Which type of insulation is known for having the least moisture permeability?

The correct answer is XPS, which stands for extruded polystyrene foam. This type of insulation is known for its low moisture permeability, making it an excellent choice for areas where moisture control is critical. Its closed-cell structure contributes to its ability to resist water absorption, thus minimizing the risk of mold and decay that can compromise the integrity of the insulation and the building materials surrounding it.

In contrast, the other types of insulation have varying degrees of moisture permeability. Fiberglass, for example, can absorb moisture if it is not properly maintained or protected, leading to reduced insulation effectiveness. Cellulose, made from recycled paper products, is also more susceptible to moisture and can become a habitat for pests if not treated correctly. Rock wool, while fire-resistant and water-repellent, still allows for some moisture movement through its fibers. All these factors highlight why XPS stands out as the insulation material with the least moisture permeability, making it ideally suited for applications where moisture management is essential.