Q: What about insulated non-glass doors – are they am important adjunct to triple glazed windows?
A: A typical wood door is only about R1-2 or only slightly better than a single-glazed window. So, having insulated doors will help cut your carbon footprint. However, I do not know of any way to improve the insulation of a door other than by replacing it. So you would need to do the calculations to see how much carbon footprint and money you would save. Very roughly, if you improved the R value of a door by R1 (say from R2 to R3) you might save about $40 a year on heating bills. Before you do this though, I would make sure the door has no drafts around it. Sealing these drafts with weatherstripping is cheap, easy and effective.