Keeping a house cool without air conditioning

Houses in cooler climates were built to retain heat, which makes them uncomfortable in the increasingly common hot spells those climates now get. Air conditioning is the obvious answer and an expensive, energy-hungry, sometimes unnecessary one.

A great deal can be achieved first with sequencing and shading, and automation is unusually good at it — because the correct action depends on comparing indoor and outdoor conditions through the day, which is exactly what nobody remembers to do at the right moment.

The principle: keep heat out, then get it out at night

Two jobs, in this order.

Prevent solar gain during the day. Far easier than removing heat once it is inside. Most summer overheating in a house is sunlight through glass, and that is the lever.

Purge accumulated heat overnight, when outdoor air is cooler than indoor air. A heavy building cooled overnight starts the next day with capacity to absorb heat again.

The mistake most households make is the opposite of both: windows open all day letting hot air in, curtains open letting sun in, and windows closed at night for security.

Shading, which is where the real gains are

The critical distinction: external shading is far more effective than internal shading. A blind outside the glass stops the energy before it enters. A blind inside absorbs it and re-radiates it into the room, which is better than nothing and much less effective.

In rough order of effectiveness:

External shutters, awnings or blinds on the sunny side. Standard across southern Europe for good reason, and rare further north for no good reason. On a hot day the difference is measured in degrees, not fractions.

Overhangs and planting. A deciduous tree or a well-placed pergola shades in summer and admits light in winter, when the sun is lower and the leaves have gone. This is passive design and it works permanently with no controls.

Reflective or solar-control film on problem glass. Cheap, and it reduces winter daylight and solar gain too, so it suits rooflights and west-facing glass rather than a main living room window.

Internal blinds and curtains, closed on the sunny side, pale on the outward face to reflect. Better than nothing and worth automating, since the sunny side changes through the day.

Rooflights and conservatories deserve a special mention: horizontal or near-horizontal glass receives far more solar energy than a vertical window, and external shading there is worth more than anywhere else in the house.

Ventilation timing, which requires measurement

The rule is simple: ventilate when outdoor air is cooler than indoor air, and close up when it is warmer. Following it consistently by hand is what nobody does.

An indoor and an outdoor temperature sensor, and a rule comparing them, handles it. In practice that usually means windows open from late evening until early morning, and closed from mid-morning until the outdoor temperature falls back below indoor — which on a hot day may not be until 9 or 10pm.

Cross-ventilation matters more than opening area. Two openings on opposite sides of the house move far more air than one large one, because air needs somewhere to leave. Add height difference — a low window and a high one, or a stairwell — and the stack effect drives flow without any wind at all.

Night purging is the single most effective free intervention, especially in a masonry house, which stores enough coolth overnight to noticeably flatten the next afternoon. Lightweight timber-framed houses hold less and benefit less, which is worth knowing before you expect miracles.

The obstacle is usually security, not physics. Restrictors that allow a window to be locked partly open, secure louvres, or ground-floor windows left closed while first-floor windows are open, all resolve it.

Fans: the intervention people underrate

A fan does not cool air, and it makes people feel several degrees cooler, because moving air increases evaporative and convective heat loss from skin.

For a fraction of the energy of air conditioning, a ceiling fan or a decent pedestal fan changes the comfortable temperature threshold significantly. Two practical notes: a fan blowing on you helps, a fan stirring an empty room does not, so fans belong on occupancy control. And a fan in a very hot room, above roughly body temperature, stops helping and starts adding heat.

Whole-house or attic extract fans are a step up, pulling cool night air through the house rapidly. Very effective for night purging where the layout suits.

Reducing what you generate indoors

Every watt consumed in the house becomes heat in the house, and in a heatwave this becomes noticeable.

Cooking is the big one. An oven for an hour is a substantial heat input; a hob with the extractor running is better; not cooking hot food at all is better still, which is why hot climates eat cold lunches.

Tumble dryers, especially vented ones, dump heat and moisture indoors. Dry outside in summer.

Old lighting. Mostly solved by LEDs, but any remaining halogen is a small heater.

Computers, consoles and televisions, which in a small room add up.

Standing load, which is heat generated for nothing at all — and reducing it is worth doing for its own sake.

Shifting dishwashers and washing machines to run overnight helps twice: cheaper electricity, and the heat is released when you are purging rather than when you are trying to stay cool.

When air conditioning is the right answer

Being fair about this: sometimes it is, and pretending otherwise is unhelpful.

Where a room faces west with large glazing and no possibility of external shading, where a bedroom is in a roof space, where someone in the household has a medical vulnerability to heat, or where the local climate now delivers sustained periods above what passive measures can handle, cooling is reasonable.

If you go that way, a few things make it much less costly. An air-to-air heat pump provides both heating and cooling, so a single unit does two jobs and can be the most efficient heating in a well-insulated room. Cool one or two rooms rather than the house. Do the shading and purging anyway, since they reduce the load the unit has to meet. And run it on solar surplus where you have panels, because cooling demand and generation peak at the same time, which is the happiest coincidence in domestic energy.

Set it sensibly too: 25 or 26 °C with a fan feels comfortable and costs far less than 21, because energy use rises steeply with the temperature difference you are maintaining.

A summer routine worth automating

1. Close blinds and curtains on the sunny side as the sun reaches them, tracking through the day rather than all at once.
2. Close windows once outdoor temperature exceeds indoor, typically mid-morning.
3. Run fans on occupancy only, in occupied rooms.
4. Open up in the evening when outdoor drops below indoor, and prompt or open secure windows.
5. Purge overnight with cross-ventilation, using restrictors or upstairs-only openings for security.
6. Shift heat-generating appliances to the night.
7. Alert if a bedroom exceeds a threshold before bedtime, so you have time to act.
8. Only then consider cooling, sized for a room rather than the house.

Items one and five do the heavy lifting, and both are free. External shading is the upgrade we would spend money on first, well before a cooling unit.