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I bought some panels from the store today as my room upstairs lets in so much heat during the Summer. Previously I had a Tan colored curtain up for years that was 100% Polyester on the Face and the Lining. Link below:

https://www.walmart.ca/en/ip/hometrends-63-sheridan-blackout-rod-pocket-window-panel-taupe/6000196944980

The new one I just bought is a Black colored curtain that is just 100% Polyester on the lining. Link below:

https://www.walmart.ca/en/ip/mainstays-100-blackout-curtain-panel-pair-linen/6000200796676

The first curtain blocked a lot of light from coming in, whereas the new one blocks much much less so.

That isn't an issue for me. I can deal with that, it's just the heat I care about. Does it matter that more light is being allowed in with this curtain? Or is it doing its job from a heat perspective?

If this is also a terrible curtain and I should make an upgrade I am down, just looking for a quick option as there's nothing around here locally and I would have to order online to get it.

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  • absorbed light converts to heat .... dark colored objects absorb more light than light colored objects .... if you are allowed, hang a screen on outside of window ... prevent light from reaching the window ... bamboo blinds are very good for that – jsotola Mar 23 at 3:58
  • No. Imagine foil curtains. They'd block all light and reflect much of it. 10 demerits for using "reflect" twice in the same sentence with disparate meanings. :P – isherwood Mar 23 at 19:32
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Dark curtains will reflect far less heat back to the outside, and thus heat up in the sun and radiate that heat into the house.

So it's not just infra-red light (a.k.a. heat light) and visible light passing through the curtain that heats the room, it's also how much infra-red light is absorbed that matters, as it raises the temperature of the material.

Unless the curtain is made of a darkening AND heat reflective material (e.g. thick curtain, facing white towards the outside), it's going to be a lot hotter inside.

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  • There are ways to not absorb light like using white. White is the least absorbing color for IR radiation white on the out side black on the inside but it all depends on the material itself how much light passes through. – Ed Beal Mar 22 at 21:42
  • @EdBeal, OP wrote that the new curtain is black. So it will block visible light but heat up. Unless I misunderstood the question. Edited my answer too. – P2000 Mar 22 at 21:44
  • The density of the fabric itself makes more of a difference – Ed Beal Mar 22 at 22:28
  • Should be six of one / half-dozen of the other. Consider what happens if there's no curtain at all. Where does the IR radiation go then? It hits stuff in the room, which gets hot. It's the same hot either way. – Harper - Reinstate Monica Mar 23 at 0:28
  • @Harper-ReinstateMonica OP is asking about black vs tan. Black will be hotter. – P2000 Mar 23 at 3:54
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I doubt either of these curtains reflect a lot of heat. They probably absorb way more heat than they reflect.

The amount of light let in shouldn't greatly effect the heat absorption of the curtains. Your new ones will still do the job even if more light passes through them. Think of a light bulb across the room, you get the light but none of the heat.

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  • Is there anything you would recommend as a better option curtain wise? I can spend more, this was just a placeholder type of deal – James Mar 22 at 21:35
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    Have you thought about putting up some reflective window film? It comes in many different ratings... – JACK Mar 22 at 23:02
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    @JACK you cannot compare a lightbulb to the sun ... lightbulb across the room also heats up surfaces with its radiation ... the effect is just too feeble to be noticable – jsotola Mar 23 at 4:04
  • @jsotola Just trying to make the point that you could have light after the heat was absorbed which is what the OP was wondering.. :-)... but yes, not a scientifically good comparison... – JACK Mar 23 at 12:29
  • I think you can make a comparison on the lightbulb but you have to keep apples to apples. Size matters 3ea 1000w light bulbs will heat up the room just the same as a 3000w electric heater in the room will. Depending on the window size a black curtain can add thousands of “watts” of heat energy just the same as an equivalent light source can. – Ed Beal Mar 23 at 13:56
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Light and IR (heat) are the same thing, so the light entering the room doesn't actually matter. Visible light itself also turns to heat, just not as effectively. Once IR is in the window, it eventually (in nanoseconds) becomes heat, period. While it's more noticeable on a black curtain, a lighter curtain scatters all (the same amount) that heat/light around the room.

To be clear; without curtains, a white room and a black room will heat the same amount from the same light (thanks newton), it's just that the white room will bounce the photons around a few times before they are absorbed and rendered as heat. That makes the white room more uniform in temp, whereas the black will have thermal contrast, but they balance and your AC still needs to remove it all regardless.

Some light (heat) gets reflected back out by a lighter curtain, so put a white sheer behind the black blackout. IR-blocking film is a better bet since that stops the heat from entering in the first place, no matter how disguised it gets by a particular set of curtains.

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