The 2015 Washington State Energy Code (R402.2.9) requires insulation at the perimeter of both heated and unheated slab on grade floors. The example shows an instance where the insulation has been installed horizontally, but the code also allows for vertical installation on either the interior or exterior sides of a foundation wall. Coverage size and R values for slab-on-grade floors can be found in Table R402.1.1 which lists a minimum R value of R10 over a distance or depth of 2 feet.
Heated slab on grade floors (R402.9.1) require R10 insulation under the entire slab. This information can also be found in Table R402.1.1, footnote d. Walls that are below grade require an R5 thermal break between a concrete wall and the slab regardless of whether the slab in heated or unheated. This information can be found under R402.2.8 Below grade walls.
Click on the link for additional information on the Washington State Energy Code.
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