foundation week: tuesday

laying insulation

gravel fill formed to make post footings

with the plumbing in and covered we began preparation of the foundation. first a layer of six inches of crushed gravel was laid down over undisturbed native soil and levelled to within 1/4". this was compacted with with a mechanical tamper. next two inches of blueboard insulation was laid flat on top of the gravel to provide a thermal break between the earth and the slab. the insulation extends six or more feet beyond the edge of the slab to keep frost from heaving up the soil beneath. this is called a frost protected shallow foundation.

next ten inches of crushed gravel was laid on top of the blueboard. this particular blueboard supports up to 25 pounds per square inch, or 3600 pounds per square foot, enough to handle the point loads created by the posts in the building. the gravel on top of the insulation is essentially formwork to provide a sixteen inch deep perimeter beam with 2x2-foot footings for the 8x8 posts.