The sunroom is probably the newest part of the entire house. It has been added on within the last 10 years. It is 2x6 construction with standard fiberglass insulation and polystyrene (not sure on the thickness) on the outside. It has blown in cellulose insulation above it, and a insulated floor with an unaccessable crawlspace under it. It has with hydronic in floor heating.
My cousin will add thermal mass to that room in the form of their furniture and other things.
I just want to come up with a way to get more thermal mass into the room without making it painfully obvious.
Ideally, I'd love to take out a basement wall to add a access to the sunroom. I would fill the crawlspace with jugs of water and insulate it if need be. Then, I would add vents and circulate the air from the sunroom through the crawlspace to add lots of thermal mass.