Slab-on-grade insulation. It seems like it should not be so complicated. Yet, slab-on-grade insulation can be arranged in a number of configurations, each with pros and cons. One arrangement may be more conducive to maintaining a continuous thermal barrier from a wall down through its foundation but it allows unsightly exposure along the building perimeter. Another configuration may better conceal the insulation yet it allows an undesirable thermal short along the edge of the slab. A particular solution may provide great continuity of the thermal barrier but be problematic in terms of constructability.
The prospect of slab-on-grade insulation can become complicated. When it does, project teams will often lean on energy codes and standards to settle the issue.
Slab-on-Grade Insulation Requirements in ASHRAE Standard 90.1
Let’s unpack the issue in the context of ANSI/ASHRAE/IES Standard 90.1 – Energy Standard for Buildings Except Low-Rise Residential Buildings.
First, the standard defines two classes of slab-on-grade:
Heated slab-on-grade floor: a slab-on-grade floor with a heating source either within or below it.
Unheated slab-on-grade floor: a slab-on-grade floor that is not a heated slab-on-grade floor.
Heated slabs-on-grade will feature hot water pipes or coils embedded within or beneath the slab to provide space heating. Heat losses from heated slabs are greater than that of unheated slabs because the temperature is warmer. 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.
The R-value specification in standard defines both the rated R-value of the insulation and the depth or width of the insulation. 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.
A 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.