The drawing shows a detail that we (at the Colorado Mountain College Solar Program) have developed to insulate foundations in cold climates. Although it uses a standard 8-in. wide concrete-block wall, our method differs from typical foundation treatments in several ways. First, we top the block wall with a redwood or pressure-treated 2×10 sill. Its anchor bolts are recessed so they don’t protrude above the sill, and the sill overlaps the blocks on the interior side, where it protects the top edge of an isocyanurate rigid insulation panel. This insulation has a higher R-value than extruded polystyrene, but it is more fragile and it can lose its effectiveness if it gets wet. By placing it on the inside of the foundation and protecting it with a vapor barrier, we avoid these pitfalls and keep the slab thermally isolated from the foundation. This detail also eliminates the extra hassle of flashing and finishing exterior insulation, sealing the gap between the foundation and the typical sill and the kind of insulation damage that can occur during backfill. When the framed wall is lifted into place, the recessed anchor bolts don’t interfere with the bottom plate.
If the slab is going to be a direct-gain heat sink or contain a radiant heating system, we place high-density beadboard under the slab for better performance. When it comes time to pour the slab, the wide sill plate serves as a screed, and if the floor is to be carpeted, it’s a convenient anchorage for carpet strips.
To build with this foundation detail you have to alter the typical construction sequence because the sill has to be installed and leveled before the slab concrete arrives. It also costs extra for the 2×10 sills, but we think that the advantages outweigh the drawbacks.
Johnny Weiss, Glenwood Springs, CO
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