We have a traditional wedge shaped casita as our lounge and it was pretty chilly in the winter, even with a good woodburner.
We fitted the 40mm polypropylene insulation available in most builders merchants. We trimmed it to fit tight between the old wooden vigas and the difference has been dramatic. It seems that all the heat was going straight out of the roof. We also paneled in the mezzanine to stop all the heat going up there.
Warm as toast now - better open the window cos it's 25 in here...