We have a house originally constructed in 1931 and a previous owner did some add-ons in the 90s.

The basement is a combination of full-basement in the original construction and unvented crawlspaces.

We are trying to address airsealing and insulation issues this year, so I just emerged from an exploratory trip to the basement.

There exists an old root cellar, 6 1/2 feet wide and 10 feet long. The walls of the root cellar are concrete block, with a vaulted concrete ceiling and a very rusted I-beam running down the spine of vaulted ceiling.

Above this ceiling is a space, and then underside of the floor of the mudroom, which someone has tried to insulate by hanging fiberglass batts, which are now sagging and falling down.

Two walls border the external soil. One side borders a crawlspace, and the curved top of the root cellar is visible and accessible from the crawlspace. The fourth wall borders the full basement, into which three rotting wood members form a makeshift door frame, onto which an even more makeshift wooden plank door is hung. Above the door "frame" is three rows of bricks forming a kind of wall, several gaps that show through to the root cellar.

How do I even begin to tackle this?

I would like to preserve the function of the root cellar (so it must be moist, dark, and cool), but insulate it from the rest of the basement and crawlspaces. This would treat the basement and crawlspaces as conditioned space while leaving the root cellar portion unconditioned, and possibly vented (there's an odd ducted hole in the ceiling that I haven't figured out the end location of--it appears to have some kind of black fabric over the top of the other end).

What to do about the rusting I beam? And how to go about insulating the root cellar from the rest of the basement? Tear out the floor above it? Tear out the concrete vaulted ceiling of the root cellar? Or go at it from the crawl space?

Any ideas will be appreciated. I will try to find a place to post photos.