How much winter sun reaches a Cambridge roof
At the winter solstice the sun over Cambridge climbs to only about 14 degrees above the horizon at noon, and the day lasts under eight hours, so the warming window is short and shallow.
Cambridge sits at roughly 52 degrees north. By the spring equinox the noon sun has risen to about 38 degrees, and by midsummer to about 61. That shift matters more for roof glass than for wall glass, because the angle at which the beam strikes the pane decides how much of its energy gets through. A low December sun hits a vertical south wall almost square on, but it only grazes a flat roof.
The flat Fenland landscape helps here. With few hills to cut off the horizon, a Cambridge roof catches the winter sun from about eight in the morning to just before four in the afternoon, unless a neighbouring house, a tree or a chimney stack stands in the way. On a terrace in Romsey or Chesterton, the ridge of the house next door can steal the lowest hour at each end of the day, and a survey checks exactly that.
Pitch and orientation decide the catch
A skylight's winter warmth depends far more on which way it faces and how steep it sits than on its size.
The geometry is plain once you picture it. A roof window on a 45 degree south slope faces the low sun almost directly, so it collects around three times the direct beam that the same area of horizontal glass does on a December noon. A flat rooflight on a single-storey extension sees the sun at a glancing angle. A north slope at 45 degrees gets no direct sun at all between late autumn and early spring, because the sun never climbs high enough to clear the plane of the roof. East and west slopes pick up a little low sun early or late, when it has the least strength.
| Glass | Direct sun at a December noon | Clear winter day |
|---|---|---|
| South slope, 45 degrees | Strong, nearly face on | Can gain more than it loses |
| Flat rooflight or lantern | Weak, glancing | Some gain around midday |
| East or west slope | Low, brief | Short morning or afternoon gain |
| North slope, 45 degrees | None | Daylight only, no sun gain |
For the north side of that table, see do north facing rooflights lose more heat?, which covers glass choices for a slope that never sees winter sun.
Weighing the gain against the loss over a whole day
A modern double-glazed rooflight loses heat every hour the room is warmer than the air outside, but it only gains solar heat for a few hours on sunny days.
Some illustrative arithmetic makes the balance clear. Take a one square metre rooflight with a whole-unit U-value of 1.4 W/m2K, a room at 20 degrees and an outdoor temperature of freezing. The glass loses about 28 watts, all day and all night, which adds up to roughly two thirds of a kilowatt hour over 24 hours. On a clear December day the same unit, with a g-value of around 0.5, might pass something in the order of 100 watts of sun at noon if it is flat, and more than twice that if it sits on a steep south slope. The flat unit roughly breaks even over a bright day. The south-facing roof window can come out ahead. On an overcast day, with only diffuse light, both lose more than they gain.
These are physics-based sketches rather than measured results for any home, and the real figures move with the frame, the upstand, the weather and the glass. The guide to rooflight heat loss in winter shows where the heat escapes, and do skylights increase heating bills? compares a modern unit with the insulated roof it replaces.
Where the warmth ends up in the room
Solar gain does not arrive as warm air; it lands as a sun patch on the floor, the worktop or a sofa, and those surfaces warm up and give the heat back slowly.
That is why a sunny winter afternoon under a skylight feels warmer than the thermostat reading suggests. Radiant warmth on your skin and a warm floor underfoot change how a room feels even when the air temperature barely moves. A dense floor, such as a concrete slab with tiles or polished screed in a kitchen extension, soaks up the midday sun and releases it into the early evening. A timber floor over a void or a thick rug warms quickly and cools just as fast.
Placement matters here too. A roof window over a stair void sends its winter sun onto a wall no one touches. The same unit over the table where you eat lunch puts the warmth where you sit. Working out where the low winter sun patch will land, month by month, is part of how we plan the light before a roof is cut.
Keeping winter gain without a summer penalty
The glass that lets in the most January warmth also lets in the most July heat, so a winter-friendly choice has to be checked against the summer sun.
In June the noon sun over Cambridge is about 61 degrees high, which puts it almost face on to a flat rooflight and a shallow south slope. A high g-value that feels generous in winter can make a loft bedroom or a south-facing kitchen uncomfortable by May. The balance point depends on the slope: a north light can take clearer, higher-gain glass without much risk, while a large south pane usually needs a lower g-value or external shading. What is a good g value for a south facing rooflight? works through that choice, and the g-value and light transmission guide explains the numbers on a glass specification.
Questions people also ask
Can a skylight replace a radiator?
No. Winter solar gain arrives for a few hours on clear days and disappears on grey ones and at night, so the room still needs its heating sized as normal. Treat the sun as a free top-up on bright days rather than a heat source you can plan around.
Does triple glazing stop the winter sun warming the room?
It trims the gain a little, because the extra pane and coatings lower the g-value, but it also cuts the loss through the night. On a north slope or a flat roof that trade usually suits winter comfort; on a steep south slope the difference is worth checking. The double vs triple glazed guide sets out both sides.
Why does the room feel cold under the skylight at night?
Older and single-glazed units have a cold inner surface that chills the air beside it, and that air sinks as a draught. A modern sealed unit with a warm inner pane largely stops it; see why a rooflight feels cold underneath.
Which rooflight gives the most winter warmth?
A roof window on a steep south slope, fitted with clear low-E double glazing and unshaded by neighbouring roofs, collects the most winter sun. Whether that suits the room in summer is the second half of the decision, which is where our energy efficient rooflights service starts. Book a light survey and we will show you where the sun lands through the year.
