Is Spray Foam Insulation Waterproof? A Complete Guide to Moisture Protection

Water damage is a major threat to any home, and insulation is often the first line of defense. One common question we hear from homeowners is: Is spray foam waterproof? When water leaks or flooding occurs, they want an insulation material that doesn’t just resist moisture but actively blocks it. That leads to another frequent question: Does spray foam stop water?

In this guide, we break down the waterproofing capabilities of spray polyurethane foam (SPF), explain the difference between water-resistant and watertight, and reveal the real answer to whether spray polyurethane foam is truly waterproof—and why closed-cell spray foam is the top choice for long-term protection.

What Does Waterproof Really Mean for Insulation?

For insulation, "waterproof" signifies an absolute barrier – it will not allow water penetration at all. This characteristic is vital because moisture severely compromises insulation performance. When insulation becomes saturated, it loses its thermal resistance (R-value), hindering its ability to block heat transfer and potentially leading to structural issues and reduced energy efficiency. 

Waterproof vs. Water-Resistant: Key Differences

Waterproof and water-resistant are commonly misunderstood. Water-resistant materials can handle brief contact with moisture but cannot withstand full submersion. In contrast, waterproof insulation offers complete and long-term protection, even in prolonged exposure to water.

Why Is Waterproofing Important for Insulation?

Waterproofing helps prevent water penetration, which can reduce the R-value of insulation, weakening its ability to resist heat flow. That’s why waterproofing your insulation is key—it keeps your R-value intact and protects your structure from mold, condensation, and long-term moisture damage.

What Happens When Spray Foam Insulation Gets Wet?

Closed-cell spray foam is considered a waterproof spray foam when properly applied—it doesn't absorb water, it repels it. If it gets wet, the moisture typically dries without causing problems. However, prolonged exposure can degrade the foam and lead to mold issues.
Open-cell spray foam, due to its structure, behaves very differently when wet (see comparison below).

Which Insulation Type Offers the Best Water Resistance?

Closed-cell spray foam provides the best water resistance among insulation options. Its dense structure keeps moisture out, ensuring durability and superior protection compared to other materials.

Closed-Cell vs. Open-Cell Spray Foam

This is a critical distinction for moisture control:

• Closed-Cell SPF: Features a dense, compact structure where the cells are sealed off from each other. This makes it highly resistant to water absorption and vapor transmission. 
• Open-Cell SPF: Has an interconnected, sponge-like cell structure. It readily absorbs and holds liquid water, making it unsuitable for areas prone to moisture, humidity, or potential leaks. 

Closed-Cell Spray Foam vs. Rigid Foam vs. Fiberglass

Here's how closed-cell SPF compares to other common insulation materials regarding water resistance:

Material	Water Resistance Ability (Qualitative)	Typical R-Value/inch	Typical Perm Rating	Key Characteristics
Closed-Cell Spray Foam	Excellent	~R−6.0−7.0	<1 (Class II Vapor Retarder)	Air barrier, vapor retarder, high R-value, adds rigidity.
Open Cell Spray Foam	Poor	~R−3.5−4.0	~10-16+ (Semi-Permeable)	Air barrier (at sufficient thickness), good sound dampening, and absorbs water.
Rigid Foam (XPS)	Good	~R−5.0	~1 (Class II Vapor Retarder)	Moisture resistant, suitable below grade.
Rigid Foam (Foil-Faced Polyiso)	Good	~R−6.0	<0.1 (Class I Vapor Barrier)	High R-value, foil facing acts as an excellent vapor barrier, less moisture-tolerant core.
Fiberglass Insulation (unfaced)	Poor	~R−3.0−4.0	>10-40+ (Vapor Permeable)	Loses R-value when wet, has potential for mold if damp, requires a separate vapor barrier.

As the data shows, closed-cell spray foam offers an excellent combination of high R-value and very low water vapor permeance, making it a top choice for demanding environments.

What Affects the Water Resistance of Spray Foam Insulation?

Several factors influence how well spray foam resists water, including the type of foam, installation quality, and environmental exposure. 

1. Type of Spray Foam: Closed-cell spray foam has a dense, closely packed cell structure that offers better water resistance than open-cell spray foam. It is always recommended to use closed-cell spray foam in areas with high humidity or that face frequent moisture.
2. Installation Quality: Proper installation is crucial to fully benefit from spray foam’s waterproof properties. Always follow professional guidelines or hire trained experts for the best results.
3. Water Exposure: Prolonged exposure to water gradually reduces the effectiveness of waterproof insulation by weakening the foam’s bond. This can cause moisture damage or mold growth in buildings. Address all water leaks that can expose your foam insulation to water. 
4. UV Exposure: Direct sunlight degrades SPF over time, reducing its effectiveness and water resistance. 
5. Physical Damage: Dents, scratches, or punctures can compromise spray foam’s water resistance. Protect the insulation with durable finishing materials.

Does Spray Foam Prevent Mold and Moisture Damage?

Closed-cell spray foam is ideal for preventing mold in damp environments. Its dense structure blocks moisture and creates an airtight barrier that inhibits mold formation.

Where Does Spray Foam Work Best for Water Resistance?

Closed-cell SPF's properties make it ideal for challenging locations:

1. Basements: Insulating foundation walls (interior or exterior).
2. Exterior Walls: Especially behind moisture-sensitive cladding or in harsh climates.
3. Foundation: Providing a durable, moisture-resistant barrier.
4. Crawl Spaces: Encapsulating floors, walls, and rim joists.
5. High Humidity Areas: Bathrooms, laundry rooms, indoor pools (with proper ventilation design).
6. Flood-Prone Areas: As part of a flood-resistant building strategy (applied above expected flood levels where practical)

Using closed-cell spray foam in these areas helps protect your home, prolong its life, and effectively combats moisture-related problems.

Can You Use Spray Foam Insulation Outside?

Closed-cell spray foam is ideal for both exterior and interior use. It's widely recognized as an effective outdoor waterproof insulation solution, offering reliable protection against harsh weather conditions, including flooding and strong winds.

When installing spray foam outdoors, remember these tips:

1. Always choose closed-cell spray foam for exterior use.
2. Apply a protective coating against UV rays to enhance durability.
3. Consider hiring a professional installer for optimal results.

FAQs

How to Choose the Right Type of Spray Foam for Your Project?

The right type of spray foam for your project depends on your insulation goals and budget. If you need maximum thermal insulation and water resistance, closed-cell spray foam is the ideal choice. For cost-effective soundproofing in interior spaces, open-cell spray foam is a great alternative. Learn more in our Open vs. Closed Cell Spray Foam Comparison Guide.

Is Spray Foam Watertight or Just Water-Resistant?

Closed-cell spray foam insulation is highly water-resistant and watertight when properly installed, creating a complete barrier against water. However, open-cell spray foam is neither watertight nor water-resistant.

Will Spray Foam Effectively Stop Water Leaks?

Yes, spray foam will prevent most water leaks in your home. However, it's still essential to resolve the source of the leak to avoid long-term damage.

What Should You Do If Spray Foam Insulation Gets Wet?

Do nothing! Spray foam will dry out when it gets wet. Nonetheless, it is important to address the water leak immediately because prolonged exposure will weaken the spray foam.

Is Spray Foam Insulation a Good Solution for High-Moisture Areas?

Yes, spray foam insulation is the best solution for areas with high moisture, dampness, or water. It can also be used as a vapor barrier to effectively prevent water damage. 

Does Spray Foam Insulation Need Additional Waterproofing?

In areas frequently exposed to moisture, combining spray foam with additional waterproof layers provides extra protection.

How Much Does Waterproof Spray Foam Insulation Cost?

The cost of closed-cell spray foam, known for its water resistance, depends on project size, thickness, and whether you choose DIY or professional installation. For a full cost breakdown, visit our detailed guide: How Much Does Spray Foam Insulation Cost?