newbie: Replacement windows questions

I am planning to replace some old (late 1800s) single pane windows with vinyl inserts. I'm interested in getting some additional information; regardless of brand, what are the key factors I should look at when making my selection? What is an acceptable warranty duration?
Also, looking at the preliminary information I've collected, it appears that I'll get substantial benefits to energy efficiency with the double pane, but only a marginal increase above that for the argon filled. Is it worth the extra money? Are any of the glass coating options worth looking at, or are they just sales tactics to up the price?
After installation, should I put the old storm windows back up as an extra layer- if so, how much additional thermal insulation are they likely to provide?
Thank you, Keith
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If your existing wood windows are structurally sound, consider instead retrofitting them with modern weatherstripping and double pane glass. For example, in the Northeast there is a service offered by the people at bi-glass.com (no affiliation).
Cheers, Wayne
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Just like someone els said, If you current wood windows are still in good structural condition you should concider installing weather stripping or energy efficiemt jamb liners instead of new inserts. The thing that the double window manufacturers dont want to tell you, ( I have gotten some very frank answeres from them) Is that the actuall energy loss through the glass its self is very minimal. Double panes only make a few point increase inefficiency. The biggest part of energy loss in old windows is Air infiltration around the sashes. Also wood windows are much more effencient than vinal. Wood is much better insulator than vinal. As well as if you keep the original wood sashes than you dont change the look of the house. The windows are the eyes of your house, they have a great deal to do with the over all look of the house. Also the tripple track storm windows are junk, they do nothing to eliminate air infiltration, all they do is keep weather off the wood windows and help them last longer. You should go with a good positive sealing single pane storm and that will fix all air infiltration issues. If you have any questions check out my website. www.fairviewglass.com, send me email if you have any questions. Hope this helps you
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To the whole house perhaps but..
http://www.greenconsumerguide.com/commercialll.php?CLASSIFICATION%&PARENT 
" The U value of single clear glass is 5.4 With ordinary double glazing this is improved to 2.6 With Low-E glass the U value is reduced to 1.8 If argon gas is used to fill the air gap, the U value will reduce to 1.6 "
Smaller the U value the better. (U value is similar to R value in the USA)
so that's a factor of 3 ish reduction in loss through the glass itself.
The biggest part of

If you seal up all the air gaps around your windows make sure the house has sufficient other sources of ventilation or you will have condensation problems.
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I agree that fenestration is always considered a major loss or gain in any thermo envelope.
Here are the U values I pulled from ASHRAE Handbook of Fundamentals (older version, they may have changed slightly, but are the U.S. equivalents - your site presented metric values).
(The values are for vertical glass on the exterior) (winter) (summer) Single glass 1.10 1.04 insulated glass- 0.1875" spacing 0.62 0.65 0.25' 0.58 0.61 0.5" 0.49 0.56 w/low emittance coating (0.6) 0.43 0.51 w/storm windows (1"-4" space) 0.50 0.50
As shown, glass is a poor insulator and a great deal of heat is lost or gained through glazing. For illustration purposes, a storm window has an equivalent R value of R-2 per sq.ft. Compare that with a ceiling with an R of 30 or higher or a wall cavity of R-19 (per sq.ft.).
So comparing std. insulating glass of .58 with one which as an emittance coating of .43, assume 100 sq.ft. of glazing - 100 x .58 = 58 UA vs. 43 UA. Assume a temperature differential of 70 def. F the std. insul glass will loose 4060 Btu per hour vs. the low emittance at 3010 Btu per hour. That's a different of 1000 Btu's per hour. Averaged over a heating season of 8,000 heating degree days and you quickly see that the better windows will pay for themselves in a few years (faster in colder climates).

That I'm afraid is complete nonsense. Glazing is always the biggest loss in any conditioned structure (normally) followed by doors and foundations. Walls and ceilings have a large exposed surface area, but now are required to have copious amounts of insulation (esp. in the new chapter 11 of the 2006 IRC) that their losses (on a sq.ft. basis) are actually quite small by comparison.
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