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Let's say it's -5 deg C outside and I'm inside a building with a moderately efficient window and a window quilt. Will I stay warmer if I take the window quilt down during the day and put it back up at night, or if I leave the window quilt up all day?

What factors affect this?

Does the answer change if I have a second window that is always "unquilted"?

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    Check out passive solar design - covers all your questions. In short let the heat in when you want it and don’t let the heat in when you don’t need it.
    – Solar Mike
    Sep 24 '20 at 5:56
  • Thanks! If it helps, the window with the quilt is not south-facing and I don't have much heat-absorbing stuff in the room.
    – capet
    Sep 24 '20 at 6:00
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    Heat absorbing stuff are also walls and floors... we designed to use black floor tiles to absorb winter sun - worked well.
    – Solar Mike
    Sep 24 '20 at 6:02
  • That's awesome!
    – capet
    Sep 24 '20 at 6:03
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If you ask this question it probably means that the structure is thermally insufficient, and thus the existing heat leaks dwarf any gains from playing with window coverings that only modify solar heating - unless you have lots of windows and lots of Southern exposure.

In my experience, when such questions get asked things are pretty bad, and the only actions to be done on the building that will really change things appreciably are: investigate how to improve the thermal efficiency of the building if you own it (including window, wall insulation, trees to shield from wind, etc.), or move if you rent. Everything else probably takes more resources to just talk about than any savings you'll get, although for sure there can be plenty of placebo effect.

The one thing that will actually make the biggest difference right this very moment is to wear warmer clothes indoors and let the temperature of the room be lower. It literally has the highest cost-to-benefit ratio that you can get in cold interiors, and thus must be investigated first. That's the only way to save significant amounts of energy and to improve your winter comfort by a lot in thermally insufficient buildings if moving or altering the building is not an option. You can always set the thermostat higher if you are OK with the added expense.

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  • Thanks! I just ordered some long underwear. :) Can you explain a bit more what you mean by "window coverings that only modify solar heating"? Would a window quilt fall into that category?
    – capet
    Sep 24 '20 at 17:54
  • @capet Yes. If you want to remove air loss through the window you must cover the window recess in a way that is impervious to air - plastic foil/sheet and proper adhesive tape to the wall surface. A quilt doesn't do it. To prevent radiative heat loss from the interior to outside you'd want a piece of aluminum foil at night, no quilt needed. The quilts have some function but they're not particularly great at anything - they don't prevent air flow, and they don't shield radiation particularly well given their thickness vs. that of aluminum foil. They are a feel-good measure mostly. Sep 24 '20 at 18:00
  • Do you have any recommendations about what kind of aluminum foil I should use and how to apply it? In particular, is it important for the foil to form an airtight seal against the wall? I was thinking of using something like this or this or just this.
    – capet
    Sep 24 '20 at 19:36
  • Also, can you help me understand why window quilts are mostly a feel-good measure? I have some guesses but I don't know: a. It is not feasible to install them in a way that seals out air b. Heat loss is already pretty much minimized with the air seal and foil c. If I put a quilt over the window, maybe I will just have the heat that would otherwise go straight out the window simply travel sideways through the wall, and then go out the window. Am I close?
    – capet
    Sep 24 '20 at 20:37

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