Which insulation type is known for high fire resistance and compressive strength and can be used in exterior assemblies?

• A. Fiberglass batts
• B. Mineral wool boards
• C. Cellulose loose-fill
• D. Polystyrene beads

Answer: (B)

Mineral wool boards stand out because they combine exceptional fire resistance with strong compressive strength in a rigid, board form. Made from molten rock and slag, they are noncombustible and maintain their shape even when exposed to high heat, which is exactly what you need for exterior wall assemblies where safety and structural stability behind the cladding are important. Their rigidity allows them to resist crushing and deformation under cladding loads and environmental factors, helping keep the wall’s insulation layer effective over time. They also resist moisture-related deterioration and don’t promote mold or pests like some organic insulations, making them a durable choice for exterior applications.

Other insulations don’t pull these traits together as well. Flexible batts, while effective for thermal performance, don’t provide the same rigid support or fire performance needed for exterior assemblies. Loose-fill cellulose can settle and isn’t a stable, load-bearing exterior layer, and polystyrene foams offer good insulation but don’t match mineral wool’s fire resistance and long-term robustness in exterior conditions.