Question about construction of a lean-to greenhouse
I have a friend who wants to construct an attached, lean-to greenhouse consisting of a wood frame and salvaged, double-pane windows and a sliding glass door. The footprint would probably be about 8' X 10' or 8' X 12'. The backside of the greenhouse would be a south facing wall. This wall is a garage wall clad in wood siding; it is insulated, and the garage is heated. We live in the Pacific NW and the winters are mild: most of the season it only goes below freezing for a few days.
Here is the issue. He wants to attach the structure directly to the existing exterior wall, but I raised a concern with him that he might end up with some kind of moisture problem within the wall. Can anybody advise on whether this is something to worry about? And, if so, what is the best way to deal with this?