How should the slats on my window blinds be turned to block the most heat?

Question

Trying to block some of the heat coming into my home. I think the slats on the window blinds should be turned so that the edge close to the window goes down, my husband thinks it will block more heat going the other way. Any thoughts?

Answer 1

I think your husband is right in this case. By having your slats facing down, all of the sunlight from the sky will still enter your home, moving downwards. Light moves in linear patterns and so you'd want to orient the slats in such a way that will allow for the most amount of light being blocked.

Answer 2

Once you block the sunlight from entering your house, the only radiant heat from sun on the the blinds is produced.  Since heat rises, it's best to have the top edge of the blind close to the window, so the rising heat moves toward the window and not into the room.

Answer 3

I think that you are right and that, unless you want to keep adjusting the slats, the angle should be equal to your latitude. For example, if you are in Philadelphia, PA (40°N), the slats should be 40° from the horizontal. That way, you will reflect the most light back to the window. It won't go back thru but it might localize the heat there.