How Waterproof Is Closed Cell Spray Foam?

Moisture control is one of the most critical factors in insulation performance. Whether you're insulating a home, a commercial building, or an industrial space, choosing the right waterproof spray foam is essential for long-term durability and energy efficiency. Among all insulation types, closed-cell spray foam is widely regarded as one of the best spray foam for waterproofing—but how waterproof is it?

According to the International Code Council Evaluation Service (ICC-ES Evaluation Report ESR-2642), certain spray-applied polyurethane foam insulations achieve a vapor permeance below 1 perm, qualifying as a vapor retarder (Class II or III), and in some cases acting as a vapor barrier (Class I) depending on thickness and product. Specifically, some closed-cell foams meet this threshold at 1.50 inches (38 mm) or 1.25 inches (32 mm) thickness. This high water resistance minimizes moisture intrusion, making it highly effective in preventing condensation and potential water damage.

But does this mean closed-cell spray foam is completely waterproof?

In this guide, we’ll break down the difference between waterproof, water-resistant, and watertight insulation, compare waterproof insulation options, and explore how closed-cell spray foam holds up under long-term water exposure.

Why Does Waterproofing Matter in Insulation?

Waterproof spray foam is vital, especially in damp areas. Water infiltration can lead to serious structural issues and reduced energy efficiency. Here are three major risks associated with moisture in insulation:

• Reduces insulation effectiveness – Water lowers R-value, increasing energy costs.
• Mold growth within 24-48 hours – Leading to potential health risks.
• Structural damage – Weakening walls, floors, and foundations over time.

Choosing the right waterproof insulation prevents these problems, ensuring long-term durability and energy efficiency.

Waterproof vs. Water-Resistant vs. Watertight: What's the Difference?

Many homeowners looking for waterproof insulation solutions often confuse these important terms. Let's clear up these commonly confused terms to help you understand what you're really getting:

Unlike water-resistant materials, waterproof materials do not absorb or allow water infiltration under normal conditions. Since the definition of waterproof is certain and no insulation is truly 100% waterproof, the real question should be: 

Which insulation provides the best water resistance?

Closed Cell Foam vs. Other Insulation: Which Is More Waterproof?

When choosing insulation foam waterproof solutions, it's crucial to understand how different materials handle water. Closed-cell spray foam for waterproofing offers major advantages over traditional materials.

Comparison of Waterproofing in Different Insulation Types:

Why is Closed Cell Spray Foam the Best Waterproof Insulation?

Closed cell spray foam excels as a waterproofing foam spray for a robust moisture barrier:

• Continuous Water Barrier: When installed correctly, it creates a seamless layer, effectively blocking moisture at seams and penetrations.
• Superior Water Damage Resistance: More resistant to water damage than fiberglass, EPS, or open-cell spray foam. According to ASTM D2842 test standards, high-performance closed-cell spray foams typically absorb less than 1% by volume when exposed to water. In contrast, open-cell foam can absorb significant amounts of water.
• Closed-Cell Structure for Durability:  The closed-cell structure minimizes water absorption, ensuring long-term insulation performance and preventing moisture-related degradation.

Important Note: While highly water-resistant, prolonged immersion in standing water can eventually lead to gradual performance degradation. Proper drainage and moisture management are still essential.

How Does Water Affect Closed Cell Spray Foam Over Time?

While closed cell spray foam waterproof properties are superior to traditional water sealing foam or insulation materials, prolonged exposure to moisture can still affect its durability.

• Saltwater & high humidity exposure – In coastal environments, constant moisture exposure may gradually weaken the foam.
• Temperature fluctuations – Repeated expansion and contraction over time can degrade its water resistance.
• Installation quality – Poorly applied spray foam with gaps or insufficient thickness will be less effective in repelling water.

Despite these risks, a properly installed closed-cell spray foam application can last for decades with minimal performance loss.

How to Extend the Water Resistance of Closed Cell Spray Foam?

Closed-cell insulation provides excellent moisture resistance, but additional precautions can further extend its lifespan. 4 key strategies to enhance spray foam insulation’s water resistance:

1. Optimal Installation Practices:

• Apply at least 2 inches of thickness for optimal waterproofing.
• Ensure seamless coverage with no gaps.

2. Implement Comprehensive Moisture Control Systems

• Install vapor barriers in high-humidity areas like basements, kitchens, and industrial spaces to minimize moisture infiltration.
• Use waterproof membranes for critical areas such as roofs, foundations, and crawl spaces to provide an extra layer of protection.
• Ensure proper drainage around the foundation and other water-prone zones to prevent standing water.
• Consider using a dehumidifier in enclosed areas where moisture accumulation is a persistent issue.

3. Physical Protection Measures:

• Prevent rodents, mechanical damage, and UV exposure.
• Avoid drilling or cutting through insulated areas.

4. Regular Inspection and  Maintenance:

• Inspect every 6 months for any damage.
• Address small issues before they escalate.

These measures will maximize the durability and effectiveness of your waterproof foam insulation.

Best Applications for Closed Cell Spray Foam Insulation

Thanks to its waterproof properties, closed-cell insulation is ideal for demanding environments:

• Basements & crawl spaces – Prevents moisture penetration and mold growth.
• High-humidity areas – Used in commercial kitchens, locker rooms, and showers.
• Exterior applications – Functions as waterproof spray insulation for walls, roofs, and marine environments, and well pump houses (Learn more about insulating a well pump house).
• Industrial & agricultural use – Provides durable moisture resistance.
• Flood-Prone Areas – FEMA classifies closed-cell spray foam as a flood-resistant material, making it suitable for construction in flood zones. It can withstand flood conditions better than many other insulation types due to its closed-cell structure and water resistance.

When correctly applied, waterproofing spray foam provides a long-lasting moisture barrier against water infiltration.

Frequently Asked Questions

Is Spray Foam Expanding Insulation Waterproof?

The water resistance of insulation foam waterproof solutions depends on the type of foam. Closed-cell spray foam is highly water-resistant, while open-cell foam absorbs water easily. You can find more details about the key differences between closed-cell and open-cell spray foam insulation in our blog.

Does Closed Cell Spray Foam Create a Watertight Seal?

Yes, when properly applied, watertight spray foam like closed-cell spray foam forms a seamless moisture barrier. Unlike traditional insulation materials, it adheres directly to surfaces, preventing water from seeping through gaps or weak spots.

Can Closed Cell Foam Get Wet?

While closed cell foam resists water well, it can get wet on the surface. Don't worry though - it maintains its insulating properties even after brief water contact thanks to its closed cell structure.

Can Mold Form Behind Closed Cell Spray Foam Insulation?

Properly installed closed-cell foam prevents mold growth by blocking moisture. However, improper installation may trap moisture, leading to mold issues.

Is Closed Cell Spray Foam Flood-Resistant?

Yes! FEMA classifies closed-cell spray foam as a flood-resistant material, meaning it maintains its structure during flooding and resists water absorption.