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I have some windows that I want to insulate at night during the winter. I have energy star windows, thermal drapes, and cellular blinds and those plastic window film kit things, but I want to reduce my heating costs even more.

So here's my idea. What if I took a bunch of fiberglass or mineral wool insulation, wrapped it in a bunch of plastic, and shoved that in between the window and blinds at night, possibly filling in gaps between the plastic and the the window frame with other material, and possibly secured on the inside end by a board or something? Then I would probably take it out during the day.

I'm wondering

  1. whether you think this would significantly affect heat loss through the window,
  2. whether it would cause any condensation problems,
  3. and about any unintended consequences I might be missing.
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    What other steps have you taken to reduce energy usage? There might be easier, more effective steps you could take first.
    – LShaver
    Sep 4, 2020 at 20:40
  • Great question! I will keep my answers to just the realm of heating/cooling. First, I will note that I'm a renter. My landlord insulated my attic and walls pretty well. I am in "negotiations" to get one of those thermometer guns so I can look for heat loss. I'm working on upgrading my fridge and water heater. I am using electric baseboard heat but haven't upgraded that for a long time. I am using all manner of sweatshirts and blankets and stuff so I can run the apartment colder in the winter. And I did the aforementioned window stuff.
    – capet
    Sep 4, 2020 at 20:50
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    Are the windows air tight? 4 decades ago I used poly sealed by removable 1/2” plastic insert strips mated and sealed into 2sided taped plastic strips to zip into. In “Winterpeg” it made the difference between frost/condensation to none and significant drop in gradient temps in the room. The slight leaks were attenuated but inflated inside from the lower pressure inside the house. It was very inexpensive. Now I make wood frame inserts with heat stretched thermoplastic for each window cheaply. Makes a big difference and almost clear
    – Tony Stewart EE75
    Sep 4, 2020 at 21:01
  • @Tony Stewart Sunnyskyguy EE75 Great question and great solution! I will pay more attention to that when it gets cold or windy and then get back to you, unless you have another way to check. I apologize that I don't completely understand your solution yet. Are you saying that you put some 2sided tape around the border of the window, used that to attach some plastic strips with some kind of recess in them, and then stuck the 1/2" plastic insert strips into those when it was cold?
    – capet
    Sep 4, 2020 at 21:06
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    Its a window kit with the flat strips in rolls that insert into edge grip strips left on all season, slightly ugly but effective and only for winter. However on a vaulted entrance sunroof I uses carpet tape and heat shrink poly permanently . They used double pane here in Toronto, when triple pane is needed.
    – Tony Stewart EE75
    Sep 4, 2020 at 21:35

2 Answers 2

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A humble solution I have seen in a temperate climate is to use whole mattresses in deep-set windows. Looks terrible but works well.

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  • That suggests that the fiberglass would help too, right?
    – capet
    Sep 5, 2020 at 6:49
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    A lot more pleasant to use sheets of polyester insulation in a cotton cover (doona cover). I saw this done, but condensation was never an issue.
    – Polypipe Wrangler
    Sep 6, 2020 at 3:50
  • Thanks, makes sense!!
    – capet
    Sep 6, 2020 at 15:16
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Fit shutters, been “standard” on chalets in Switzerland for over 200 years.

You could make the shutters perform even better by making them with an insulation layer as well. Fitted well that would start to get closer to the r value of the wall.

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  • Very cool, thanks! So you're talking about exterior shutters?
    – capet
    Sep 4, 2020 at 20:54
  • @Solar - good answer but FWIW - I am in France/Belgium/Switzerland alot (precovid) and they do outside blinds (metal). Old ones are pull and new ones are mechanical. The shutters sometimes go over these but are often just for show.
    – DMoore
    Sep 4, 2020 at 21:00
  • @capet yes, keeping the cold air off the windows makes a big difference to my winter heating bill.
    – Solar Mike
    Sep 4, 2020 at 21:00
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    @DMoore up in the mountains, they did not have the money for “fancy” mechanical blinds, that could get frozen etc so if I look at all the chalets then it is shutters...
    – Solar Mike
    Sep 4, 2020 at 21:02
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    @capet then you failed as mine was first by nearly 3 hours - check the times they were posted.
    – Solar Mike
    Sep 11, 2020 at 4:46

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