I would guess if they're that old, and double-pane then they are likely R2. Todays super efficient windows (Low-E + Argon) are R3 with the most heavily insulated ones being around R3.5 (or u-value of 0.28)
Windows are terrible at insulating or lowering ones heating bills... even todays best. I have a program that calculates savings based on previous to new. Want to know what each of my 3x3 windows is saving me now that I replaced them?
My old 35 year old windows were just double-pane (U-Value 0.5) and my new Low-E + Argon were U-value 0.32 (R3). Improving 9 square feet from R2 to R3 for my Heating Degree Days area (New England) is saving me $6.05/year in oil for each window but each cost me about $200. Payback = not my lifetime. I changed them because some were broken, my wife wanted something easier to clean, and I hated the mildew that formed on the bottoms of the frame under the glass. I still get the mildew even with these new vinyl windows but it's a simple clean come spring.
If you want to save $ look to sealing air leaks, penetrations, adding insulation, but don't replace the windows to save on heating replace them only because of something else.
Last edited by Piedmont; 10-28-2009 at 06:04 PM.
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