Rim joists can usually be sealed after framing, unless access will be blocked. For example, access would be limited if the last joist bay was only 12 inches wide or the joists were only 2x8s (which would leave 71/2 inches of vertical space in which to work once the subfloor above was laid). It’s best to handle the insulation, air sealing and moisture diffusion all at the same time.

Cut a piece of rigid foam insulation and a piece of fiberglass batt to fit in the cavity. Put the fiberglass up next to the rim, then cover it with the foam. The combined thickness of the fiberglass and foam should bring the foam flush with the inside face of the plate. The R-value should be equal to the typical above grade wall. Caulk all four sides of the foam, plus the crack under each floor joist. This will give you a continuous seal.

Option 2
Gasket and Adhesive

Before setting the rim joist, tack a resilient rubber gasket onto the plate. The gasket will fill the gaps due to movement and flaws in the wood. The gasket forms an excellent seal, because it’s not interrupted by each floor joist like some other methods.

Even when compressed, the gasket will hold the rim up by 1/16 in. to 1/8 in. You may want to use scraps of gasket as shims to bring the floor joists up to the same level. Lay gasket along the full length of interior bearing walls, so the building loads distribute evenly. Setting the rim will be harder, because the gasket prevents the rim from sliding on the plate. You’ll have to lift the rim into place. This may also make it harder if you find yourself trying to square the floor framing.

Use adhesive to seal between the rim joist and the subfloor. Simply run a continuous bead of adhesive on top of the rim as you lay the subfloor. This creates a long lasting seal with very little extra effort.

Option 3
Offset Plates

In cold climates, it's critical to keep the rim joist warm, because condensation can collect on cold lumber. The well-known trick of kicking out the bottom plate of a 2x6 wall allows room for rigid insulation on the outside. You may need to add fiberglass insulation on the inside to match the R-value of the typical above grade wall. For air sealing, this detail creates a nice corner between the top plate of the first floor wall and the rim. That corner can hold a continuous bead of high-quality elastomeric sealant. Then a continuous bead of construction adhesive seals the rim to the subfloor above. This method uses inexpensive materials, and doesn’t interfere with the work flow during framing.

Option 4
Outside Gaskets

Here’s a promising idea. Gaskets four to six inches wide span the gaps on the outside. Wide gaskets made of EPDM, closed-cell foam or saturated open-cell foam should work. Because the gasket doesn’t prevent the rim from sliding on the plate, it’s no trouble to position the rim and square the floor framing. Outside gaskets will be easy if you tip up your wall framing and then nail up the sheathing. If you install sheathing on the deck, you’ll have to tack the strip of gasket to the top plate, then lay the sheathing on top.

Basements

If a basement will be heated -- now or in the future -- you'll need to seal the rim joist. Even if a basement will never be heated living space, blocking cold drafts could prevent pipes from freezing during frigid weather. The principle is the same as with the other options. In addition to the sealing the framing, you'll want closed-cell polyethylene sill sealer between the foundation and the mud sill. Although often used, fiberglass under the mud sill doesn’t block air movement. Polyethylene conforms to the rough, uneven surface of the concrete, blocks air and protects the wood from moisture originating in the concrete.