A lot of old houses just leak heat,
Yep, this old house has a sun room with single pane windows. I put up thick curtains between the two access points between the sun room and the main part of the house to close them in times of extreme heat and extreme cold. It works well enough for now.
It's below freezing here tonight, so I'll be sure to close all the curtains as it helps. My sister lives in an old house in Scotland. I think they are limited on what alterations they can make to 'preserve the character', but homes have to be liveable.