As soon as they were placed there was a marked, noticeable difference in the warmth in the room. Since we have placed them we have seen at least a five and in some cases ten degree increase in the average temperature in the various rooms, all of which had vintage single-pane windows. We live in New Hampshire. You have to be sure your measure correctly, yes, but the rubber flange around them makes up for any minor errors in your measurement or minor variations/irregularity in old wood window frames (and was a deciding factor in our purchase since the competition requires carpentry to get the inserts in place). Even better, it is like they are not there, the view is still the same without any distortion and I still find myself sometimes trying to put my hand on a windowsill only to realize I can't since the Indow is there. Yes, they are a bit expensive but we believe we will, in the long run, save money since our heating oil costs will be less.