Saturday, June 2, 2012

Retrospective #7: Window Film Insulation

Images courtesy of Space Window Insulation, an excellent source for bulk, inexpensive materials. SWI also accepts our local currency, REBS. Good on you!

Dead air is not good on the radio, however, it is absolutely fabulous in your home. Specifically, it is great in your building envelope: the walls, windows, floor and ceiling. Another name for dead air is insulation. As my friend the rocket scientist likes to say, “In double glazing the second piece of glass is not the insulation, it is the air in between.” He went on to show me a graph of the R-values for double-glazing when the two panes of glass are set different distances from one another from 1mm through 50mm. Now you may think that only a rocket scientist can get excited about insulation. Not true, I have managed to get four columns in a row out of dead air! (Maybe that’s why I’m in the newspaper and not the radio.) 

You will recall that last week I described how to make an eco-thrifty version of a window quilt that I call a “window batten.” Before that I wrote about the special relationship between curtains and pelmets, and before that it was pink batts. I promise this week will be the last on the topic…for now. 

While not everyone is in a position to insulate their walls, ceiling or floor, or even to install pelmets, I think anyone could make eco-thrifty window battens. (By the way, did anyone make one after last week’s column? If so, please write a letter to the Chronicle and share your experience.) I also think that anyone could install plastic window film insulation that you can pick up in kit form from a number of places around Wanganui. Just as the second piece of glass is not the insulator in double-glazing, the piece of plastic is not the insulator, it is the air gap in between the plastic and the glass. My rocket scientist friend tells me that a 20mm gap is optimum. Anything wider does not improve performance, but anything narrower reduces performance. And by performance I mean R-value: the Resistance to heat flow. 

For instance, the R-value of an insulated wall is about 2, while the R-value of a single-glazed aluminium window is 0.15. That is so low that even a 5mm air gap that you will get by applying the plastic film to the inside of a standard aluminium window may double that window’s insulating ability to about 0.3. That’s still a lot less than an insulated wall, but one of the mottos of eco-thrifty renovation is “Every little bit helps…so long as it has a short payback period.” 

And while it may take a rocket scientist to calculate the exact payback period of plastic window film insulation, it does not take one to compare its cost versus purchasing new double glazed windows for an entire home. We are talking a few hundred dollars versus many thousands of dollars. And as long as the air gap is well sealed, the performance should be equivalent. As a matter of fact, I learned recently that tight fitting thermal curtains can be just as effective as double-glazing. I might add that window battens are just as effective as tight fitting thermal curtains. In our home we are piggy-backing many of these strategies. For example, 1 pane of glass + 1 air gap + 1 sheet of plastic + 1 air gap + 1 window batten + 1 air gap + 1 thermal curtain = 1 very low power bill. This type of horizontal lasagna of window treatments may not appeal to everyone, but I’m confident that saving money does. To quote my friend who is a solar engineer in the Himalaya mountains, “Warm is always beautiful.” 

Peace, Estwing


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