Keeping a home cool during a heat wave

The most effective way to keep a home cool during a heat wave is to stop heat from getting in and to remove heat that has already gathered inside. Most of the work happens before the hottest part of the afternoon: closing the house early, shading windows that face the sun and reducing heat from appliances and people. These steps cost little, and they help an air conditioner keep up or help you manage without one.

A building gains heat through its windows and roof during the day and releases it slowly after dark. Keeping the house shut during the hottest hours and opening it when the outside air cools is the main strategy.

Keep the heat out during the day

Close windows and blinds on the sunny sides of the house in the morning, before the rooms warm up. Exterior shading is more effective than interior coverings, because it blocks sunlight before it passes through the glass. Awnings, shade cloth or trees placed to shade the windows can help, and west-facing windows usually take the hardest hit from afternoon sun. Light-colored interior blinds or curtains reflect some sunlight before it turns into heat in the room, so they are still worth using when exterior shade is not possible. Seal leaks around windows and doors with weatherstripping or temporary tape, since warm outside air can seep in through small gaps.

Then ventilate at night. Open windows when the outside air is cooler than the air inside, usually in the evening and early morning. Opening windows on opposite sides of the house creates a cross-breeze that moves heat out quickly. Close everything again once the outside temperature rises, and avoid leaving the house open to hot air during the afternoon.

Move air and cut heat sources

Fans cool people by moving air across their skin, but they do not lower the temperature of the room. Run ceiling and box fans when someone is in the room and switch them off when you leave. A fan in a closed room only circulates warm air. When the temperature reaches the high 90s °F or above, a fan alone may not prevent heat-related illness, so move people to an air-conditioned space, such as one of the cooler public spaces described in the final section. When you ventilate at night, open interior doors so cooler air can move through the house. While the air conditioning runs, close the doors to rooms you do not use, so the cooled space stays smaller.

Appliances add heat, so reduce their use on the hottest days. Use a microwave, slow cooker or outdoor grill rather than the oven, and run the dishwasher and clothes dryer in the evening or early morning. Keep showers short and cool, and switch off electronics that are not in use, since they still give off heat. Run kitchen and bathroom exhaust fans briefly and only when needed, because they pull cooled air out of the house along with the moisture and odors they are designed to remove.

If you use a window air conditioner, make sure it is sealed into the window frame with its side panels or foam insulation, and plug it directly into a wall outlet rather than an extension cord. Clean or replace the filter regularly so the unit can move air efficiently. For central air, a common energy guidance figure is 78°F (about 26°C) while you are at home and awake, raised a few degrees when you are away. Setting the thermostat much lower does not cool the house any faster. It only makes the system run for longer.

Check on people, pets and the house

Heat illness can develop quickly, especially among older adults, young children, people with chronic illnesses and people who work or exercise outdoors. Watch for heavy sweating, headache, dizziness, nausea and weakness, which can signal heat exhaustion. Move the person to a cooler place, loosen tight clothing, apply cool wet cloths to the skin and offer water to sip if they are alert. Get medical help right away if they vomit, if symptoms get worse or if they last longer than an hour. Heat stroke is an emergency. Confusion, slurred speech, fainting or a seizure call for an emergency number right away, and while help is on the way, cool the person as quickly as you safely can. People who take medications that affect body temperature or fluid balance should ask a doctor how hot weather may affect them.

Pets need the same attention. Never leave a pet, a child or a vulnerable adult in a parked car, even for a few minutes, because the inside temperature can climb dangerously fast. Make sure animals have fresh water and shade indoors and outdoors. Check on the house itself by measuring the indoor temperature with a simple thermometer. If the inside stays hot even with the cooling running, confirm that the thermostat is set correctly, the filter is clean and the outdoor unit is not blocked by plants or debris. If the system runs constantly without cooling the house, a licensed HVAC technician should inspect it, because refrigerant and electrical problems need professional repair.

Plan the day around the heat as well. Schedule outdoor errands, yard work and exercise for early morning or evening, when temperatures are lower, and keep drinking water within reach throughout the day. Know where the nearest cooler public spaces are, such as a library or a community center with air conditioning, in case the home becomes too hot to stay in. Spending a few hours there is a practical option when indoor temperatures climb and stay high.

Preparation pays off most before a heat wave arrives. When hot days first appear in the forecast, test the fans and air conditioner, clean or replace the filter and set up shade for the windows that take the afternoon sun. Then list the neighbors who live alone or have trouble with stairs or transport, and agree on who will check on each one during the hottest days.