Cork panels?

My son, DIL and granddaughter live in a tiny house that we all built here in NC and they moved to the west coast. All interior walls are pine T&G, knots sealed and painted white except in the bathroom they have a clear finish, possibly a marine varnish - don't remember for sure but can ask if you're interested. They too were on a tight budget and did not want to be surrounded by plastics and foam, hence the build and not a motor home purchase.

Thanks, but trying to avoid T&G pine. With material prices the way they are, specifically wood, T&G pine is really expensive

Here is a very informative site that you might want to read in detail.

https://www.mychemicalfreehouse.net/2019/09/modular-homes-for-chemically-sensitive.html

How about birch plywood usually not badly priced and can be painted if you need to.

PureBond formaldehyde-free plywood is fine. ... Baltic Birch plywood is made from solid birch veneer, cross-banded, and laminated with exterior grade adhesive. Exterior grade adhesive is less toxic than interior grade.Jan 12, 2015

I like the cork idea. Or you could do an environmentally-friendly clay plaster finish, too. Here's a link to some ideas: https://www.livingetc.com/whats-news/4-eco-friendly-wall-finishes-to-know-about-for-2020-237017

Not sure what the materials costs are on these; you'd have to do some background research.

@Jud Lang you're right about prices - didn't even think of that because they built 4 years ago and things were very different. Have you looked into the masonite wall panels that have a decorative skin? That might be an option - thin, light weight and I think they are formaldehyde free.

@User Sheet metal for interior walls?