Hi there, I hope someone has had a similar situation and can share what they did.

We have a house built in 1950 over a full basement. The basement isn't insulated, but it stays comfortable enough in winter, and nice and cool in the summer. I am planning to seal the rim joist spaces between the floor joists with blocks of foam insulation and caulk to cut down on some air infiltration. Otherwise, I like the basement just fine the way it is. Right now, for example, it's 68 degrees in my living room, and 65 degrees in the basement. Pretty good, I think.

Sometime in the 1950s, a 10' by 30' room was added on to the back of the house, over a crawl space. The crawl space is accessible from the basement through a basement window that originally opened to the outside. The crawl space has a concrete floor and is dry. The walls are block walls.

However, it is vented, and fiberglass insulation was stuffed in between the floor joists of the room above. That room is freezing in winter despite having four heat registers. The air ducts that supply the room run through the crawl space BELOW the insulation and are not themselves insulated. So of course, the air is not warm by the time it comes out into the room.

I have been reading up on crawl space sealing and insulation. Basically, the advice is to make a crawl space like mine part of the building envelope. I understand that, and am willing to crawl in there and remove the insulation from between the joists and air seal the space (caulk along the sill, foam blocks in the rim joist spaces, seal and insulate the heat ducts, permanently seal the vents), and leave open the window into the basement. The crawl space would then just be another part of my basement. What I question is the advice to insulate the walls of the crawl space. If my basement walls are not insulated, why would I add insulation to the crawl space walls? I'm willing to do it if necessary, but can't really understand why it would be, if it isn't necessary to insulate my basement walls.

Any thoughts? Is air sealing the space and making it part of the building envelope enough, or do I need to insulate the walls too? Since I'm the one crawling in there and paying for it, I want to make sure it's really necessary! Thanks!