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How do window blinds help to reduce the temperature in a room? On the one hand, I know that drawing the blinds helps my room to stay cool in the summer. On the other hand, just as much sunlight is coming into the room with the blinds or without them (just as much energy, I guess). My blinds are white, and so perhaps more effective at reflecting light than most things in my room; but black curtains would have a similar effect, I imagine. Am I just keeping the heat in the small space between the window and the blinds, rather than having it distributed through the room?

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    $\begingroup$ On the other hand, just as much sunlight is coming into the room with the blinds or without them Isn't one of the points of blinds to block sunlight? You might need to buy better blinds $\endgroup$ – Aaron Stevens Jul 11 at 22:36
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    $\begingroup$ I think the point is that the blinds themselves are inside the room, so the energy from the sun enters the room before hitting them. @AaronStevens $\endgroup$ – Brick Jul 12 at 0:29
  • $\begingroup$ One thought is that most window blinds are made of plastic or wood, which are pretty good thermal insulators. So even for dark colored blinds, the hot side of the blinds is facing the window and insulates the hot air between in at the glass. $\endgroup$ – KF Gauss Jul 12 at 2:27
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Because of color or, more accurately, albedo and reflection.

Sunlight enters your room through your window, because your window is largely transparent. It hits your floor or other things in your room and gets absorbed or reflected inside your room. The amount of heat varies depending on location, where the sun is, how big your window is, but direct sunlight, filtered through a glass window, you're talking a couple hundred to a few hundred watts per square meter. If the window is 2 square meters, that's not as much as a space heater, but not too far off.

The blinds are in the room, but they are white and reflective, so much of the sunlight that comes into your room hits the blinds and bounces back out the window. If your floor is white, the sunlight hits the floor but gets reflected in all directions so it stays in the room. The blinds being close to the window reflect a considerable percentage of the heat carrying sunlight back out the window. Any white covering next to the window would do the same.

I heard a story/saw a photo once about a couple who custom built a house in Maine with huge windows to take advantage of natural solar energy in winter and they did it a little too well. The inside of their house would reach 90 degrees during winter.

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If the room is dark there is less light. Less light = Less energy. Window Blinds = Less light = Less energy.

Window blinds block sunlight from entering the room through the window and energizing the air molecules in it, due to thermodynamics the pre-existing heat dissipates, and with less new energy added into the system temperature decreases.

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    $\begingroup$ As written, I think this begs the question. The blinds are inside the room, so the light enters the room before hitting them. The energy is in the room. Pretty sure the OP wants to know why the energy entering and hitting the blinds is different than it continuing into the room. As written, this kind of suggests the energy never makes it into the room. $\endgroup$ – Brick Jul 12 at 0:31
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The blinds are inside the room, but very near the window panes. Thus they will reflect much of the light and heat back out of the window, keeping it from entering farther into the room where it would heat up the air and objects it shined on.

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protected by Community Sep 25 at 16:39

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