Will interior (polycarbonate or vinyl plastic) storm frames significantly improve U- .30 windows?
I read on a separate thread a discussion of the difference between U-factor .30 and .20 windows (conclusion was that .30 was usually sufficient given the cost — .20 means essentially triple-pane). I’ve got new Anderson fibrex casements with U- .29/ SHGC 0.25 but I’m not feeling snugger than I did with the original leaky 1950 single panes, storms, and the annual plastic-and-hair dryer topping. Is it worth it to create a ‘storm insert’ with polycarbonate or vinyl, in the manner of ‘energy savr window inserts;’ might that approach achieve a U-.20 solution? What if I covered both sides of a 1″ x 2″ wood frame; would that “quadruple-paned” solution in theory impact our energy bill significantly? Wondering if such storm window inserts would be worth the effort, if it matters significantly what type of ‘glazing’ we use, and if they would keep heat out in the summer. Thanks for any experience or scientific perspectives!
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According to ASHRAE, each 1.5" air space will add about R-0.9 if you are mounting them on the interior. The R-value will be slightly higher on the exterior, maybe R-1.1, depending on the temperature.
That won't have a big impact on your heating bills, but will make a difference in comfort when sitting near the window.
Most of the insulating is done by the air between panes, not by the material of the panes themselves. Polycarbonate will probably perform a LITTLE better than glass here, but the air gap will perform the same either way, and the insulating value of the air gap will dominate.
Using the R1.1 (best case) number Frank mentioned, the storm window will get your overall window perfance to around around U 0.22, and in reality will likely perform somewhat worse than that. You'll have around a 25% reduction in absolute energy loss through the window. If you have a lot of glazing in relation to wall area, you might see a reduction in energy use, but probably not a significant one.
Bill
My calculations show about 10% which I'd say is significant. Take it with a grain of salt though, those results are only as good as their assumptions.
Profuse gratitude for these helpful responses. Armed with this invaluable information I will build some test frames for our skylights and progress through some 45 windows. Thank you for taking the time to provide the sanity checks. You all have confirmed that it's likely worth the effort to build my own double-sided frames rather than to purchase the single frame options I find online. We recently got window replacements so I won't be able to supply 'before/ after' energy information (e.g. the new windows, themselves, will hopefully contribute to a smaller carbon footprint). But if I notice a difference in comfort, will report back.
Here is a product I had researched in looking to create my own plexiglass interior storms that might be of interest
https://www.grainger.com/product/10D042
I wanted to create something like the Indow interior storms
https://indowwindows.com/
What if I covered both sides of a 1″ x 2″ wood frame; would that “quadruple-paned” solution in theory impact our energy bill significantly?
Use the "annual plastic-and-hair dryer topping" on both sides of a tight 1x2 frame rather than plexiglass or thicker vinyl. The number of layers is more important than the material used. .
I have successfully used these ...
https://www.windowinserts.com/
You can test your idea cheaply using the "window film" systems that you apply and then shrink with a hair dryer. Frost king makes these, I think 3M too. A kit for a window is usually under $20, and comes with the film and double sided tape. Expect to get about one season out of these films -- they aren't a permanent solution. The film holds an air barrier near the glass, and acts like another pane. If you see a big difference, then make up the permanent assembly you describe. If you don't see much/any difference, then you're not out much time or money testing the idea.
Bill