What material to use for replacing interior porch panels?
I've had great luck with expanded vinyl beadboard.  It comes in sheets that can be used as paneling, pieces cut to smaller sizes for wainscotting, and planks.  Some are reversible, with a bead on one side and a V groove on the other.

Expanded vinyl is the material used for most "permanent" house trim.  It is a little more expensive that wood-based alternatives, but it is durable, lasts forever, and has better characteristics in virtually every respect.  It's weatherproof, moistureproof, and unaffected by UV.  It doesn't warp, rot, or split.  The material inhibits the growth of mold, mildew and bacteria.  It doesn't require painting but is paintable.  It's easy to work with, and looks like the wood product.  The rigidity is closer to thin wood boards than to paneling or plastic sheets.  I've used it for exposed exterior, protected exterior (screen porch ceiling), and inside use (wainscot).  

The big trim manufactures like Azek sell it through building supply stores: https://azek.com/products/trim/beadboard-trim.  



Stores like Home Depot and Lowes have in-house brands: http://www.homedepot.com/p/House-of-Fara-W96WP-12-sq-ft-White-Vinyl-Reversible-Interior-Exterior-Paneling-3-Piece-per-Pack-W96WP/205437105