Can anyone help settle this family debate - Does a room heat up significantly more if indirect sunlight is allowed in through a window rather than keeping the curtain closed? I think the room will not heat up significantly more. The room would definitely heat up more if direct sunlight was allowed through a window where the curtain is open.

In the summer months when it is hot and humid here, I close all the windows and draw all the curtains during daylight, this helps to keep the room cool. I never used to do this, but did notice that the room is significantly cooler as a result of keeping the windows closed and the curtains drawn, so yes, I would say that even indirect sunlight will warm the room more if the curtains are open. I open the windows and curtains at night in the summertime to allow the cooler night air in.

Science isn't decided by polls.

 

Basically the issue is radiative heat transfer, thich varies as the fourth power of the temperature. If the objects outside are hotter, they will radiate more energy into the window than is radiated out of it. If a large fraction of the solid angle is sky, then overall that's probably cooler than inside.

 

If it's direct sunlight, then it's a ~6000K source, which is probably hotter than the inside.

the other thing to take into account is heat sources indoors. humans, tvs, computers etc. radiate heat. i dont know about windows, but in the winter i put a sheet over the entrance to the "game room" which contains my computer, and it gets TOASTY in there. so much that it was even uncomfortably warm on the year our heater was broken. (and uncomfortably cold outside the sheet)

I feel that sunlight will make a small difference.