Posted By ChrisJ on 26 Jan 2014 02:25 PM
"Which takes us back to the concept of storm windows? Can one even purchase storm windows?"

Dana1 has posted links or names of companies that make them, so they are out there.

Chris

The tightest storms in the industry are the Harvey TruChannel, and they have a hard-coat low-E glazing option.  But they're not sold through retail channels, only contractors, which can make them hard to get in some areas.  (Harvey Building Products is a regional player in the northeastern US- good luck finding them south of PA, or west of NY.)

The Larson Low-E storms are distributed through the major DIYer box store chains and available almost everywhere in North America. It's worth the up-charge to order up the Silver or Gold series for significantly better air-tightness plus nicer hardware than their low-end Bronze series.

There are others. Resources about low-E storm windows, including independent performance verification can be found here.

A tight hard-coat low-E storm over a reasonably tight wood sashed single pane double-hung performs about as well as a U0.35 replacement window (but at a lower price point).  If applied over a U0.5 clear-glass double pane you'll hit ~U0.30, and over a pretty-good U0.32 window you'll be in the mid 0.20s.

Thanks Dana!

Larson has an interior panel I am looking into.

I tried to read the numbers on my Anderson 200 series window, U0.59 could that be right?

My backside hurts from kicking myself!

Chris

Most clear-glass dry air (no argon) double-panes of the 1980s often ran about U0.6-U0.7. By the 1990s they were more likely to fall in the U0.5-U0.6 range as they got better about thermal breaks on the metal components of the sealed glass.

U0.59 windows doesn't meet IRC 2006 code-min anywhere north of US climate zone 2, and doesn't meet subsequent IRC versions anywhere, but they're still out there. Recent-years Andersen 200s sold in my neighborhood come with argon filled single hard-coat low-E glazing, and run in the U0.32- U0.34 range (and thus meet IRC 2009 code min which is current code in MA, but not always IRC 2012.)

I keep hoping to see vacuum-insulated glass show up in a big way, since it SHOULD be cheaper & simpler to build than triple-panes, and has half the U-factor. (Not holding my breath on this one though... :-) )

My eye sight isn't what it used to be so I will have to look into it further. We built the house ourselves in 2009 so I should find U0.34 on them somewhere.

I don't know what I was thinking when I ordered double hungs.

Chris in RI

Bob, they were new construction windows, although I don't think much of their doors either. My take was the warm, humid conditioned air escaping and condensing on the storm glass. We installed the same windows except in casements in a few rooms and they had no trouble at all but the sliders iced up so bad they couldn't see out of them. They had to crack the storm (also a slider) to solve the issue. There was no seal of the storm which was held by clips against the frame, real cheesy. I thought the casement to slider comparison was obvious proof but I think they had it in their heads I somehow botched the slider installs.

The windows were wood framed extruded clad, I can't remember what line, I think Ultras or equivalent if it was something else.These were the first and only windows I've ever seen with a factory storm, the homeowners got them from a relative who worked for Kolbe.

Posted By Dana1 on 28 Jan 2014 01:21 PM

...snip...
I keep hoping to see vacuum-insulated glass show up in a big way, since it SHOULD be cheaper & simpler to build than triple-panes, and has half the U-factor. (Not holding my breath on this one though... :-) )

I am surprised that vacuum-insulated glass can be made in a practical and cost competitive way. The thermal conductivity of air is essentially independent of pressure until the air pressure is reduced to about 1/10,000th of an atmosphere, and to get a factor of 10 reduction in thermal conductivity requires a reduction in pressure to about 1/1,000,000 atmosphere (https://science.nrao.edu/facilities/alma/aboutALMA/Technology/ALMA_Memo_Series/alma554/memo554.pdf)

At these very low air pressures, most of the remaining gases are water vapor and hydrocarbon contaminants, and these require heat and time to pump out of the system. Maybe with close spacing of the two panes, the vacuum requirements are not quite as drastic (since the mean free path approaches the spacing of the glass), but close spacing makes it even more impractical to evacuate the space between the two glass panes.

Of course, the glass panes cannot retain their small gap with atmospheric pressure pressing on the outside of the two panes, so microballs must be used to hold the panes apart. That would make drawing the vacuum exceeding difficult and time consuming, since the surface area containing contaminants is increased by the zillions of microballs, and since the remaining gas molecules must diffuse through the minefield of microballs.