Answer

XPS (extruded polystyrene) insulation is considered water-resistant, but not completely waterproof.

Water Resistance vs. Waterproofing:

• Water-resistant: A material that can resist the penetration of water to some degree, but not entirely. Prolonged exposure or submersion will likely lead to water absorption.

• Waterproof: A material that is completely impermeable to water, regardless of exposure duration.

XPS and Water:

• Closed-Cell Structure: XPS has a closed-cell structure, meaning the cells within the material are tightly packed and not interconnected. This characteristic significantly hinders water absorption compared to open-cell insulation like EPS (expanded polystyrene).

• Capillary Action: The closed-cell structure prevents water from being drawn into the material through capillary action (the ability of a liquid to flow in narrow spaces against gravity).

• Reduced Absorption: Due to its closed-cell structure, XPS absorbs water very slowly and in significantly smaller quantities than other insulation types when exposed to moisture.

• Performance Impact: Even though XPS resists water absorption, prolonged exposure to moisture can still slightly degrade its thermal performance (R-value) over time. The extent of degradation depends on the amount of water absorbed and the duration of exposure.

Factors Affecting Water Resistance:

• Density: Higher density XPS tends to be more water-resistant.

• Surface Imperfections: Cuts, punctures, or damage to the surface of the XPS board can create pathways for water to enter.

• Joints and Seams: Water can penetrate through gaps between XPS boards if they are not properly sealed.

Typical Applications and Considerations:

• Below-Grade Applications: XPS is frequently used in below-grade applications (foundations, basement walls) due to its relatively good water resistance compared to other insulation types. However, it’s crucial to use proper waterproofing membranes and drainage systems in conjunction with XPS to prevent prolonged exposure to water.

• Exterior Insulation: XPS can be used for exterior insulation applications.

• Roofing: XPS is sometimes used in roofing applications.

• Importance of Drainage: Regardless of the application, proper drainage is essential to minimize water exposure and maintain the long-term performance of XPS insulation.

Comparison with Other Insulation Types:

• EPS (Expanded Polystyrene): EPS is less water-resistant than XPS due to its open-cell structure, which allows for greater water absorption.

• Polyurethane/Polyisocyanurate: These foam insulations also have closed-cell structures and offer good water resistance, though the specific performance varies depending on the product.

• Mineral Wool/Fiberglass: These insulation types are not water-resistant and lose their insulating value when wet.

In summary: XPS insulation offers good water resistance due to its closed-cell structure, making it suitable for applications where moisture exposure is a concern. However, it’s not completely waterproof, and prolonged exposure to water can reduce its thermal performance over time. Proper installation techniques, including sealing joints and ensuring adequate drainage, are crucial for maximizing its effectiveness and longevity, especially in high-moisture environments. The use of a waterproofing membrane with any below grade application of XPS is highly recommended.