When should insulation be installed under a basement slab?

Installing insulation under a basement slab is particularly important when there is in-floor heating. In-floor heating systems heat the space from the ground up, and proper insulation beneath the slab helps to maintain the heat within the living area rather than allowing it to dissipate into the ground. This not only enhances the efficiency of the heating system but also improves comfort levels in the basement, as a well-insulated slab prevents cold from penetrating through to the living space above.

In scenarios without in-floor heating, insulation may not be as critical, especially in warmer climates where heat retention is less of a concern. However, in colder climates or if a homeowner specifically requests insulation, considerations can vary. Thus, option B correctly highlights a key condition under which insulation beneath a basement slab is particularly advantageous.