How to cool a house in a heat wave

A hot day and a heat wave are different problems, and most cooling advice is written for the first one. On a single hot day the house starts cool, warms up, and recovers overnight. In a heat wave it does not recover, and by day three you are fighting a building that is hot all the way through.

Everything solid in your house — framing, plaster, floors, furniture, the slab — holds heat. On a normal night that stored heat bleeds out to cooler outdoor air and the house resets.

When overnight lows stay high, that reset does not happen. Each morning starts warmer than the last. The thermal mass that was helping you on day one is working against you by day three, radiating heat into rooms even with the sun off them.

Two things follow, and both are counterintuitive:

• Whole-house cooling gets harder each day, not easier, because the structure is now a heat source rather than a heat sink.
• Concentrating on one room gets easier each day, relatively speaking, because a single closed room is a far smaller volume to hold against a losing battle.

That is the strategic shift. Stop defending the whole perimeter and hold one room properly — cooling a house against cooling a room.

Sequence matters, because the free measures multiply the effect of the expensive ones.

1. Shade every window the sun reaches, from outside if you can. Solar gain is the largest single heat input in most houses. An external shade stops the heat before it enters; an internal blind stops it after it has already arrived — shading and film compared.
2. Close the house while it is hotter outside than in. Every open door is a partial air exchange in the wrong direction.
3. Purge overnight, but only if the outdoor air is genuinely cooler. Check the forecast low rather than guessing. Open up when outdoor temperature crosses below indoor, close at first light — the night purge in detail.
4. Kill indoor heat sources. The oven, the tumble dryer, the dishwasher’s heated dry, incandescent lamps, a gaming PC. Cook outside or cold if you can — the kitchen is the worst room.
5. Pick the retreat room. Low, shaded, small, with a door that closes. A ground-floor or basement room on the north side beats a large bright one every time — why basements hold.
6. Move air over people, not through the house. A fan pointed at a person does more than a fan pointed at a hallway.
7. Deal with humidity separately if it is high. At high humidity the same air temperature feels significantly worse and sweat evaporates poorly — a dehumidifier is doing a different job from a fan.

This is the most important safety point on the page and it is widely not known.

A fan cools you by evaporating sweat. It also moves air across your skin by convection. Below skin temperature, air moving over you carries heat away. Above it, air moving over you delivers heat, the way a convection oven does.

Around 95°F the convective gain starts to outrun the evaporative loss, and in very humid conditions the crossover comes lower still, because sweat cannot evaporate efficiently. Past that point a fan pointed at a person is adding to their heat load.

The practical rules in that situation: use the fan to move air around a room rather than directly at a still person, keep skin wet with a damp cloth so evaporation continues, and treat the fan as a supplement to actual cooling rather than a substitute — the full mechanism.

Heat waves stop being a comfort problem for some people well before they do for others. Plan for this before the forecast, not during.

• Older adults, who sweat less efficiently and often feel less thirsty.
• Infants and young children, who cannot move themselves or say what is wrong — cooling a nursery safely.
• Anyone on medication that affects sweating or fluid balance, or with a heart or lung condition.
• People working or exercising in the heat, including in a garage or workshop.
• Pets, which cannot sweat and cannot open a door — keeping animals cool.

Heat stroke is a medical emergency. Confusion, stopping sweating, a very high body temperature or loss of consciousness means call emergency services immediately — not a fan, not a cold shower, a phone call. A cooling center, a library or a shopping center beats anything on this page if the house is losing.

If the heat wave is a long one, a few things shift:

• Your unit will run more or less continuously. That is normal and not a fault, provided the room is holding. If the room is not holding, the unit is undersized for these conditions rather than broken — how to tell the difference.
• Filters matter more than usual. A clogged filter costs you capacity precisely when you have none to spare, and it takes ten minutes to deal with — cleaning a window unit.
• Grid strain and outages become a live risk. Have a plan that does not depend on mains power — cooling during an outage.
• Raise the setpoint rather than abandoning it. 78°F with a fan is achievable and sustainable; 68°F is neither — what to set it to.