Slab insulation using EPS
I am in the process of designing a new home construction project. Naturally, I am interested in minimizing costs and maximizing insulation/efficiency. The home is slated to be a ranch-style that is slab-on-grade. I have been planning on using high-density EPS foam (6″ thickness) underneath the slab, and 6″ foam on the exterior of the slab and footings. The EPS foam would also go underneath the footing as well so that there is a continuous layer of insulation.
I was recently reading that the majority of the energy is lost through the foundation’s exterior walls, so I am questioning the cost effectiveness and performance gain by doing a continuous layer of insulation. In other words, it seems that there is a higher cost versus efficiency gain using this scheme.
An alternative approach would be to insulate the exterior perimiter of the foundation down to the footer, and then install EPS foam over the slab. This would reduce the complexity of the foundation so there is nothing particular interesting going on that would scare (increase price) the foundation contractor (i.e., it would be a conventional foundation). We would, of course, need to increase the ceiling height to compensate for the thickness of the foam. The disadvantage that I see here is that the insulation is not continuous around the foundation footing so there would be some heat loss through the foundation wall.
In case this is relevantm we intend to insulate the walls to around R-35, and the cathedral ceilings to around R-45.
I would like to hear other’s thoughts on these two approaches.