Organic Cotton Batting works well as a foundation layer in upholstery, and is often the only padding used in antique chairs. It generally yields a firmer seat than foam, with firmness depending on what’s underneath it. If the chair has springs it can yield a very comfortable seat. If it has only webbing, it will feel firmer.
I prefer to use the original materials (or replace with equivalent material) whenever possible to retain the integrity of the chair’s purpose and history.
Wool batting is NOT suitable as a foundational support layer – it replaces polyester batting, which serves to ‘soften the edges’ on the surface rather than providing a substantive padding layer.