I was wondering how much of a difference going from a .20 window to a .24 is going to make. What would the payback time be if the difference is almost 10k? The house would be an ICF so it is very tight and the doors are only a .24.

What is the area of the windows?

Located in the New York area.

I ran three window cases for Albany, NY, using some window specs for Serious Windows for a well-insulated 1600 sq. ft. house with 66.7 sq. ft. of windows on each side, with each of the four windows on each side being 40" x 60". Costs are for natural gas heating.

U-value SHGC Annual Heating bill with natural gas
0.19 0.22 $625.13
0.24 0.22 $658.24
0.24 0.41 $596.98

These were with "typical" solar gain as defined in RESFEN, so some shading. A five point change in U-value changes the annual heating bill by $33.11. If you paid $10,000 to reduce the U-factor by 0.05, may you have a long life... since it will take 302 years to pay it off. On the other hand, if your house is not badly shaded, going with higher solar gain windows, in this case a SHGC = 0.41 in place of 0.22 even with a U-factor of 0.24 REDUCES your heating bill relative to the lower U-value window. If your house is completely shaded, then just focus on the U-value for thermal performance.

Lee, Thanks for putting it in numbers for me. This makes my answer clear because like most I have a budget. So if I go with a geo thermal system my savings will be even less depending on its efficiency of course.