Posted By wes on 07 May 2010 08:45 AM
The floor system will be entirely within the envelope formed by the ICF walls, correct? That said, the insulation values of the walls at the floor system will be the same as the rest of the walls. Why add extra insulation? Upgrading the R values of the 12" tall floor system will have little effect on the 'whole wall' R values of the house. If you want to increase the R values, use a form with thicker exterior foam insulation.
If we're talking about the edge of intermediate floor slabs, YES, the
thermal break where the floor meets the wall is a good idea, but may be
difficult to implement.
If we're talking a basement slab, eat loss through the floor can be substantial in places where the
deep subsoil temps are under 50F, or even with higher subsoil temps if the local water table is close to the slab. In places with deep subsoil temps above 60F the thermal mass of the soil at the slab level can be a net benefit moderating both heating & cooling loads. The R-value/thermal-mass of
dry soil can be used as a seasonal thermal store with self-limiting loss, but with a high water table, not so much.
In much of IA ~R4-5 of EPS or under the entire slab would likely be a cost-effective net-benefit even 8' below grade, but probably net-negative in warmer parts of KY. Local site conditions can & will affect this balance considerably, and rules of thumb are to be discounted heavily.