Dr. Caroline Engel Purcell:

Yes. U-value testing for windows is much more complicated than for walls. Historic Environment Scotland has done a few technical studies on this. It did the work about ten years ago. One paper, by Dr. Paul Baker, contains a list of measures that can be taken to improve the thermal efficiency of traditional windows. Part of that involves decreasing draughts. Another option is secondary glazing. An option for highly-protected structures that one does not wish to mess with is to have heavy thermal curtains. Historic Environment Scotland calculated the U-value improvements from that mix of steps to improve windows. Those studies have not been done here, but they could be.