Summer overheating and thermal mass

The phenomenon

Heavy structures — brick, concrete — store a lot of heat. That slows the change in internal temperature: they take up heat during the day and release it at night.

A light structure has less such mass, so the internal temperature follows the external one more closely. In winter and for heating that is an advantage — the house warms up quickly — but in summer it has to be managed.

What actually causes the overheating

Worth being clear about: the primary cause of summer overheating is not the structure but the radiation coming in through the glazing.

A large south- or west-facing glazed area without shading causes overheating in any structure. Thermal mass only delays and dampens the effect; it does not remove it.

The order of the solutions

1. External shading

The most effective: stop the radiation before it gets in. Shutters, louvres, external blinds, canopies, planting.

Internal shading is worth much less, because the heat is already inside.

2. Opening size and orientation

The cheapest design tool: how much glazing, facing which way.

3. Night purge ventilation

Cool night air flushes out the structure warmed during the day. It works faster in a light structure, because there is less mass to cool.

4. Time lag in the build-up

Particularly important at the roof: denser insulation gives a longer time shift, so the heat has not arrived by the hottest hours.

5. Adding mass deliberately

Where it is warranted — through a wet floor build-up, for example — mass can be brought into the structure, which improves the acoustics at the same time. Floor build-ups.

What the technology gains

The fast response is not only a drawback: a light building warms up fast and cools fast, which is a distinct advantage for intermittent use — a weekend house, an office — and it responds better to control by the building services.

The point

Summer comfort is decided by shading and the layout of the openings, not by the mass of the structure. In a light structure those decisions are only more important, not harder.