I did a huge amount of research on this topic a few years ago before replacing my windows. The gap in question is the gap between the two panes of glass in the unit, as you said. From what I remember from checking a number of websites, for the optimun performance, the unit should be 24mm, ie, a 16 mm gap between two panes of 4mm glass. I can understand why a narrower unit would be less efficient but I can't remember why the wider one was less efficient also but it made sense when I read the explanation. But beware of cowboys! I went with a company who told me that I was getting windows with K-glass. That's glass with a microscopic coating of metal on the inner pane which reflects heat back into the room, thus acting like triple glazing. When they arrived for installation, the sticker that should have been on the inner pane of every window indicating that it was K-glass and that this pane should be facing into the room wasn't on any one of them. I protested but was assured it was K-glass (which I had paid 10% extra for) and that sometimes they take these stickers off when assembling the windows in the workshop. I was convinced I was being had and after the windows were installed, I got a loan of a K-glass detector which indicated that-yes, you've guessed it, I had ordinary windows without a hint of K-glass in them! I know of two other people this happened to. The company had to come back and take out and replace all the units in each window. No apology given but I don't think they made much profit out of the job!