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I have been told (including in comments by awesome user Unslander Monica) that a foil sheet placed on the interior side of an air-sealed window can help prevent radiative heat loss through that window. I have a few questions about this:

  1. Does it matter what kind of foil I use? Is anything better than the "normal" foil used for food storage?
  2. Does it matter whether the cavity between the foil and the window is not completely sealed? (I already have plastic film that should be blocking outside air from getting in through the window)
  3. Does foil work significantly better than any white opaque surface?
  4. Are there common conditions that I might not think of where this would not work?
  5. Can foil also prevent radiative heat loss if I use it to cover a wall/ceiling? If so, is is partially effective if the wall/ceiling is only partially covered?
  6. If the answer to #5 is "yes," then I am guessing that I will need to make sure that any pipes running through the covered walls do not require heat from my living space to prevent freezing. Right?
  7. If the answer to #5 is "yes," are there any special precautions I should take in a room heated by baseboard heat?
  8. Can I also use foil on my walls/ceiling to increase solar heat gain when condition are appropriate? If so, is there something better I could use?
  9. Catch-all question: Is this a giant waste of time? I don't intend to work on this until I have upgraded/insulated windows/doors/attic, sealed small cracks, and improved my personal habits. But I am renting so I have limited ability/desire to tear open walls or buy major appliances.
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  • A few things that I think I have learned on this site, for anyone planning to go down this road. Anybody, please tell me or take this comment down if I'm wrong: 1. Covering surfaces with foil will prevent you from covering surfaces with, e.g., black tile or something else that can absorb and then reemit heat, so it could create a missed opportunity under certain conditions. 2. Covering windows will prevent solar gain during the day, so ideally I would only do that at night unless I have some other fancy system. 3. If I do this, I will need to dust the foil regularly to maximize reflectivity. – capet Sep 25 '20 at 16:08
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    I think you've violated the "one" question rule for this site. I could be wrong but I don't think so. – JACK Sep 25 '20 at 17:11
  • @JACK Thanks, sorry about that. I may have gotten too excited. I didn't know there was such a rule, I won't do it again. – capet Sep 25 '20 at 17:20
  • You should add that list of findings to your question post or provide it as an additional answer. It doesn't belong in a comment. – isherwood Sep 25 '20 at 18:32
  • I will if you think those findings are true. :) – capet Sep 25 '20 at 18:52
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Does it matter what kind of foil I use? Is anything better than the "normal" foil used for food storage?

Foil reflects heat by reflecting light that would normally be absorbed by the surface (or what's behind the surface, in the case of glass). By bouncing that light back into the room the energy is retained. Obviously this means that the more reflective (shiny) the surface, the more effective it will be.

Does it matter whether the cavity between the foil and the window is not completely sealed?

Not with respect to radiant heat reflection. Obviously it would help with insulation and heat loss in general.

Does foil work significantly better than any white opaque surface?

Yes. It better reflects all of the light spectrum (visible and not). A very glossy white surface could come close, I suppose.

Are there common conditions that I might not think of where this would not work?

Not really, assuming typical household circumstances. All matter emits heat energy, so the more heat and light in the room the more there is to contain through reflection.

Can foil also prevent radiative heat loss if I use it to cover a wall/ceiling? If so, is is partially effective if the wall/ceiling is only partially covered?

Sure. This is why foil-faced insulation panels exist. It's not usually practical for an interior surface, though, for reasons of aesthetics and durability.

Will I need to make sure that any pipes running through the covered walls do not require heat from my living space to prevent freezing?

That is a concern which varies with climate, existing insulation, etc.

Are there any special precautions I should take in a room heated by baseboard heat?

Not really. You'll still need to maintain safe clearance to combustibles, of course.

Can I also use foil on my walls/ceiling to increase solar heat gain when condition are appropriate?

This would probably be counter-productive. You want your interior surfaces to absorb light and heat in winter, not reflect it.

Is this a giant waste of time?

The benefits are unlikely to outweigh the detriments (cost, effort, loss of visibility and natural light) unless you're facing a severe energy crisis. For that reason it's rarely done. There are films that reflect some heat while maintaining partial visibility, but they would obviously cost more than basic foil.

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