I was told this company made the windows for montana and I emaild them and here is there answer.
"I received your request for information on the R factor of our 8600 series windows. We haven't had the window lab tested. However, comparable windows with a slightly bigger air space between the glass panes have R factors of 1.09 (U value 0.92). Therefore these windows would have an R factor slightly lower than 1.090".
Thanks you for your inquiry
Daniel McEnery
Kinro Engineering
(817)473-4421 ext 134
dmcenery@kinro.net