Exterior insulation at slab level for precast concrete foundation
Our Zone 4 design features a daylight basement with one wall exposed to the weather. The basement uses a precast concrete foundation from Superior Walls, and it will be a conditioned basement.
The precast wall system is easy to insulate, except at the edge of the slab floor. For structural reasons, the slab is poured inside the precast wall and the slab needs to be tied into the bottom plate of the precast wall. Thus there is no good way to do “picture frame” insulation around the perimeter of the slab.
To provide at least a little protection for the slab edge, we specified that the precast wall on the exposed side penetrate below grade (see dwgs attached); the wall “sticks down” into the ground to a point 16″ below the top of the slab, and the upper 12″ of this has R-5 insulation on the inside for a thermal break.
We are now trying to decide how to place additional insulation, outside the precast, to help keep the edge of the slab warm. (We elected to omit sub-slab insulation, since most of the slab is well-buried and the average ground temp here is about 65 deg F.)
Our plan is to place skirt insulation outside the precast wall, approximately as shown in the photo attached.
Is this the right approach? Any suggestions on materials or placement?
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The approach you suggest -- a skirt of rigid foam, a few inches below grade -- is a good solution under the circumstances.
GBA readers who are considering the Superior Wall approach should know that, in addition to the potential thermal bridging problem at the slab perimeter -- the problem identified by Mark -- there are other thermal bridges through the walls as well.
Although Superior Wall claims an R-value of R-12.1 to R-12.5 for one of its wall versions, the actual R-value is about R-7. For more information, see "What would a weighted average R-value be for a Superior Wall Xi?"