I'm new here & I'm a new home owner and new to DIY'g so please be gentle on me.
Our situation: We bought a 1960 ranch home that's got good bones but needs updating. Our current project is a 500 sq foot sun-room (or may be a 3 seasons room? IDK but I've just seen different names for these type of attached rooms) Its attached at the back of our home - you exit the kitchen through sliding glass doors into it. We use it as a family room and so far its been a great asset for entertaining. The way the previous owners built it - it is three walls of sliding glass doors (two walls have 2 doors and third wall has 4 doors). There are no brick/motar walls except for the main one off the home that the sun-room is built off from. We live in So California so we can use the room about 75% of the time - but in the hottest summer or coldest winter (ok, so its only 40 degrees that still cold to us!) its just unusable. The sun-room ceiling is exposed (meaning the roof is the ceiling) so there is no insulation or drywall finish to it. So I was thinking if we insulated the sunroom ceiling we might be able to get some control of the heat/cold. It has a separate A/C unit but no heat. Ideally we'd like to do recessed 'can' lighting up there too (right now there are a pair of mismatched ceiling fans) but that might be too advanced for our DIY skills. Anyway, I've been searching high and low online to figure out a way to do this and I can't find any instructions. I could be phrasing this wrong but its the best way I can describe it - thanks in advance for all of your help!
It sounds like you are a candidate for closed cell spray foam in the lid, though with all the walls being glass I am not sure you will be gaining much. Drapes and ensuring that the sliding glass doors are "air tight" (most apparently leak like sieves) will also help somewhat. Do you have an insulated concrete slab in there? That will act as thermal mass if the sun gets to it, radiating some heat back out at night. BUT, with glass walls, you have a tough row to hoe. If you insulate the roof on the inside w/ foam, make sure whatever the roofing is made of will take the heat, 'cause it is gonna get hot (may need ventilation between the foam and the roofing). Good luck to ya, and don't freeze in that 40 degree "cold".