When, if ever, should you consider installing a continuous layer of rigid foam under a concrete slab?
The answer to this question depends on at least four factors: (1) your climate zone, (2) the height of the slab with respect to grade, (3) whether the room above the slab is used for living space; and (4) whether the slab has hydronic heat.
The most common insulation materials for use under a concrete slab are extruded polystyrene (XPS) and expanded polystyrene (EPS). Of these two materials, green builders prefer EPS, because EPS is manufactured with a more environmentally benign blowing agent than XPS. (For more information on this issue, see “Choosing Rigid Foam.”)
If you prefer to avoid the use of rigid foam insulation, you might want to consider installing mineral wool insulation under your slab. For more information on this approach, see “Sub-Slab Mineral Wool.”
Insulation requirements for slab-on-grade floors can be found in section R402.2.9 of the 2012 IECC and 2015 IECC, as well as section N1102.2.9 of the 2012 IRC and 2015 IRC. Code requirements for slab insulation are very poorly written; in fact, they are almost unintelligible. Here’s my translation: