Final answer:
The recommended insulation to install under a slab is foam board, such as polystyrene, due to its poor thermal conductivity and the advantages it offers in terms of rigidity, moisture resistance, and ease of installation.
Step-by-step explanation:
The kind of insulation recommended for installation under a slab is foam board insulation. This choice is based on the thermal properties of the material, as referenced in Table 1.5 which indicates that polystyrene foam is a very poor conductor and therefore an excellent insulator. The same principle applies to other materials with a high resistance to thermal conductivity, such as fiberglass, wool, and goose-down feathers; these materials are characterized by their numerous small pockets of air, which exploit air's poor thermal conductivity to insulate effectively.
While fiberglass is also a good insulator, foam board made of polystyrene or similar materials is typically used under slabs due to its rigidity, durability, moisture resistance, and ease of installation. Reflective foil is less commonly used in this application as its main function is to reflect radiant heat rather than insulate against conducted cold from the ground. Moreover, neglecting to install any insulation under a slab can result in significant heat loss, increased heating costs, and discomfort, negating the option of no insulation being required.