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In summer, that is when it's warmer outside the house than inside, one want to prevent the air in the house to warm up too much. Let's consider that all the windows are kept closed, that they are double panes and made of "Low-E" glass. During the day, does closing the curtains on the windows reduce the heating of the house? If so, does the color of the curtains have a significant impact?

Intuitively, I would think that once the sunlight goes through the window, it's too late to prevent the greenhouse effect. However, the curtains could create a layer of warm air that would improve the isolation, but maybe the convection would prevent this layer to form.

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  • $\begingroup$ Should you wear all white or all black clothes? google.ie/amp/io9.com/5903956/… $\endgroup$ – user154420 Jun 19 '17 at 14:59
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    $\begingroup$ @Countto10 I'm not sure I agree with that article... You would have to analyze the heat transfer better; but I get the feeling that the strength of the radiation from the sun actually beats out the radiation from the body. I've definitely felt black shirts heat me up on warm days (but that's purely anecdotal). As far as this question goes; I believe the curtains could heat your house in the summer. I cannot remember the name; but there is a type of residential heating that uses large curtains to assist with radiating heat into the room. $\endgroup$ – JMac Jun 19 '17 at 15:07
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    $\begingroup$ @Countto10 I appreciate your comment, but your link does not answer the question as the considered parameters differs. Moreover, regarding the color of clothes, you'll find on the web many sourced answers that proves the opposite. PS: I read somewhere that black is recommended to prevent sunburns, not to prevent heating. $\endgroup$ – A. Hersean Jun 19 '17 at 15:11
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The heat from direct sunlight is about 1000 watts per meter squared in hot countries. If sunlight passes the glass, the percentage not reflected will heat the room .

During the day, does closing the curtains on the windows reduce the heating of the house?

Yes , depending on the color of the curtain , its reflectance/albedo.

If so, does the color of the curtains have a significant impact?

Yes. Black curtains will have very low albedo, this means all the energy coming in will be absorbed by the curtain and transformed to infrared radiation, a heat source at the window.

Here is a link for reflectance/albedo of various materials.

White curtains would be close to the reflectance of" white paper sheet - 0.6-0.7".

The 30% absorbed will add to the heat of the room. In hot countries it is wise to have shutters, shadowing the window, which also reduce heat entering the house even if there is no direct sunlight. Of course if the glass is double glaze or particularly reflective the improvement will be smaller, but there will always be improvement in using external shutters, particularly of white colors.

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Windows are transparent to visible light, but opaque to infrared. If your curtains are dark, they will absorb the visible and emit infrared. This will cause your house to warm as the light doesn't escape so easily. If they are lighter, they will be more reflective to visible light and will bounce it back through the window. This is a rather gross simplification of the process, but this is the basic idea.

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  • $\begingroup$ Do you mean "omit" or "emit"? Does the amount of visible light absorbed is significant over the infrared? $\endgroup$ – A. Hersean Jun 19 '17 at 15:10
  • $\begingroup$ Emit, I'm not sure I understand your follow up question. $\endgroup$ – Greg Petersen Jun 19 '17 at 15:17
  • $\begingroup$ My comment was poorly formulated. White curtains reflects some light through the window. Do you have an idea of how much, relative to the absorbed light? BTW, thanks for your answer. $\endgroup$ – A. Hersean Jun 19 '17 at 15:24
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    $\begingroup$ It depends on the glass and wavelength of light. I'm not sure what commercial grades are used in most household windows, but I know that when I look through our infrared camera at work, our windows are opaque. If you can figure out the material, you can search for transmittance graphs such as one here: thinkalpen.com/resources/… $\endgroup$ – Greg Petersen Jun 19 '17 at 15:32
  • $\begingroup$ Again, this is a bit of a simplification. Windows in particular have other devices that help with insulation such as double panes. If you want a solid number, then you are asking a much more complicated question. However, the main physics behind closing the blinds is the transmission, absorption and reflection of light between the glass and the curtain. $\endgroup$ – Greg Petersen Jun 19 '17 at 15:38

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