We don't have insulation curtains on windows. Didn't even know they exist. Would be an idea. Recommended material? Felt is kind of expensive here. We have 2-3 thick cotton curtains on the doors but plan to add even thicker ones.
Double glazing from the 90s.
Haven't crawled the crawling space on the attic but it was insulated in the 90s, I recon it would need some looking in to.