Which type of insulation is commonly used for exterior foundation walls?

Rigid sheet insulation is commonly used for exterior foundation walls due to its high insulating value and moisture resistance. This type of insulation is specifically designed to be installed in challenging environments, such as the exterior of foundation walls, where it can effectively prevent heat loss and manage moisture intrusion. Rigid insulation boards are typically made of materials like expanded polystyrene (EPS), extruded polystyrene (XPS), or polyisocyanurate, which provide excellent thermal resistance.

The use of rigid sheet insulation also allows for easier installation alongside waterproofing systems, ensuring the foundation remains dry and insulated. This is particularly important in preventing problems related to cold weather and frost heave, where inadequate insulation can lead to costly damage.

Other insulation types, such as fiberglass batting and loose-fill cellulose insulation, are generally more suited for interior applications or in areas that do not experience significant moisture exposure. Spray foam insulation, while effective in sealing gaps and providing thermal resistance, might not be as commonly used for exterior foundation walls due to its cost and application requirements in large surface areas. Rigid sheet insulation stands out as the optimal choice for this specific insulation need.