Sleeping through a heatwave

Every UK summer follows the same pattern. A warm week arrives, the house feels fine for two nights, and then something changes. The furniture and the walls have quietly absorbed heat all day, and even the loft insulation itself is now part of the problem rather than the solution; by the third night that stored heat is being given back to the bedroom faster than the open window can carry it away. This is thermal mass working against you, and it is the single biggest reason British homes feel worse on day four of a heatwave than they did on day one.

The intervention that works is boring, and free. Close the house up before the outdoor temperature rises above the indoor temperature. In most homes that is somewhere between eight and nine in the morning. Draw curtains on the sunward side, particularly west-facing rooms that will bear the brunt of the afternoon. Open everything again once the outdoor air has fallen below the indoor air, which on a typical heatwave night is around ten in the evening.

None of this requires equipment. It requires the household to notice the temperature difference and act on it, which most of us do not, because our homes were designed for the opposite problem.

• Move the fan. A ceiling fan running counter-clockwise, or a floor fan aimed across the bed rather than at it, moves air over the skin and lets the body's own cooling do the work.
• Shade before dawn. On any day forecast above twenty-five degrees, close blinds and curtains on the east side before the household gets up, and on the west side by mid-morning.
• Cook cold. A hob adds hundreds of watts of heat to the kitchen and, in most terraced homes, to the rest of the ground floor. Salads, wraps and a slow cooker in the utility room keep the house liveable.
• Sleep low. If the upstairs bedrooms are unbearable and there is a sofa in a cooler room downstairs, use it for the worst two nights. Nobody needs a sleep debt on top of a heatwave.
• Open the loft hatch. In many houses the loft is the hottest space in the building, but opening the hatch is only useful if there is a route out for the warm air; otherwise you are venting heat into the landing.

The single most useful under-a-hundred-pound purchase is a thermal blackout blind for the worst-affected window, fitted inside the reveal so it stops light from spilling round the edges. A good one drops the peak surface temperature of the glass by several degrees and, more usefully, holds it there through the afternoon.

The second is a decent room thermometer with a humidity reading, placed at pillow height. Most people wildly overestimate their bedroom temperature during a heatwave, which leads to bad decisions about opening windows at the wrong hour. A five-pound sensor tells you when the outdoor air is actually cooler than the indoor air, and often the answer is later than you thought.

British homes were built to conserve heat, and the loft is the frontier where that policy is enforced. Two hundred and seventy millimetres of insulation sits on the ceiling of the top-floor rooms, which in winter is exactly what you want. In summer, that same insulation traps warm air in the loft void above it, and any warmth that does make it through the ceiling has nowhere obvious to go.

This is why upstairs bedrooms run hotter than downstairs living rooms, even in houses with well-fitted external shading. The problem is compounded in loft conversions, where the roof itself has become the ceiling and the insulation sits directly above the sleeper's head. A properly ventilated cold roof helps; a warm roof without ventilation does not.

The Boiler Upgrade Scheme continues at seven thousand five hundred pounds for an air-source heat pump, unchanged. If you were planning to apply this autumn, get quotes now; installers who are quiet in July are booked solid by October.

In the UK, on almost any afternoon hotter than about twenty-three degrees, opening a bedroom window makes the room warmer, not cooler. The outdoor air is hotter than the indoor air, and the sunward wall of the room is being warmed both by radiation and by the incoming breeze. The room is quietly acting as a very slow oven.

The correct rhythm in a UK summer is closed by day, open by night, and the window position that helps most is not the one nearest the sleeper. It is the one furthest away, ideally on the opposite side of the house, so that air is drawn across the room rather than sitting still against the pillow.