What type of insulation should be installed under a slab?

When insulating under a slab, extruded polystyrene (XPS) is the most suitable option. XPS is a type of rigid foam insulation that provides excellent thermal resistance and is particularly effective in preventing heat loss from concrete slabs. Its moisture resistance is another significant benefit, as it helps to prevent issues related to water infiltration that can compromise the insulation's effectiveness over time.

Additionally, XPS insulation supports structural loads without compressing, making it ideal for applications beneath concrete where structural integrity is crucial. Its durability and long lifespan further enhance its suitability for this purpose, ensuring that it maintains its insulating properties over time and under varying environmental conditions.

Other types of insulation, while useful in different applications, do not match the performance characteristics of XPS for slab insulation. For example, fiberglass insulation is less effective in high-moisture areas and not suitable for direct contact with concrete due to its susceptibility to moisture and degradation. Foam board insulation generally refers to a broader category, which includes XPS, but may imply other types that do not have the same moisture resistance. Mineral wool insulation is also excellent for soundproofing and fire resistance but is not designed for beneath-slab applications due to its overall thermal performance limitations in that context. Therefore, XPS is