When sunlight heats the air in a truly sealed room, the air pressure in that room increases.

Why the pressure goes up

Think of the room as a rigid, closed box:

  • The amount of air (number of molecules) stays the same.
  • The volume of the room stays the same.
  • The only thing that changes is the temperature of the air as sunlight warms it.

From the ideal gas law PV=nRTPV=nRTPV=nRT: with nnn and VVV fixed, raising the temperature TTT makes the pressure PPP go up.

In more everyday terms:

  • Warmer air molecules move faster.
  • They hit the walls, floor, ceiling, and each other more often and harder.
  • Those more frequent, harder impacts are exactly what we call higher pressure.

So as the sun shines through the windows and heats the air, the pressure in the sealed room rises slightly. It would not stay the same, decrease, or “disappear”; it just becomes a bit higher than it was before the heating.

Tiny story version

Imagine you’re in a big bouncy-castle that can’t leak any air.

  • At first it’s cool, and everyone inside is walking slowly.
  • Then the sun heats it up, and suddenly everyone inside starts running faster.
  • They bounce off the walls more often and with more force.

From the outside, the castle would feel tighter and more “puffed up” – that’s like the air pressure going up in the sealed, sun-warmed room.

TL;DR:
If the sun shining through windows heats the air in a sealed room, the air pressure in that room increases because the warmer air molecules move faster and hit the walls harder and more often.

Information gathered from public forums or data available on the internet and portrayed here.