I am adding a room to my house. North side, so I'm tempted to get really well insulated glass. The operable windows are bought and installed, but I've got some fixed glass too, and wondering how fancy to get. But when I do the arithmetic I'm not so sure it's worth it. The basic options are low-E/argon filled with an R-value of 2.04 and plain ole' insulated glass (two panes of regular glass with air in between) with a R-value of 1.69. My area has about 3000 heating degree days, which is 72,000 degree hours. I have about 40 sq-ft of the fixed glass I'm buying. So I believe the btu's lost through the glass in a heating season is: 72000 degree-hours * 40 sq-ft / R The units of R (reciprocal of U) are degree*sq-ft per btu/hour, so the units work. So the difference between R=2.04 and R=1.69 is about 300,000 btu. I'm going to install a mini-split ultra-efficient heat-pump: http://www.fujitsugeneral.com/wallmountedRLS2_specs.htm The 12RLS2 has an HSPF of 12 btu per watt-hour. So providing that extra heat will cost me about 25 kilowatt-hours. For a season. For my regular electric rate this is less than $3; for the peak billing program, and I plan to consume no electricity during peak, it's a little more than a buck. For summer-time, the degree days are only about 1500 and the SEER is 25 (same units as HSPF, btu per watt-hour). It seems like a no-brainer. Anyone want to check my math ?