Sill plates and rim joists are usually poorly insulated (if at all) and very leaky. So if you have an unfinished basement, grab some silicone or acrylic latex caulk to seal the sill plate. If you simply have fiberglass insulation stuffed against the rim joist, pull it out. Run a bead of caulk between the edge of the sill plate and the top of the foundation wall. Use expanding spray foam anywhere there are gaps larger than 1/4 in. between the sill and the foundation.