What material is typically discouraged from use in insulating concrete and masonry foundation walls?

Fiberglass batt insulation is typically discouraged for use in insulating concrete and masonry foundation walls due to its susceptibility to moisture. In these environments, where there is a risk of water intrusion from the soil or other sources, fiberglass can absorb moisture, losing its efficiency and potentially leading to mold and mildew growth. The insulation's effectiveness relies on its ability to remain dry and maintain an intact thermal envelope.

Other insulation materials, like rigid sheet insulation or closed-cell foam, are often more appropriate because they provide a higher level of moisture resistance and are better suited for below-grade applications. Reflective insulation can also be beneficial in specific scenarios, but fiberglass batt insulation lacks the necessary moisture management properties for foundation wall applications.