Sit by a sunny window on a cold day and you will feel warm, even though the air outside is freezing. That warmth is solar gain: heat from the sun entering a building, mostly through its glass.

Solar gain can be a free source of heat in winter and a real problem in summer. Either way, it changes how much heating and cooling a building needs, so it has to be part of the design.

How solar gain works

Sunlight passes through glass and lands on floors, walls, desks and people. They absorb it and warm up. That heat then warms the air in the room. Glass lets sunlight in easily but does not let the heat back out as easily, so the room keeps warming while the sun shines on it.

How much gain a room gets depends on:

Which way the windows face. In the UK, south-facing glass gets the most sun through the day. East-facing rooms warm up in the morning, west-facing rooms in the afternoon and evening.
How much glass there is. Large glazed areas and rooflights let in a lot of sun.
The type of glass. Some glazing is designed to let in less of the sun’s heat.
Shading. Overhangs, blinds, trees and nearby buildings all reduce gain.
The time of year. The summer sun is higher and shines for longer.

Solar gain and heating

In winter, solar gain helps. On a bright day, a south-facing room may need little or no heating in the afternoon, while a north-facing room on the same floor still needs it.

But you cannot rely on it. Heating systems are designed for the worst case: a cold, dull day or a long winter night with no sun at all. So solar gain is not normally used to make a heat pump smaller. Instead, it affects how the system is controlled.

Solar gain and cooling

In summer, solar gain is often the biggest reason a building overheats. Heavily glazed offices and showrooms can become uncomfortable quickly, even on mild days. In well-insulated buildings the heat has less chance to escape, so it builds up.

This is where an air-source heat pump that can cool as well as heat earns its place. The cooling side needs to be sized with solar gain in mind, room by room.

What this means for design

Work out gains for each room, not just the building as a whole.
Group rooms with similar orientation into zones, so a sunny side can be cooled while a shaded side is heated, if the system allows it.
Use controls that respond to each space, so the system does not keep heating a room the sun is already warming.
Reduce unwanted gain first, with shading or better glazing, before adding cooling capacity.

A useful match

There is a helpful side to all this. The sunniest hours, when cooling is needed most, are also when solar panels produce the most electricity. A building with solar panels and a heat pump can often cool itself with its own power at exactly the time it needs to.