[JandC]

I am not sure how much money the dual pane windows are as an ordered option, but as already has been mentioned they are not sealed units, or argon gas filled. You would have the same result if you took some construction glue and stuck an extra thin piece of glass over each of your existing windows. They will cut down slightly on outside noise and possibly make your unit a little easier to heat/cool. With the existing R values in the sidewalls of Montana's I wouldn't want to carry the extra weight.