How many hours of direct sun does a room get?
Sun hours in a room depend on three things working together: which way the window faces, how large the window is relative to the floor, and what stands in the way outside. Most people can estimate it themselves with a compass and a couple of rules of thumb.
Short answer: a south-facing room with nothing in the way can have direct sun in the window for 6-9 hours on a clear summer day and 3-5 hours on a clear winter day. A north-facing room typically has no direct sun at all — only daylight. The exact figure depends on the window's orientation, the season, the latitude and whether anything blocks the sun. There is no shortcut around working it out for your specific address and windows.
What determines the number of sun hours?
- The window's orientation. The single most important factor. South gets the most sun overall, east gets morning sun, west gets evening sun, north gets essentially no direct sun.
- The season. The sun sits higher in the sky in summer and lower in winter. A south-facing window therefore gets longer sun hours in summer, but a window with a low sill can actually get more direct sun deep into the room in winter, because the sun sits low and shines further in.
- The latitude. The further north you live, the bigger the difference between summer and winter — and the lower the sun sits in winter, even in the middle of the day.
- Obstructions. The neighbour's house, trees, a tall hedge or your own roof overhang can cut away several hours of sun, even with a perfectly south-facing façade.
What is a good window-to-floor ratio?
Daylight and direct sun are two different things. Direct sun is when the sun's rays hit the window unobstructed — that can be counted in hours. Daylight is the general light level in the room, even on an overcast day, and it depends mostly on how large the window area is relative to the floor area. A rule of thumb used widely in construction is that the glass area should make up at least around 10 per cent of the floor area to give a room that feels bright. A bedroom of 12 m² should therefore have at least around 1.2 m² of window glass to sit on the good side of that rule of thumb. Below that, the room starts to feel dark, even with sun outside.
How do you work out which room is darkest in a home?
The simplest measure is to compare the window-to-floor ratio for each room. A room with a small window relative to its floor area — or no window at all facing south, east or west — will typically feel darkest, no matter how much sun hits the rest of the house. North-facing rooms with small windows, and rooms with the neighbour's house close by, are the classic candidates. It can be surprising that it is not always the smallest room that is darkest — it's the ratio of glass to floor that decides it, not the room's size on its own.
How to estimate sun hours yourself for a specific room
- Find the window's orientation with a compass — preferably measured outdoors, since a compass indoors is easily thrown off by metal in walls and fittings.
- Look up sunrise and sunset for your address on the date you want to know the sun hours for. Most weather and astronomy services have this.
- Work out when the sun is within the window's field of view. A window facing due south "sees" the sun from roughly when the sun is 90 degrees east of south until it is 90 degrees west of south — broadly speaking, a large part of the day. An east-facing window typically only sees the morning sun, a west-facing one only the afternoon and evening sun.
- Subtract obstructions. If the neighbour's house, a tree or your own roof overhang gets in the way at some point in the day, those hours have to be subtracted. This often requires observing the room on site at different times if you want to be completely certain.
Why do two neighbouring homes with the same window orientation get different sun hours?
Because obstructions are local. Two houses can have an identical south-facing façade and yet experience completely different sun if one has a tall building or a large tree closer in the direction the sun sits low. That is one of the reasons you cannot read sun conditions directly off a compass direction in a property listing — the direction only tells half the story.
What cannot be worked out in advance?
No calculation can predict cloud cover. A sun-hours calculation shows the potential sun under a clear sky — how much sun you actually experience on any given day also depends on the weather. The calculation also cannot capture seasonal obstructions, such as a deciduous tree that shades in summer but lets the sun through in winter, unless you factor that in yourself.
How do window height and sill height affect sun hours?
Two windows in the same façade, same orientation and same width can let in different amounts of direct sun if the sill height — the distance from the floor to the bottom of the window — differs. A low window that almost reaches the floor catches low-lying sun earlier in the morning and later in the evening than a high-set window, because the angle into the room is different. In winter, when the sun never gets particularly high, this matters extra: a high-set window can lose the low winter sun entirely, while a low window catches it far into the room.
Are several smaller windows better than one large one, or the other way round?
For the total glass area — and therefore the daylight ratio — it makes no difference whether the area is spread across one large window or several small ones. But for direct sun hours it can make a difference if the windows face different directions: a room with one window facing east and one facing west can experience sun both morning and evening, where a single large window facing only one direction gives a more concentrated sun period. Corner rooms with windows in two walls therefore often have more sun hours in total than a room with the same glass area in a single façade.
What HouseSense does
For each scanned room, HouseSense calculates two figures: daylight as glass area relative to floor area, and how many hours of direct sun the windows get on the chosen date, based on the house's actual position and north orientation. The requirement is that the room has at least one window registered — without that, there is nothing to calculate on. The figures make it possible to compare rooms within the same home and see which one is genuinely darkest, instead of guessing from memory.