http://blog.sls-construction.com/2012/building-science-nfrc-label#.UFJ3vELKMkc

and

http://www.energyvanguard.com/blog-building-science-HERS-BPI/bid/55519/a-good-window-is-still-a-poor-wall?source=Blog_Email_[A%20Good%20Window%20Is%20Sti]

it isn't only about how the window looks.

condensation causes a lot of damage rotting

from sills down to sole plates.

best of luck. and remember if nfrc sticker

is not on brand you are shopping...shop another brand.