The ways that homes overheat in summer:
- Heat conducts through the roof/walls from outside to inside, due to poor insulation and made worse by dark colours and direct sun on the outside surfaces.
- Hot air gets inside through uncontrolled gaps/leaks or open doors/windows.
- Solar gain (heat from sunlight) comes through windows/openings to the interior and heats up the inside floor/walls/air.
- Internal heat gains from people, cooking, lighting, washing, computers, fridges etc
These can be combatted by a combination of insulation, keeping windows closed, external shading and being conscious of how much heat you're producing inside (difficult to avoid, most things we do generate heat).
Fortunately even in the summer, the UK is relatively cool at night. One of the most effective ways of preventing overheating is to make use of thermal mass to 'store coolth' or buffer heat gains. When outside temps cool down in the evening, open things up and get the heat out of the house, cool down the fabric. Materials like stone, concrete, brick and to some extent plaster, if they start the day cool, will then absorb the heat energy during the day keeping the ambient temp down and giving you more of that radiant thermal comfort by keeping the surface materials cooler for longer. Water also has high thermal mass but it's not that practical to have lots of it sitting around inside.