among the higher figures in England. The roofs here are collecting through more hours of it than most of the country manages.

None of that is a reason to build a darker extension. It is a reason to specify the roof glass for the aspect it actually has. A room that would be perfectly comfortable in Manchester with clear glass can be genuinely unusable in Leigh or Thorpe Bay with the same glass, purely because of where the sun is and how long it stays there.

Before you spend anything, work out what you are actually dealing with. Overheating from solar gain has a signature, and it is worth checking your symptoms against it.

• It tracks the sun, not the thermostat. The room is fine in the morning, climbs through the afternoon, peaks around four to six in the evening, and is still warm well after sunset. If it is hot first thing regardless of weather, that is a different fault.
• It is worst on clear days. A bright cloudless afternoon is far hotter than a warm but overcast one, even at the same outdoor temperature. Solar gain needs direct beam, so cloud takes the edge off it. If overcast warm days are just as bad, suspect poor ventilation rather than glazing.
• The heat comes from above. Stand under the glass on a sunny afternoon and you can feel the radiant warmth on your head and shoulders. That is near-infrared coming straight through the pane, and it is the tell that the glass is the culprit.
• It is worse than the rest of the house. If the extension is five or six degrees above the living room next door, the difference is the roof glazing, because that is the only thing the extension has that the older room does not.

A comfortable room sits around 21 to 23 degrees. People start to notice discomfort past about 25, and a south-facing extension with clear glass can run to 28 or higher through a July afternoon and hold there for hours. Bedrooms in a south-facing loft have the same problem with a sharper edge, because a room that peaks at 28 in the early evening is still shedding that heat at bedtime.

If solar gain is the problem, solar-control glass is the largest single lever, and it is worth understanding what you are buying. G-value is the fraction of solar energy the glass lets through. Lower means less heat. Here is the range you will meet in rooflight glazing, with the light transmittance alongside, because a good unit blocks heat without turning the room brown.

• Clear double glazing, G-value around 0.60. Maximum light, maximum heat. Right for a north-facing plane, wrong over a south-facing extension.
• Standard solar-control double, G-value around 0.35. A low-emissivity coating plus a solar-control layer. Passes roughly a third of the sun’s energy while keeping light transmittance up around 0.66, so the room stays bright. This is the sensible default for most south-facing extensions.
• High-selectivity solar-control double, G-value around 0.28. For large lanterns and big runs of flat glass over a single room. Still lets around 60 per cent of the daylight through, so you lose very little brightness for a real drop in heat.
• Body-tinted glass, G-value around 0.42. Usually the wrong answer. It cuts heat by absorbing it, so the pane itself gets hot and radiates a share of that back into the room, and its light transmittance collapses to around 0.44. You get a dimmer room that is barely cooler.

The point that most quotes miss: a selective coating separates the light you want from the heat you do not. Near-infrared, which carries the solar heat, is invisible, so a good coating can reflect it while letting the visible light through almost untouched. You do not have to accept a gloomy room to get a cool one. If someone offers you tinted glass to solve overheating, they are trading away your daylight to do a job the coating should be doing. Choosing the specification properly is the heart of an energy efficient skylight installation, and on a south roof it starts with the G-value, not the U-value.

One thing the glass will not solve on its own is a room that never lets heat out. Which brings in the second lever.

Glass controls what arrives. Ventilation controls what leaves, and in a room with a glazed roof it does something a wall window cannot. Hot air rises and stratifies, so the warmest air in an extension pools against the ceiling, which is precisely where the glass is. An opening at the top of that glass lets the hottest layer escape and pulls cooler air in low from the garden doors. That stack effect shifts far more heat than a window at head height ever will.

For a room that overheats, an opening rooflight is not a luxury. On a lantern or a high flat roof where nobody is going to reach a manual pole twice a day, an electric opening rooflight with a rain sensor is the practical answer: it can be set to purge automatically on warm evenings and shut itself when the weather turns. Pair a high-level opening with a low-level inlet, a door or a window on the shaded side, and you get genuine cross and stack ventilation through the room.

Timing matters as much as the openings. The trick in summer is to purge overnight and through the early morning, flushing the cool night air through the room to drop the fabric temperature, then close up and keep the heat out through the middle of the day. Left open through a hot afternoon, a rooflight lets hot outside air straight in. Ventilation is a tool with a schedule, not a switch you leave on.

If you are planning the extension now, the order of decisions is simple: orientation first, then the glass, then the unit, then ventilation, then shading as a finishing layer rather than a rescue. Survey which way the roof plane faces, put solar-control glass on the south and west planes, keep clear glass for any north plane, and design in an opening at the high point. Get those right and shading becomes optional comfort rather than a remedy for a mistake.

If the extension is already built and cooking you every summer, you have a few routes, in rough order of how much they cost and how well they work.

1. Replace the glazing units. On many lanterns and flat rooflights the sealed units can be changed for solar-control glass while the frames stay in place, which is the most direct way to cut solar gain in a room that is otherwise finished. Where the frames are failing or the whole assembly is dated, replacing the complete unit with a modern roof lantern or flat-roof skylight in the right glass does the same job and resets the seals and the U-value at the same time.
2. Add powered ventilation. Retrofitting or upgrading to an electric opening rooflight gives you the overnight purge that a sealed pane cannot.
3. Internal blinds. The cheapest option and the weakest. A blind sits behind the glass, so the heat is already in the room before the blind catches it. Reflective blinds help with glare and take a little off the peak, but they cannot compete with glass that stops the heat outside the pane.
4. External shading. Anything that blocks the sun before it reaches the glass, an external blind or a brise soleil, is far more effective than an internal one, though it costs more and changes the look of the roof.

For most people the honest answer is that the glass is where the money works hardest, because it deals with the heat at the point it tries to enter. Everything else manages the heat once it is already inside. If the underlying problem is a failed or leaking rooflight rather than an overheating one, that is a job for a roofer to look at first; a glazing change assumes the unit itself is sound.

Whether you are building new or improving a room you already have, the conversation with whoever quotes should cover the same short list.

• Which way does the roof plane face? Orientation decides everything downstream. A quote that treats every plane the same has not looked at the compass.
• What is the G-value of the glass? A number should come back. On a south-facing extension you want it down around 0.30, with the light transmittance still up near 0.6.
• Is the U-value centre-pane or whole-unit? Whole-unit is the honest figure and the one Building Control uses. Centre-pane numbers always flatter the glass.
• Is there an opening at the high point? If overheating is the brief, a purge vent belongs in the design, not as an afterthought.
• Who notifies Building Control? New and enlarged rooflights are notifiable under the Building Regulations, with Part L covering thermal performance and Part K covering the safety glazing that overhead glass requires. We make that notification to Southend-on-Sea City Council, or the relevant authority for your address, as part of the installation.

One regulation worth understanding. Part O of the Building Regulations addresses summer overheating, but it applies to new residential buildings, not to an extension on a house you already own, and Part L sets no G-value limit for a rooflight in an existing home at all. So for most extension work nothing in the rules obliges anyone to think about solar gain. That is exactly why so many installers do not, and why a roof full of fully compliant glazing can still leave you with a kitchen nobody wants to sit in on a bright afternoon.

Skylights On Sea has been installing skylights and rooflights across south Essex for over fifteen years, every job backed by a ten-year workmanship guarantee, and every survey starts by working out which way each roof plane faces before any glass is chosen. If your extension already overheats, or you want to make sure the next one does not, ask us for a specification with the G-value written on it for each plane, alongside the U-value, so you can see on paper what your summers are going to feel like.