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I am insulating a single-pane window with plastic film (http://www.amazon.com/gp/product/B002GKC2GW?keywords=plastic film insulation&qid=1453679450).

This window can't be opened, so I will keep the plastic even in summertime. This also implies that there are no drafts through this window.

Is it worth to add more layers, separated by ~1 inch? Is it worth the money? Is it worth the effort?

My guess is yes (slightly). This may create a barrier of still air that will get a bit warmer by the sunlight (although it is not facing south). More convincing arguments?

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  • With air as the insulation I don't think the 2nd layer of plastic will be worth the effort. Most double or triple pyne windows are gas filled not air to reduce the heat transfer, looking at the cost of double pane to triple the difference in cost just doesn't justify the additional pane, then look at replacement cost if the seal fails or a cracked window from a baseball or tree limb , I personally don't see the advantage,,, IMHO
    – Ed Beal
    Jan 25, 2016 at 0:19
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    Oh no, it defintitely makes a difference. Other gases are better but air is still a great deal better than nothing. The trick is having the layers close enough that boundary-layer effects prevent convection. Without, of course, the plastic just pulling in and kissing. I recall hearing the ideal was around 1/2”, don't recall exactly. Some sort of intermediate slats to help protect the gap might help, especially between 2 plastic layers.
    – Harper - Reinstate Monica
    Jan 25, 2016 at 0:59

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There are, as always, tradeoffs. First, 1 inch is more than is generally considered ideal - 1/2" or so is preferred as it's less prone to internal convection currents. Yes, plastic window film kits are often installed with considerably larger spacing, but they are also commonly installed on less than ideal windows where they stop actual drafts...

Any multi-pane system comes with a very basic tradeoff - more panes, less light. Neither glass nor plastic lets all light through, and multiple panes/layers have a multiplicative impact on the light transmission - if a single pane lets through 90 % of the light, two let through only 81% and 3 let through only 73%...

The additional layers do provide improved insulation value over a single pane, though even a triple-pane system is still a poorly insulated area compared to the rest of the wall (if the rest of the wall is properly insulated, anyway.)

I suspect hard data on the effectiveness of plastic film layers would be difficult to source (though I could be wrong, I don't feel like looking for it right now based on that suspicion) but you'd probably see far more difference from REPLACING the old single pane window with a modern, low-E double-pane unit than from covering it with 2 additional layers of plastic film - but the cost would also be higher.

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  • Given how thin said plastic is, would there really be all that much to gain? Seems that heat transference between those thin layers would be pretty high.
    – DA01
    Jan 25, 2016 at 6:27
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    It's not the thickness of the film (or glass) that matters. The 1/2" of dead air between the films is the major benefit.
    – Ecnerwal
    Jan 25, 2016 at 13:35
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I'll give another answer from my own experience.. We did 2 layers with the first layer underneath being bubble wrap. I have an unfinished basement with two old single pane windows. They simply had a curtain over them, but were still losing a lot of heat.

I saw this idea online and it seems to work great for us and provides an easy and cheap secondary layer. Good luck.

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A single pane window has a R-value of about 0.91 (mostly due to the interior and exterior boundary air layers, not the actual glass, which is an almost useless thermal insulator). A double pane low-E window has an R-value of about 3.1. Each ½" layer of air space you create (with plastic film or glass or plexiglass) adds an additional R=1.0. So adding two layers of window film (with a minimum ½" air space) to a single pane glass window will result in an R value of about 2.91, almost equivalent to the low-E double pane, but at MUCH, MUCH less cost.

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I put mirror mylar on my single pane windows and it made a HUGE difference. It has lasted over 20 years. I also put it on my double-pane patio doors and it makes a huge difference there as well. Much less expensive and more efficient at heat reduction.

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    Are you referring to the completely opaque material or the "half-silvered" that lets just enough light through that you can see outside in the daytime?
    – fixer1234
    Jun 18, 2017 at 18:26
  • @Catherine That's awesome!! +1 to fixer1234's question. Also, did you put it on the inside or the outside of the windows? Did you adhere it to the glass or the frame? Is there an air gap between the window and the mylar? And if the two faces are not identical, which side faces out?
    – capet
    Sep 24, 2020 at 14:57
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If your window structure allows, you might also consider making an "interior window insulating panel". Essentially an interior "storm" window that can be installed and removed as needed. This site has all the details: http://www.arttec.net/Thermal-Windows/

I went this route for one window in my house where the plastic just didn't want to stick to the frame. It's a little more cost & effort up-front but if all goes according to plan it should be worth it.

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Here is an idea for you. Build a wood rectangle out of 1 x 1/2 inch wood that just fits into the window frame with a 3/16 inch under size. Cover this on both sides with plastic sheet. Put a 1/8 inch foam strip around the outside and put two screws in on one side for hand holds.

You place this into the window, compress the foam on one side using the screw holds, slide the screw side in and then let the compression of the foam hold it in place. Position an inch away from the pane to achieve for best insulation. An even simpler version of this is to install 3/4" spring curtain rods at the top and bottom of the window and run a loop of plastic sheet around them abut an inch wider than the window frame.

Some windows have channels for fitting exterior screens, these can also be used to install exterior panes. In one case I just covered the exterior window screen with plastic

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