XPS foam board is one of the most commonly used insulation materials in both residential and commercial construction. Known for its durability, moisture resistance, and high thermal performance, XPS (Extruded Polystyrene) has become a go-to solution for various insulation challenges. However, a frequently asked question by homeowners, contractors, and builders is: "Can XPS foam board get wet?" This article takes a deep dive into the water resistance of XPS foam board, its performance in wet conditions, comparisons with other insulation products, and the latest trends and data in the insulation industry.
What is XPS Foam Board?
XPS foam board is a rigid insulation material made from extruded polystyrene. It is manufactured through a continuous extrusion process that creates a closed-cell structure. This structure gives XPS its superior properties, including:
High thermal resistance (R-value)
Excellent compressive strength
Long-term durability
Moisture resistance
Lightweight and easy-to-cut design
Commonly used in foundation walls, under slabs, roof insulation, and exterior wall insulation, XPS foam board is a preferred material for applications requiring consistent insulation performance and moisture resistance.
Can XPS Foam Board Get Wet?
The Short Answer
Yes, XPS foam board can get wet, but with certain limitations. Thanks to its closed-cell structure, XPS foam board is highly resistant to water absorption. However, it is not completely waterproof. Over time, especially when submerged for extended periods or exposed to repeated freeze-thaw cycles, XPS foam board can absorb small amounts of moisture.
Understanding Water Resistance vs. Waterproof
It's important to distinguish between water resistance and waterproofing:
Water-resistant materials can resist water penetration to a certain extent but may eventually absorb some moisture.
Waterproof materials are impervious to water, even under prolonged exposure.
XPS foam board falls into the water-resistant category. It can withstand wet environments better than many other insulation materials, but it is not 100% waterproof.
Water Absorption Data and Performance
Let's look at some technical data that illustrates how XPS foam board performs in wet conditions:
| Property | XPS Foam Board | EPS (Expanded Polystyrene) | Polyiso (Polyisocyanurate) |
| Water Absorption (ASTM C272) | 0.3% to 0.7% by volume | 2% to 5% by volume | 1% to 3% by volume |
| R-Value Retention in Wet Conditions | Excellent | Poor | Moderate |
| Closed-Cell Structure | Yes | Partially | Yes |
| Freeze-Thaw Durability | High | Low | Moderate |
As the data shows, XPS foam board absorbs far less water than EPS or polyiso. The closed-cell structure prevents water from easily seeping into the material.
Applications Where XPS May Get Wet
Certain applications expose insulation to moisture, either due to environmental conditions or direct water contact. Here's how XPS foam board performs in these scenarios:
1. Below-Grade Insulation
XPS foam board is widely used in below-grade insulation, including:
Foundation walls
Basement walls
Under slab insulation
In these environments, groundwater and soil moisture are persistent threats. Fortunately, XPS foam board has been tested and proven to resist moisture intrusion, making it ideal for below-grade applications.
2. Roof Insulation
Flat roofs and green roofs often experience standing water or moisture buildup. XPS foam board installed in roofing systems retains its compressive strength and thermal performance even when exposed to wet conditions temporarily.
3. Exterior Wall Insulation
In Exterior Insulation and Finish Systems (EIFS) or continuous insulation systems, XPS foam board acts as a thermal barrier and a moisture buffer. When properly sealed and flashed, it effectively resists water penetration.
Pros and Cons of Using XPS Foam Board in Wet Environments
Pros
Low water absorption rate
High long-term R-value retention
Excellent durability
Resists mold and mildew
Good compressive strength in wet conditions
Cons
How to Protect XPS Foam Board from Water
While XPS foam board is naturally water-resistant, additional steps can further enhance its performance in wet environments:
Use a vapor barrier or drainage mat behind the XPS board in foundation walls.
Seal all joints with compatible tape or spray foam to prevent water infiltration.
Install a weather-resistant barrier (WRB) when used in exterior wall insulation.
Use protective coatings or cover boards in roofing applications.
Avoid prolonged submersion in standing water or flood zones.
Product Comparisons: XPS vs EPS vs Polyiso
Here's a comparative breakdown of three popular rigid foam insulations:
| Feature | XPS Foam Board | EPS | Polyiso |
| Water Resistance | Excellent | Poor | Moderate |
| R-Value per Inch | 5.0 | 3.6 | 6.0 (degrades in cold) |
| Cost | $$$ | $ | $$$ |
| Compressive Strength | High | Moderate | Moderate |
| Moisture Retention | Low | High | Moderate |
| Environmental Impact | Moderate | Low | High |
| Best Use Case | Below-grade, Roofing | Walls, Packaging | Above-grade dry walls |
Latest Trends in XPS Foam Board Usage
1. Green Building Certifications
With the rise in LEED and Passive House certifications, XPS foam board is seeing increased use due to its energy efficiency and durability. However, environmentally conscious builders are pushing for XPS made with low-GWP blowing agents.
2. Hybrid Wall Assemblies
Builders are now combining XPS foam board with spray foam or mineral wool to create hybrid insulation systems that balance moisture control, fire resistance, and thermal performance.
3. Prefabricated Panels
Pre-insulated panels using XPS foam board are becoming popular in modular construction, where speed and performance are critical.
FAQs
What happens if XPS foam board gets wet?
XPS foam board can tolerate getting wet without losing much of its performance. It absorbs very little water and retains its R-value even in damp conditions. However, it should not be submerged for long periods.
Can XPS foam board be used in bathrooms?
Yes, with proper sealing and waterproofing layers, XPS foam board can be used behind tile in wet areas like bathrooms and showers. It is commonly used as a tile backer board in modern bathrooms.
Will mold grow on wet XPS foam board?
No, XPS foam board does not support mold or mildew growth due to its closed-cell structure and lack of organic material. However, trapped moisture behind the board may promote mold on adjacent surfaces.
How do I dry out wet XPS foam board?
If exposed to water, remove the board and allow it to air dry. Since it absorbs minimal moisture, drying is usually quick. Avoid heat sources as they may deform the board.
Is XPS foam board better than EPS in wet environments?
Yes, XPS foam board is significantly better than EPS in terms of water resistance and R-value retention. EPS absorbs more water and breaks down faster in humid or wet conditions.
Can I install XPS foam board directly on concrete?
Yes, XPS foam board is ideal for installing against concrete surfaces. It can be used under slabs, on foundation walls, and in basement floors. Use construction adhesive and seal all seams to prevent water intrusion.
Conclusion
So, can XPS foam board get wet? The answer is YES, but it handles moisture exceptionally well compared to other insulation types. Its closed-cell structure, low water absorption rate, and high R-value retention make it an ideal choice for environments where moisture is a concern.
Whether you're insulating a basement, building a green roof, or creating a continuous thermal envelope for energy efficiency, XPS foam board offers the durability, performance, and reliability needed in demanding conditions. With proper installation and moisture management practices, it remains one of the best insulation choices for wet environments.
By understanding how XPS foam board behaves in wet conditions and how it compares to other insulation materials, homeowners and contractors can make informed decisions that lead to long-lasting, energy-efficient, and resilient building envelopes.
