Foam board insulation doesn't absorb water, which essentially limits the amount of moisture build-up in your insulation. Polyiso and extruded polystyrene foam boards have the strongest waterproof properties.

What insulation is waterproof?

Waterproof rigid insulation is used on roofs. Expanded polystyrene, or XPS, extruded polystyrene, or EPS, and polyisocyanurate, or polyiso, are the three main types of rigid, closed-cell insulation materials.

What happens if foam board gets wet?

If your spray foam gets wet, it will eventually dry out again. Spray foam that is wet for an extended period is likely to deform and potentially expose insulation in your wall. It would require a considerable amount of water for most spray foams to deform.