Foundation with freshly-poured concrete
I expanded the footprint of the house to the rear and side of the old foundation. I chose to go with a frost-protected shallow foundation. Unlike traditional foundations, this type does not require footings down 42” to frost depth. The installation began with a 6” layer of crushed stone, followed by a 12” layer of EPS rigid foam insulation (EPS=expanded polystyrene), with short EPS stem walls around the perimeter, forming a shallow basin that is filled with concrete. The EPS isolates the concrete from the ground and the outside air, which prevents “thermal bridging” (heat loss through structural materials that don’t insulate well, like concrete). The addition was then framed directly above the concrete slab. The insulation not only prevents heat loss to the ground, but also heat loss from the ground – which is 50-55 degrees year round – to the ambient air. That’s why traditional footings down 42” aren’t required.
The walls were framed with 2×6 studs to create 5-1/2” cavities for insulation. The studs are spaced 24” apart, instead of the normal 16”. This wider spacing means that there is less wood and more insulation in the wall assembly. The roof is framed with engineered trusses, which are structural wood assemblies manufactured off-site. The rear roof trusses are 18” tall and filled with insulation. The spaces between the top and bottom wood members of the trusses are filled with insulation which reduces thermal bridging. The attic is also framed with trusses. These have “raised heels”, to provide ample space for insulation directly over the outside walls, which eliminates the principal cause of ice dams.