Overheating in a glazed extension: solar control options
Too much glass facing the wrong way, with no plan for the heat. Orientation, g-values, external shading and the ventilation that keeps a glass room usable in August.
- Glass area, orientation and g-value decide how hot a room becomes.
- External shading works better than internal blinds because it stops heat before it enters.
- Openable panels and a cross-flow of air are part of the specification, not an afterthought.
A glass room that is comfortable in December can be unbearable in July, and the cause is almost always the same: a large area of glass facing the wrong way with no plan for the heat.
Orientation and g-value
A south facing wall of glass gains the most heat. East and west elevations gain it at low angles, when blinds struggle to help. The g-value of the glazing tells you what fraction of that solar energy passes through, so a low g is valuable on a sunny elevation and largely wasted on a shaded one.
Shade from outside, not inside
External shading stops the heat before it reaches the glass. A canopy, an overhang sized to the sun angle, an external blind or a solar control coating all work this way. Internal blinds are useful for glare, but the heat is already inside the room by the time it meets them.
Air movement
Design for a cross-flow: a low level opening or a low opening panel on one side, and high level or roof vents on the other. Two small openings on the same wall do very little.
Approved Document O deals with overheating in new homes, and the principles apply just as well to an extension: limit the solar gain, provide ventilation, and do not rely on an air conditioning unit as the first answer.
Glass specification
Where a large area of glass faces the sun, a solar control coating lets visible light through while reflecting more of the infrared. The room stays bright and the temperature stays manageable. On a north elevation the same coating earns nothing, so it is worth specifying per elevation rather than across the whole house.
Written by the Minimal Frame Projects UK team. If you would like advice on a specific opening, send us the dimensions and a few photographs and we will come back with a specification.
