Hi everyone,
I am planning to build a 8' x 12' greenhouse this spring, next to my garden. I plan on building it out of wood, with 1/2 the roof, and a couple walls having double-wall polycarbonate panels. I am keeping it small so I might be able to heat it, or keep it up above 40*F or 50*F with a small electric heater. My questions is, do you think I can insulate the part of the ceiling and North wall with Fiberglass insulation (kraft-face or un-faced?) between studs and rafters, and cover it with knotty-pine interior boards? I don't suspect there will be a lot of humidity in the summer, as I will have some power roof vents on a thermostat, and a couple lower venting windows. Not sure about the winter, but I suspect it to be on the dry side. So can I use the fiberglass insulation? And should I use kraft-face, or unfaced? Thoughts?

Hi @rod725020 and welcome!

A greenhouse is a very humid environment many days of the year. If you have never owned one, you will find out after you build it, so make sure you do it the right way, or you're in for a heap of trouble later on with mold and rot. Before picking any insulation or taking advice from someone who may not be knowledgeable enough to give you a good answer (like me), I would check about fiberglass insulation and moisture.

Do your research first. Check YouTube, read articles, speak to people that do insulation for a living.

For me. I know I would want a vapor barrier and would probably use pink insulation board and even double-bubble, double-sided reflective insulation. One to insulate the wall and the other to reflect any heat away from the wall and back into the greenhouse, but there are variables in every greenhouse setup.

~Jim

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