Can you use 2" (underground rate) foam insulation between the cement foundation wall & the 2 x sill plate? Assuming that you use 2 foam on the exterior that extends down below the rim joist & over part of the cement foundation, wouldn't this provide a real good thermal break? Thanks, Rob.

Unless that's one very wide sill (2x12 or larger) it's unlikely that you'd have the compressive resistance ratings for carrying the dynamic weight of the house, but that's something that could be engineered. To what end would be the question.

While 2" of exterior foam over the edge of 2x6 or 2x4 sill plate is a decent thermal break on the sill, unless that exterior foam extended below the local design frost line depth the thermal bridging of the foundation wall would be substantial. In many homes with uninsulated foundation walls the heat load of an unconditioned crawl space or basement exceeds 20% of the total heat load for the house.

Posted By Dana1 on 23 Nov 2012 10:19 AM
To what end would be the question.

Hi Dana1: I saw a drawing of a super insulated wall that showed a 2" foam sill plate gasket - no longer can locate the link. Rob.

I found the drawing. My mistake, it is an ICF foundation drawing, not a pure cement foundation: http://edgewaterhaus.com/wp-content/uploads/2011/06/1008_Normand_10-A3-2-1-2-Wall-X-Section1.pdf Rob.