Which insulation type is most water-resistant?

• A. Fiberglass
• B. Polyiso
• C. Cellulose
• D. XPS (Extruded Polystyrene)

Answer: (D)

Water resistance in insulation comes down to how much moisture a material will absorb when exposed to water. The structure of the insulation matters a lot: closed-cell foams trap tiny air pockets and resist liquid water better, while materials with open paths for water wick and hold moisture more readily.

Extruded polystyrene is a closed-cell foam, so its cells are packed tightly and water has difficulty penetrating or wicking through. That makes it the most water-resistant choice among the options, helping it keep its insulating performance even if a leak or splash occurs.

Fiberglass is made of glass fibers with open porosity. Water can move through it easily, leading to absorption, a drop in thermal performance, and potential issues like mold if it stays wet. Cellulose is a paper-based material that readily soaks up moisture from the air or leaks, which also compromises its insulating value and structure when wet. Polyiso is a rigid foam with good moisture resistance, but it can still absorb some moisture over time, especially if faces or seams allow moisture entry, so it isn’t as resistant as XPS in wet conditions.

So, the most water-resistant option is the extruded polystyrene, thanks to its closed-cell structure and low moisture absorption.