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I'd like to keep room(facing east) cool during mornings. The room covered with textured glass on the east. By how much can I reduce heating by sticking the aluminum foil on the inside behind the glass. I understand that the glass/foil will be heated and radiate inside the room, So lets ignore that for the moment. the total surface area of the window is 108sqFt if it helps. Note: I can't afford to put the foil in from of the glass as this is in the 3rd floor.

user42766
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2 Answers2

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The aluminum has an albedo of $95\%$ in visible light, so this is the proportion of incoming light that you will be able to redirect. Glass is usually opaque to IR, so that should be absorbing them.

So, the energy input in your room will come from three sources:

  • The $5\%$ of visible light not reflected by the aluminium. Probably neglegible.
  • The IR emitted by the glass, as it has been warmed up by the IR absorbed from the sun.
  • The light that you manage to reflect from the aluminium, but gets absorbed by the texture in the glass.

I would say your biggest problem is the second point. The problem here is that once the energy is in the IR range, your room is a trap with no way out. You can easily estimate how important this factor would be by just touching the glass in the morning, is it warm? If not, then your aluminium would probably work very well.

Nevertheless, even if the glass is warm, the aluminium barrier would shield it, and hopefully deflect it to the walls, making the sensation less pressing.

If you really want to get the room cool, you need some sort of cooling system. A water bag behind the thermal shield could go a long way, as the energy required to heat water is much higher than for air.

Davidmh
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@Davidmh 's answer is pretty good, but in fact many glasses transmit well into the infrared. Take a look at Google Images for "infrared glass transmission."

You get the most bang for the buck by blocking light and IR light from getting in -- see those "low-E" tinted glass installations in industrial buildings. So anything which reflects visible, and preferably IR as well, will do a good job of reducing the heat load in your room.

Carl Witthoft
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