Is it really hotter inside?
Often, yes. A weather report gives the temperature in the shade. Your room is not in the shade. Sunlight comes through the window and warms the floor, furniture, and air, so a room in full sun can pass the number in the forecast.
Why a room gets hotter than the air outside
Sunlight comes in through the windows
Glass lets sunlight in. When that light lands on floors, walls, and furniture, it turns into heat, and the heat has a hard time getting back out through the glass. One sunny window can warm a room faster than the outside air ever could.
Walls and roof store heat
Concrete, brick, and roofing soak up heat all day and give it back slowly. That is why a room can stay hot into the evening, after the air outside has started to cool. Rooms right under the roof usually get it worst.
The heat has nowhere to go
A closed room with no air movement keeps whatever heat enters it. Shutting the windows is right while it is hotter outside. If you leave them shut after the evening cool-down, the room holds on to the day's heat.
Things inside give off heat
Computers, TVs, chargers, ovens, bulbs, and people all release heat. In a small room, a gaming PC or an hour of cooking can noticeably warm the air.
Humidity makes it feel worse
Sweat cools you by evaporating, and it evaporates slowly in humid air. The same temperature feels hotter, and still air makes it feel stuffier. What is a comfortable room temperature explains how humidity and other things change what feels right.
Rooms that heat up the most
- Top-floor rooms and rooms directly under a roof
- Rooms with large windows facing the sun for much of the day (usually west or south in the northern hemisphere, west or north in the southern)
- Rooms with only one window, so air can't pass through
- Rooms with no shade from trees, awnings, or nearby buildings
How to cool it down
If the room is a baby's, check the best room temperature for babies first, because heat is a bigger risk for them.
During the day
- Close curtains or blinds on sunny windows before the sun reaches them. Light-colored or reflective ones work best, and shades outside the glass beat shades inside.
- Keep windows shut while it is hotter outside than inside.
- Turn off devices you are not using.
In the evening and at night
- Open windows once it is cooler outside than inside. Open two on different sides of the room so air can pass through.
- Put a fan in one window facing out. It pushes hot air outside while cooler air comes in through the other window.
- A fan pointed at you helps you feel cooler, but it does not lower the temperature of the room.
Trying to sleep in a hot room? The best room temperature for sleeping guide has more ideas.
For the long term
Window film, outdoor shades, a lighter roof color, and better roof or attic insulation all reduce how much heat gets in.