How Cool Should Your House Be When It’s 100°F Outside?

Start at 78°F when you’re home, then lower the setting a degree or two if you need to. A 75°F setting may feel better during extreme heat, but setting the thermostat to 65°F will not make most central AC systems cool the house faster. Give the system time to run, and measure the temperature in the rooms where you spend the most time.

On a 100°F day, your AC may run for long stretches, especially during the hottest afternoon hours. If the indoor temperature rises despite a reasonable setting, replace a dirty air filter, make sure supply vents are open, and keep furniture clear of return vents. If cooling remains weak or the system struggles after sunset, arrange for an HVAC professional to inspect it.

Close blinds or curtains on sun-facing windows before the rooms heat up. A ceiling or portable fan can make you feel cooler while you’re in the room, though it does not lower the room’s temperature; turn it off when you leave. If the air feels sticky, use your AC’s normal cooling mode or a properly sized dehumidifier to improve comfort without continually lowering the thermostat.

Keep doors and windows closed while the AC runs, and limit heat-producing tasks such as oven use during the hottest part of the day. If your home remains uncomfortably hot, prioritize a cooler space rather than relying on a low thermostat setting alone. For personal comfort outdoors, our guide to what to wear in humid heat covers breathable clothing choices for summer.