With the tax credits that have been given to homeowners this year, people are looking even closer than in the past to the quality of windows that they are buying. Something that most windows have in common is the Argon gas. Argon gas is denser than plain air, so in theory the window unit should be more energy efficient. Many people these days are looking very close to the NFRC ratings on the windows because windows that have both U-Factor and SHGC ratings under.30 will give the homeowner up to a $1500.00 tax credit. Upon looking at the NFRC web site there seems to be a 2 to 5 point swing in a window built without gas.  A window with a certain kind of glass pack option rated at a .29 might be .32 with out the gas (The exact window, but with out the gas). The point is, all windows with gas will lose the gas in it at some point and the ratings on your new windows will not be as good tomorrow, as they are today. If you can get a window that has a .26 with out the gas in it, than tomorrow it will still be a .26. If you buy a window that comes with gas and has a .26, tomorrow it could climb to over thirty, which does not meet the requirement for the tax credit.