A stand-alone greenhouse should have a well-insulated north wall. An alternative to wood framing would be to build the north wall of cinder block with insulation on the inside. Fiberglas batts are intended for framed walls that are sheathed outside and covered inside with drywall or panelling (both vulnerable to moisture). You'd be better off to insulate with non-absorbent material: foamboard, Reflectix, etc. Inside the north wall, a single-layer covering would be fine, such as the plastic sheets used to line dairy stalls.
The wood frame for the other three walls and the roof should be painted (oil-base) and/or waterproofed. Three-layer poly is fine for the walls. The roof panels might be multi-wall (I used 6-layer for my roof, R 3.8).
One feature I'd recommend is a row of clerestory vents, with louver vents low on the walls, to help with summer ventilation. I posted quite a few photos and descriptions of a severe-climate greenhouse here:
The thread "Low-tech Solar Radiant Heat" in
Greenhouses forum
