The "rim joist" is the outside edge of the floor -- a piece of wood typically 10 to 12 inches tall sitting on top of the concrete foundation wall, accessible from the basement or crawl space. Until just a few years ago, it was common practice to not put any insulation on this joist at all, which means that there's nothing between the basement air and the outdoors but 1½ inches of wood! Adding insulation to this joist is usually very cost effective. We recommend using solid foam board rather than fiberglass batts.
