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So we're about to have a cold snap (20 degrees, cold for us here in Central Texas). I have old windows on the house in need of replacement, but for now, I did a hack for the next week/month: I placed 2.5 mil plastic over the window banks (groups of 2 windows) and securely duct taped the edges. The gap is about 2 1/2 inches.

The windows themselves are standard 1/16" thick single pane windows, double hung, been around forever.

My question is, how much value is there in doing this? I am doing this for approximately 140 square feet of window surface. Is there an estimated flow differential between with and without in terms of BTU's/hour, assuming an outside air temperature of 20 degrees farenheit, a an inside air temperature of 78 degrees farenheit? Thanks!

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  • it's not the R-value that matters here as much as the stopping of drafts. you can stuff insulation behind the plastic if you want r-value, but it looks terrible. – dandavis Dec 31 '17 at 16:03
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A 0.0025 inch thick sheet of any solid has negligible R-value in-and-of-itself (for purposes of home insulation).

The (relatively) calm air next to it does have some R-value, on the order of R 0.7.

Heat flow is calculated using conductivity (U-value). U-value is the inverse of R-value. R-value has units of (square foot)(hour)(Fahrenheit degree)/(British Thermal Unit). A British Thermal Unit is enough heat to raise the temperature of one pound of water by one Fahrenheit degree, or about 1055 Joules. 1 BTU/hour is about 1055 Joules / 3600 seconds, or about 0.3 Watts.

If you have pulled your sheet of plastic tight against the window, then you have not trapped any air between the plastic and the window. Thus, you have not improved the R-value of the window.

If you have pulled your sheet of plastic taut, but with a layer of still air between the plastic and the window, then you have replaced an air-pane assembly (with a total R-value of about 1.2) with an air-pane-air-plastic assembly with a slightly higher R-value (perhaps 1.9). If the effective R-value of the original assembly was higher, your savings would be less.

Here are the ΔU-value calculations for going from an R-value of 1.2 to 1.9, and from an R-value of 2.2 to 2.9:

  • 1/1.2 - 1/1.9 = U 0.83 - U 0.53 = ΔU 0.30
  • 1/2.2 - 1/2.9 = U 0.45 - U 0.35 = ΔU 0.10

Suppose you reduced the heat conduction by 0.10 - 0.30 BTU/sf/hr/F°. For 140 square feet, with a temperature difference of 58 F°, this is a savings of 800 - 2400 BTU/hour, or 250 - 750 Watts.

Citation (for materials available in 1950): http://wps.prenhall.com/wps/media/objects/2513/2574258/pdfs/E09.4.pdf

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    The improvement may be even more than this, because old windows tend to leak, and if the plastic goes around the window molding it should do a decent job of stoppoing the air leaks. On the other hand, the 2 1/2 inch gap between the plastic and the window is much larger than ideal because it’s wide enough for natural convection to occur which will increase the heat transfer beyond just conduction. Double pane windows typically have a space between panes of about 1/2 inch to get maximum thermal resistance. Tough to realistically estimate the impact, but you’ve made it better. – Mark Dec 31 '17 at 13:35
  • Your R value increase of 0.7 is too low. Even if you neglect the bulk air insulation between the layers adding two additional air film layers will do quite a bit. From the souce you cited: h = 1.8 T^¼ (in scientific units) which translates to R = 3.65 T^-¼ in imperial units for each side. Even if you have a 60 degree temperature difference that would come out to an increase of ≈3. Reality is probably somewhere between these two estimates. – Rick Feb 1 at 12:35
  • @Rick -- The outermost surface is subject to wind buffeting. If that surface is glass, we can count the interior surface of the glass as having a layer of calm air. If that surface is a sheet of plastic so thin that it can ripple in the wind, we cannot trust that either surface of the plastic will have a layer of calm air. As listed in the answer, the extra insulation value I am counting is for adding a layer of calm air on the exterior side of the glass (which is now protected from the wind by the plastic that is further out). – Jasper Feb 1 at 20:41
  • I was assuming the plastic was on the inside. With the plastic on the outside, that would certainly complicate matters and makes the R value highly dependent on the wind due to the buffeting. – Rick Feb 1 at 21:26
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It's not the R-value so much as reducing the number of cubic feet per minute of warm air flowing to cold, or vice versa. A couple of old, drafty, double-hung windows may be like having a large, open hole in the outside wall. Shrink-wrapping the window frame can make a huge difference, noticeable immediately, by reducing the draft and unwanted air exchange.

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