Should you allow air flow through a crawl space with a dirt floor?

A crawl space with a dirt floor is always a problem as the dirt will be a continuous source of moisture and risk mold, mildew and rotting floor joists if left as is. So that was a good move to put down a poly vapour barrier on the ground, but the rest is a little suspect to me as it is to you. First though I would point you to this page in case you haven’t seen it yet, it discusses the issues with crawl spaces and how they are best insulated and kept dry.

Crawl Space Insulation Tips for Insulating Crawlspaces Properly

So a few questions – is it now just flat ground under the cabin?

If not, now deep is it?  How much space between the dirt and the bottom of the floor joists?

Is it now just uninsulated walls made of plywood? 

What I’m getting at here is that it really isn’t, or doesn’t *have* to be a crawl space anymore. If the walls (or ‘sides’) are just plywood then it seems to be it may be doing no good but only harm. I will try to elaborate :  Insulating the ground on top of the poly and not having air sealed and insulated walls is pretty much pointless, that fiberglass is doing very little to retain heat.

I imagine the floor is pretty cold in winter? Based on what I know so far - if it were me I’d remove the plywood so you don’t trap moisture, I would then insulate between the floor joists below. Normally we don’t recommend venting an otherwise closed in crawlspace, but letting air flow freely on all sides makes the most sense I think.

You could spray foam underneath it as you suggest, (see here to find spray foam companies with the most eco-friendly blowing agents) as that will insulate it, act a a vapour barrier and prevent air leaks. You could then cover the spray foam with your existing plywood if its still in good shape,  or to keep animals out entirely you could lattice or diamond lathe metal around perimeter. Let me know if I misunderstood anything, happy to help more.