as far as I know, a greenhouse has the same thermal performance as a window. In the passivhaus modeling software I've used, it seems that in cold climates like the northeast US, south facing-windows can sometimes (but not all the time) allow more heat energy in than they lose, and windows in other directions typically do not. When they are allowing net gain, the south-facing windows usually have really great insulation values. For example, a typicall insulated wall has an "R" value in the area of R15 to R30, and an excellent window with triple-pane glass might have an overall R7.5. A standard window installed in a normal home might be R2 installed. So I'm thinking that a greenhouse that has net solar gain would have glass only on portions of the south side and still have insulated walls in the other directions.
how hard is it to get other than 5 star as a rating choice, eg upvote downvote, or even descriptive rating choices like "complete" "incomplete"