We enclosed our previously screened-in carport, placing two small conventional windows on the SW side, facing our tiny sideyard. Both the SE side (facing our back garden) and NW side have sliding glass doors, the NW doors having glassed-in miniblinds. Both sliding doors have thermal drapes that can be used to cover them completely in weather that is too hot or too cold, and I have both a portable dehumidifier in the SW corner and and a large "end table" automatic quartz heater in the NW corner. I placed three large area carpets over the concrete slab, giving me essentially three different areas in which to arrange things. The slab absorbs both heat and cold, so if I am attentive to the room's needs it is entirely possible to keep it comfortable year round by adjusting the drapes, ventilating, and running the dehumidifier and heater. My sliding glass doors are screened, so on a good day I can open the doors at both ends and have a wonderful breeze through the entire room, and almost have the look and feel of a sun porch *and* a screened in porch, but without the inconveniences. The NE side opens into our tiny dining area, and we have an air conditioner in there. On very hot, humid days, our "sun room" is cooled sufficiently from that and a ceiling fan we added. This is a fairly low cost solution for anyone with a roofed-over carport or garage that they never park in; it's incredible the amount of living space that can be added to a home--it's now the largest room in the house, and the most attractive in terms of view, lighting, and general comfort.

chisue - How big is BIG?? lol We have a small family so I don't see a 25-person gathering situation. The screen porch will be off the great room which will likely have french doors flanked by floor-to-ceiling windows. Our current deck is 12x16 which seems roomy. I haven't spent much time thinking about arranging a screen porch but I am thinking I would not have an eating table on it. It may be just a space for relaxing. I can picture my husband and me sitting on lounge chairs enjoying the pond view on nice afternoons. lsst - Yes, the idea of a 'conditioned' sun room has fallen by the wayside. Our breakfast area/sitting area off the kitchen will have floor-to-ceiling windows so I'll consider that my "sun room". Between the building costs of a true sun room and the on-going costs to heat and cool, that idea is officially abandoned. Thanks for your reply which re-enforces my thoughts.

We were mulling over the same question a few years ago. We started out thinking screened porch but then realized how much we needed a 4-season gathering space when we entertained. Friends who had gone the 3-season route told us they were essentially 'no season' rooms because they were too hot in the summer and too cold in the winter (you know Minnesota extremes). So we decided to ante up for a 4-season room and we are so glad we did! We went 16'x24', put in 2 sliding glass patio doors to the deck, plus 8 good-sized windows, and a double French door to the dining room. It is a room of light all year round! We followed our furnace guy's recommendation to put in a gas fireplace for heat and a ductless minisplit for air conditioning. They both work splendidly and we can just turn them off and shut the door to the rest of the house when we're not using the room. We love the room in all seasons. With the help and advice of some very skilled siblings, we did almost all the work ourselves and it cost around $40,000.

IRC Section R316 covers Foam Plastic (also R302.8). The applied thickness does not govern a thermal barrier for this use. There are other characteristics that determine that, like where it is used, what it is exposed to, and if there is other cover like the batt insulation. If you choose to enclose the under deck space to a crawl space, or whatever your AHJ will define it as, will determine a thermal barrier. However, there is functionality of enclosing and air sealing the bottom of the deck. Many DIYers choose to use foam board under the joists as the air seal plane, with batts fully between the joists, then covering with a soffit material.