This won't answer your question, but when my parents built their first house, the concrete porch in back got screened, then enclosed, then heated by the time house construction was complete. The dining room sliders still led out to it, the solid door from the living room was eventually replaced with double louvered doors, and the windowed laundry room door stayed in place. The main house exterior brick walls remained, too. We always called it "the porch" though it was our main living area. The next 3 families who lived there called it the "family room", and wondered what was with the dining room sliders. When one owner took out the sliders but left the tracks, the next family was really confused lol. So, I guess I'm asking, is a four-season porch really a kind of living room? Sounds like yours will be a "sun room"?

A lot will depend on your windows and doors. Something like Loewen triple glaze or maybe Optiwin or Zola will perform better than others.

You'll also want to reduce or eliminate thermal bridging as much as possible. In a room that small with 3 sides, floor and roof exposed it will be a significant issue.

Is it shaded or will it receive a good bit of solar warmth in the winter? Is your architect familiar with passive solar techniques to take advantage of that (and to avoid it being an unplanned sauna in the summer)?

My first thought was, "If it faces South, then sure! Instant Four Season Porch. If it faces West, then take out Summer for a Three Season Porch." :-D

My only concern would be the floor. My house had what must have been a small screened in porch off the LR - it is built on a slab, whereas the rest of the house has a basement. Previous owner, installed sliders and a small gas furnace. Gas furnace finally died, couldn't find one small enough for the space. Windows lost their seal. We had the glass in the windows replaced about 14 years ago. Then, about 10 years ago, I installed baseboard heat with a thermostat. The porch gets sun all day - front faces east, side (longest wall) faces south, and back (shortest wall) faces west. The house AC keeps it comfortable in the summer and the baseboard heat keeps it about 70 except when it gets in the single digits. Even then, it's in the high 60's. I bring in many of the plants from my terrace for the winter - they love all that sun. There is a nice size attic above the ceiling and the ceiling is insulated. It is wood bead board. It's my favorite room in the house - SO pleasant on a cold day to sit in the sun and see "green" instead of "brown" outside.

If you have lots of down jackets and pants for winter, and lots of swim suits for summer there's no problem. Otherwise, you need to consult your architect. Happy temperatures/!

When we define "warm enough," we hope it will be a livable space like the other parts of the home vs. a space with a cold uncomfortable floor, a room that is a cold space which requires the main heating system to continue to run.

With that said; we don't expect it to be a comfortable as main house with a full basement.

Thank you

I would change the insulation plans to foam insulation for better comfort. A room with lots of windows and all but one exterior wall will always feel a little less comfortable than another who doesn't have those qualities.