Same issue here. I have newer construction so I don’t think adding more insulation will help. My issue is mainly the newer contractor grade cheap-o windows and patio door that leaks massive air. I’m thinking about adding a curtain to prevent the cold air from entering into the main room and just leave the sunroom cold as it is. I’ve already added a curtain to the patio door and cellular shades on all of the windows. I bought a thermal imaging gun and it’s cold from the floor to the ceiling since this is a raised sunroom. I think the only thing that can help keep the heat in is bubble wrapping the entire sunroom from the outside lol. Plastic wrapping the windows help a bit but it takes too long to do per window. I’m thinking about getting those snug removable plastic window panel inserts that I can re-use.