Q: What about adding film with low E coating to existing windows? We have beautiful old windows we want to keep.

A: I have never seen a low-E film. The films that are common are shading films that just block all light. They do not selectively block UV light or IR light in the way that low-E coatings do. I have used the sun blocking films on exposed south-facing windows and it works pretty well, but unless you remove it in winter it does darken the room in winter. A better solution would be to buy a window insert made from glass (such as from Innerglass in CT) and get the low-E coating on the glass.