Are there any opinions on these R10 + windows here in the group?
Bet they want some serious money for them:
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http://www.seriouswindows.com/
I looked it over but could not easily find how they get to R10.
4 windows and 3 air spaces would do it.
Several years ago I did a study using an energy analysis program (Wattsun) that calculates yearly solar gain through windows and energy losses through the skin materials to come up with an overall energy use projection for a model house.
For the cool but not particularly sunny Pacific NW you can build an all glass walled house (4 sides) with R-5 glazing (leading edge at the time) that will have energy gains about equal to energy losses. The main challenge in such a house would be storing the solar and internal gains to even out internal temperature fluctuations.
From a look at the Serious Windows chart (http://www.seriouswindows.com/html/numbers.html) the 925 series (R-9)- this window has a foam insulated fiberglass frame, two internal films suspended between dual glass panes (probably w/ heat reflective coatings), and the unit is filled with Krypton gas. This and the single film 725 series (R-7) might be cost effective in some projects.
With all sealed windows the limit of the cost effectiveness is often determined by the lifetime of the seals. My own standard dual glazed windows are averaging about 25 years. I certainly hope newer tech sealants are doing better than that. :-\