Low-e Glass in Primary Sash vs. Storm Window
I’m a window restorer in Cleveland, OH, and am finally working on my own windows. I know much has been said here about low-e glass, but I haven’t quite found an answer to my query.
Would it be more energy efficient to install low-e glass in the primary sash, or in the storm window? Or both?
It would seem to me that on the prime sash, it would reflect more heat back in to the home, but result in a colder interstitial space which might reduce the insulating value relative to if the low-e had been installed in the storm window instead.
If low-e in both is best, should the coating on both prime and storm face into the house, or should they both be facing into the interstitial space as they do in an insulated panel?
Every window is getting the full work down, including new storm windows I’m building, so swapping out glass in the prime sash isn’t a biggie. Our old glass barely has any wave to it and is badly scratched, so we would be sacrificing minimal historic character.
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When you have two panes facing each other, and you coat one of the surfaces, one of the ones facing the gap, which one you coat does not matter at all for the U-factor. The fundamental physics formula for the radiant heat transfer between two objects only needs the two emissivities, but it doesn't distinguish between them.
In this case, I'm guessing the original windows have divided lites. In that case, coating that glass isn't as good as coating the storm, because you are only coating the glass and not the wood. So the average emissivity over the whole unit is higher than the glass. Unless you put aluminum tape over the wood muntin bars to make them low-e too, and leave the aluminum unpainted.
If you are willing to do a second coating, the only place you would get a significant benefit is surface 4, the surface of the original windows facing the room. The downside of that is that it will increase the condensation potential, something you probably want to avoid on historic wood windows.
Putting coatings on both the storm and the original, facing each other, wouldn't have a significant benefit because once you have one coating, essentially all the heat transfer between them is by convection in the air. Reducing the radiation heat transfer further wouldn't provide a measurable benefit at that point.
If you want to do better than the low-e storm, you could consider interior and exterior storms, with low-e coatings on both, both facing towards the original window.