These F-factors are acceptable for all slab-on-grade floors, but you need to make sure the insulation is rated and applied accordingly. The F-factors for slab-on-grade floors are defined in Standard 90.1. Consult Appendix A (Table A6.3.1 in the 2016 edition) for a tabulation of assembly F-factors for slab-on-grade floors based on the arrangement and rated R-value of the applied insulation. Vertical Insulation: Continuous insulation is applied directly to the slab exterior, extending downward from the top of the slab for the distance specified.įully Insulated Slab: Continuous insulation extends downward from the top of the slab and along the entire perimeter and completely covers the entire area under the slab. Horizontal Insulation: Continuous insulation is applied directly to the underside of the slab and extends inward horizontally from the perimeter for the distance specified, or continuous insulation is applied downward from the top of the slab and then extends horizontally to the interior or the exterior from the perimeter for the distance specified. The F-factors are provided for three insulation configurations (verbatim below from the 2016 edition with my emphasis): F-factors are provided for both heated and unheated slabs. In contrast to the U-factor for floors, the F-factor for slab-on-grade floors is expressed per linear foot of building perimeter. For example, "R-10 at 36 in." means that insulation with a rated thermal resistance of 10 must be installed and that the insulation must extend a distance of 36 inches form the top surface of the slab.Ī review of the Standard 90.1's building envelope prescriptive requirements will reveal a reference to a "maximum assembly F-factor" rather than a U-factor, as one would see for other envelope components. The R-value specification in standard defines both the rated R-value of the insulation and the depth or width of the insulation. For unheated slabs, insulation may or may not be required depending on your climate zone, whether or not the project is residential, and which edition of Standard 90.1 is being referenced. Heat losses from heated slabs are greater than that of unheated slabs because the temperature is warmer. Heated slabs-on-grade will feature hot water pipes or coils embedded within or beneath the slab to provide space heating. Unheated slab-on-grade floor: a slab-on-grade floor that is not a heated slab-on-grade floor. Heated slab-on-grade floor: a slab-on-grade floor with a heating source either within or below it. Let's unpack the issue in the context of ANSI/ASHRAE/IES Standard 90.1 - Energy Standard for Buildings Except Low-Rise Residential Buildings.įirst, the standard defines two classes of slab-on-grade: Slab-on-Grade Insulation Requirements in ASHRAE Standard 90.1 When it does, project teams will often lean on energy codes and standards to settle the issue. The prospect of slab-on-grade insulation can become complicated.