Hi –
I have read a lot about air sealing here and via Department of Energy.  Lots of great information but I am a little bit more challenged deciphering best practice principals to my century home.  The house is a 1922 kit house in Tacoma, Washington.  I’m finally getting around to air sealing the rim joists in our basement area.  Half of the basement is about 7 feet of head room and basically a concrete box interrupted by piers and housing our duct work. 

The other half of the basement is a concrete enclosure filled with formerly exposed dirt that is now encapsulated with 12-mil plastic, mechanically and adhesively attached to the concrete foundation.  The headroom on this side of the basement is about 3 feet.

We have no intention of finishing the basement any further but it is conditioned for the most part (albeit terribly inefficiently and leaky) due to the current HVAC set up which we are hoping to improve upon post air-seal.

At one point, the foundation was definitely failing on the “finished” side and the house was sinking.  It probably still is; but, after seven years, I’m not noticing obvious signs. Previous owners poured a new concrete wall around the interior perimeter of the back half of the house, the side that is now a concrete box, and new post and beams to level things out.  Pressure treated wood shims were shoved into the gap between the sill plate and foundation and a water table trim piece was applied to the exterior to cover up the ghastly details and divert the eyes from the lopsided appearance. So there is not just one point of air leakage between the cement foundation and mud sill but several layers due to the various shims.

I share all of the above details because to reach the rim joist is going to be a real pain in the butt as I have to reach past the new foundation wall and up behind the beam and posts resting on that foundation wall into the rim joist cavities of the old foundation wall.  I thought about repeated applications of spray foam in that cavity but was concerned about creating a hidden moisture problem.  The more labor intensive (but maybe impossible due to access issues) option is to insert rigid foam sealed with spray foam. 

EDIT: My image was from the DOE website manual.  I want to confirm that I will be using rigid foam insulation boards to fill most of the cavity.  But I do have one concern: For the places that I absolutely can’t reach because of the “double foundation” issue, would it be an issue to use only spray foam to seal that cavity, especially, since I do not have a way to install a capillary break between the foundation and mudsill? Would I be inviting a rot issue by spraying these sections of mostly inaccessible rim joist directly with spray foam? As said earlier, the basement is not finished but conditioned (albeit poorly).

And then the exterior detail.
The house still has original shingles and shiplap.  There is no sill gasket to interrupt capillary action but we will not be lifting the house.  I can’t apply the tape as shown in the attached illustration (again NOT USING fiberglass fill but will be using rigid foam) because the exterior detail is not as straight forward as the picture here.  In some places the rim joist assembly sticks out a little over the foundation and other places it is more recessed.  The last row of shingles hides all of these details. I was thinking I should at least caulk all the gaps between the sill plate, shims, rim joist, etc., for air sealing purposes only.  Is that completely pointless?  If not, what might be the danger in doing this?