Hey Julian
The walls are 45mm thick, the walls are insulated with 50mm celotex I think it is, then plasterboarded on top of that. The floor is just the supplied floor bearers, 18mil ply and then the floor on top of that.
The roof was professionally done, cost me £1000 with membrane, under layer and then top layer. Should last 25yrs!
My garden is south facing, quite lucky, during the day in the winter the temp was 25 degrees in the day, this helped keep the cabin warm at night, I do have two of these in the cabin
View attachment 7303
They kept the cabin warm, winter this year was harsh and the cabin didn't drop below 15.
Now the temp during the day 30!
I also have a air probe in the cabin to a display in the house,
View attachment 7305
The temp is in the house, the bottom is the cabin with lowest and highest temps of the day