Final answer:
Insulation under a basement slab should be installed before pouring concrete to prevent heat loss, manage moisture, and improve energy efficiency. It is a standard construction practice, not typically an optional homeowner request.
Step-by-step explanation:
Insulation should be installed under a basement slab before pouring concrete. This is an essential step in basement construction as it prevents heat loss, helps manage moisture, and improves energy efficiency. Installing insulation after framing or drywall would be impractical, as the slab foundation needs to be insulated before it is covered with concrete. The insulation is not typically an optional feature requested by a homeowner; it is a standard building practice designed to conform to building codes and ensure a comfortable, energy-efficient home.