We used hi R value double glazing in DC. Further North as in Finland , Canada, etc. you would need to add a 3rd layer in the room and use the 2-3 foot extension as a walk out, yet still inside. Also, an insulated modern high R , Roman Shade could do.

I like this a lot. Well done. How is the floor made that divides the upper and lower part of the addition. Wonder if a shipping container would be less expensive to attach to an existing home in this way.