There is also that fiberboard that can be used in showers then covered. And there are all kinds of sealants in that aisle as well.

What we did was use OSB then caulk all the seams and paint it white using semi-gloss paint. It's been through a summer season and winter season and has held up perfectly. (I have to use misters in the summer to keep the greenhouse usable here in Nevada.)

Granted, it doesn't look at pretty as a nice sheetrock wall, but my concern is that the moisture resistance may not be enough. I don't have any experience with that type of sheetrock, but would definitely suggest researching it. No matter now hard you try, water will eventually get splashed on the wall.

Here is a link that might be useful: The Build of My Greenhouse

What about exterior MDF siding? It's rigid enough to be used with no backing, comes pre-primed, and a 4 x 8 ft. panel is usually priced under $25.00, give or take, and unlike OSB; has a smooth and easy to paint surface. We used it on our large garden shed 20 years ago and it's still solid. Of course it's been repainted several times since, but it's also exposed to the worst weather the Midwest can throw at it.

It would also allow you to drill into it to mount shelving, or hooks to hold tools, etc., and if/when you decide to change things about, the holes can be filled in and painted over.

The important thing is to seal it completely at all seams with a high quality, paintable, exterior grade caulk, and use only galvanized, exterior grade fasteners to mount it.

I found the Olympic acrylic soild stain to work best on it. It covers exceptionally well and dries to a soft, satin sheen, although for a solid, streak free finish, would likely require 2 coats for the initial application.

H
A material I would highly recommend is Hardiboard. made of fiberglass and cement Comes in 4x8 sheets stucco, vertical wood or stone finishes. You can also get it in siding type of clapboard. with the same finishes. I've even used this underwater!! . lol Holds latex paint like a champ.. In the land of eternal rot(florida) it's the only way to go. Might mention that it only costs around 25 bucks a sheet as cheap as sheating or plywood.
gary

On north wall I put 2" styrofoam between 2x4s, caulked with foam goop, covered with green 3/8-in plywood, primed, and painted 2 coats with white exterior paint. Good light reflection into GH, heat retention, and water resistance.

We just used a white vinyl paneling (from Home Depot) that comes in 4X8 sheets to cover our kneewall in the greenhouse. The joints are all sealed and I believe it will work out just fine.