Why does a south-facing room get too hot in summer?
A south-facing room is rarely overheated by the south window alone — it's the roof windows, west-facing glass, missing external shading and a 17-hour summer day that do the damage.
Short answer: A vertical south-facing window actually lets in less solar energy in summer than in winter, because the high summer sun grazes the glass at a shallow angle. When a south-facing room still overheats, the cause is usually something else in the same room: a roof window or sloped skylight, which receives over four times as much sun in June as in December; west-facing windows, where the low evening sun shines deep into the room; internal curtains instead of external shading; and the fact that at northern European latitudes, the day is over 17 hours long in June.
The surprising part: the south window isn't the main culprit
It sounds wrong, but it can be calculated: a vertical south-facing window receives more direct solar energy in winter than in summer. The reason lies in the angle at which the sun strikes the glass.
A vertical south-facing window faces horizontally out towards the horizon. When the sun is due south, the angle between the sunbeam and the window is set by the sun's altitude, and the share of radiation that hits the glass follows the cosine of that altitude.
- Summer solstice, sun at about 57° (Copenhagen): cos(57°) = 0.54
- Equinox, sun at about 34°: cos(34°) = 0.83
- Winter solstice, sun at about 11°: cos(11°) = 0.98
In the middle of a clear winter day, the sun hits the south window almost head-on, while the summer sun only grazes it. The south window is the best-behaved of all window orientations when it comes to summer heat — and at the same time the only one that delivers anything at all in December.
So when a south-facing room becomes unbearably hot in July, it's worth looking at what else is in the room.
Culprit number one: roof windows and sloped skylights
A horizontal or near-horizontal window faces upward, towards the part of the sky where the summer sun sits. Here the share of radiation follows the sine of the sun's altitude — the exact opposite of the vertical window:
- Summer solstice, sun at 57°: sin(57°) = 0.84
- Winter solstice, sun at 11°: sin(11°) = 0.19
In other words, a roof window receives over four times as much direct sun per square metre of glass at midday in June as in December — the exact opposite of what you'd want. That's why a loft room with roof windows is typically the hottest room in the house in summer and the coldest in winter, regardless of which compass direction it otherwise faces.
A sloped roof window sits somewhere between the two extremes, depending on the roof pitch. The flatter the roof, the more the window behaves like a skylight.
Culprit number two: west-facing windows in the same room
Many rooms described as south-facing also have a window facing west — in the gable end, in the corner, in a door out to the terrace. The west window is what delivers the afternoon heat.
In summer, in northern Europe, the sun sets around north-west rather than due west, and it stays low for hours beforehand. A low sun in the west shines deep into the room — all the way to the back wall instead of onto the floor just inside the window — and it hits the vertical west window close to head-on, exactly as the cosine calculation above predicts for low sun altitudes. On top of that, the building has already been absorbing heat all day by the time the west sun arrives.
A roof overhang barely helps on the west side. Overhangs work against a high sun, and the west sun is, by definition, low when it shines in.
Culprit number three: the curtain is on the wrong side of the glass
Internal curtains, roller blinds and venetian blinds block the light, but they don't block the heat very well. The solar radiation has already passed through the glass and into the room by the time it hits the curtain; the curtain heats up and releases that heat into the room air. You get shade, but keep a large share of the heat.
External shading — screens, awnings, external shutters, external blinds, a pergola or a deciduous tree — stops the radiation before it passes through the glass, and the heat that's generated stays outdoors. The difference between internal and external isn't a detail; it's the single most important decision for summer comfort in a sun-facing room.
A deciduous tree to the south or west is the only kind of shading that adjusts itself to the season: full leaf cover in July, bare branches in January, when you actually want the sun in.
Culprit number four: the summer day is 17 hours long
Even if each individual hour is milder in summer, there are more of them. Day length can be calculated from cos(hour angle) = −tan(latitude) × tan(declination). For Copenhagen at the summer solstice: tan(55.7°) = 1.465, tan(23.4°) = 0.434, and the signed product gives cos(hour angle) = −0.635, i.e. an hour angle of 129.4° and a day of 2 × 129.4 ÷ 15 = 17.3 hours.
A seventeen-hour day means the house doesn't have time to cool down at night during a warm spell. Nights are short and often mild, and if the windows stay closed, the room starts the next morning warmer than it ended the day before. After three or four hot days in a row, it's the accumulated heat — not that day's sun — that produces 27 degrees in the bedroom.
What actually works — in order
- Shade from the outside, not the inside. Put screens or an awning on the window that's causing the problem — usually the roof window or the west window, not the south window.
- Give the south window an overhang. The depth can be calculated: overhang = window height ÷ tan(sun altitude). For a 1.5-metre-tall window and a summer sun at 57°, that's 1.5 ÷ 1.54 = 0.97 metres. The same overhang shades only the top 19 cm of the window in December, when the sun is at 11° — so the winter sun still gets in.
- Air the room out at night. Open two opposite windows to create a cross-draught during the cool hours between midnight and 6am. Heavy surfaces — concrete, brick, tile, masonry — can store that coolness and pull the temperature down the following afternoon.
- Keep the windows closed during the day when it's hotter outside than in. Open windows in the midday heat let heat in, not out.
- Move the function instead of fighting the physics. A loft room with roof windows makes a fine winter study and a poor July bedroom. Some rooms are best solved by using them differently.
- Consider solar-control glazing when replacing windows. Glass is available with different solar heat gain coefficients (g-values). Use a low g-value to the west and in roof windows, and a higher g-value to the south, where the winter sun is a benefit.
How to find out which window is the problem
The method is to go room by room and note four things for each window: which compass direction it faces, whether it's vertical or sloped, how many square metres of glass it is, and whatever might shade it from outside. A sloped window of 1.2 m² can easily deliver more summer heat than a vertical south-facing window of 3 m².
If you have a measured floor plan with the correct north direction, the question can be answered without guesswork. In HouseSense, the home is placed on the map so the north direction is correct, and the 3D model then shows genuinely calculated sunlight for a chosen date and time — say, 5pm on 21 June, when it usually turns out that it was the west window, not the south window, that put the heat into the room.