Common Myths Debunked: Open vs. Closed Cell Spray Foam Insulation in Home Renovations

Introduction

Spray foam insulation has become one of the most talked-about upgrades in modern home renovations, promising everything from lower utility bills to enhanced structural integrity. Yet, for many homeowners, the choice between open cell and closed cell spray foam is clouded by conflicting advice, persistent myths, and technical jargon. If you’re planning a renovation or retrofit and want to make the smartest investment for your home, it’s crucial to separate fact from fiction. In this post, we’ll break down the key differences between open and closed cell spray foam, debunk the most common myths, and provide practical guidance to help you select the right type for your specific needs. Whether you’re insulating an attic, crawl space, or whole-house remodel, understanding the realities of spray foam can save you money, time, and headaches down the road.

Understanding Spray Foam Insulation: The Basics

What is Spray Foam Insulation?

Spray foam insulation is a versatile material made by mixing two chemical components (isocyanate and polyol resin) that react to expand and harden. It’s applied as a liquid, quickly expanding to fill cavities and create a seamless, air-tight barrier. Its popularity stems from superior air sealing, high R-values, and adaptability to irregular spaces compared to traditional batts or blown-in insulation.

Types of Spray Foam: Open Cell vs. Closed Cell

• Open Cell Spray Foam: Has a soft, spongy texture. The cells are not completely encapsulated, leaving them open. It’s lighter, expands more, and is less dense than closed cell foam.
• Closed Cell Spray Foam: Denser and more rigid. The cells are fully closed and packed together, making the material harder and more moisture-resistant. It’s heavier and offers a higher R-value per inch.

Debunking Common Myths About Spray Foam Insulation

Myth 1: All Spray Foam Is the Same

This is one of the most pervasive misconceptions. Open and closed cell spray foam differ dramatically in density, R-value, cost, application, and performance. Using the wrong type for your project can lead to subpar results or even building code violations.

Myth 2: Spray Foam Insulation Is Always Waterproof

Only closed cell spray foam is classified as a water vapor barrier. Open cell foam is vapor permeable and can absorb moisture. This makes closed cell foam suitable for basements or below-grade walls, while open cell is better for interior spaces where breathability is important.

Myth 3: More Is Always Better

Spraying thicker layers than recommended won’t always improve performance. For closed cell foam, exceeding the maximum thickness per pass can cause off-gassing and improper curing. Always follow manufacturer guidelines and local codes for application thickness.

Myth 4: Open Cell Foam Causes Mold Growth

This myth comes from misunderstandings about vapor permeability. Open cell foam doesn’t promote mold growth when installed correctly with proper moisture management. It can even help prevent condensation by air-sealing gaps.

Myth 5: Spray Foam Is Only for New Construction

Spray foam is an excellent retrofit solution. It can be injected into existing walls, attics, rim joists, and crawl spaces. Professional installers can advise on safe and effective retrofit techniques, even in older homes.

Open Cell vs. Closed Cell Spray Foam: Detailed Comparison

R-Value and Insulating Performance

• Open Cell: R-value of about 3.5-3.7 per inch. Suitable for interior walls, attics, and soundproofing applications.
• Closed Cell: R-value of about 6-7 per inch. Ideal for exterior walls, basements, crawl spaces, and areas prone to moisture.

Air Sealing and Structural Strength

• Open Cell: Excellent air barrier. Does not add significant structural strength but effectively reduces drafts and air leakage.
• Closed Cell: Superior air barrier and can increase a wall’s structural rigidity by up to 300%, which is valuable in hurricane or earthquake-prone areas.

Moisture and Vapor Control

• Open Cell: Vapor permeable; allows moisture to pass through. Not a vapor barrier—best for spaces where you want the building to “breathe” (e.g., attics in humid climates).
• Closed Cell: Acts as a vapor barrier; repels water and prevents condensation. Suitable for foundations, basements, and flood-prone zones.

Cost Differences

• Open Cell: Less expensive per board foot, but you need more thickness to reach the same R-value as closed cell. Often used in applications where space is not a constraint.
• Closed Cell: More expensive, but higher R-value per inch. Can be more cost-effective in walls or tight spaces where maximum insulation is required in minimal thickness.

Soundproofing

• Open Cell: Superior sound absorption due to its soft, porous structure. Excellent for media rooms, bedrooms, or offices.
• Closed Cell: Less effective at soundproofing but offers better air sealing.

Application Scenarios: Choosing the Right Type for Each Project

Attics and Roof Decks

Open cell foam is typically preferred for unvented attics in moderate climates due to its breathability and expansion, which can fill irregular cavities. Closed cell foam is a better choice in areas with extreme weather, as it adds structural strength and moisture resistance.

Basements and Crawl Spaces

Closed cell foam is the only safe choice in these areas. Its vapor barrier properties prevent moisture intrusion and protect against mold or rot, essential in below-grade environments.

Exterior Walls

Closed cell foam is often chosen for exterior walls due to its high R-value and ability to provide a continuous air and moisture barrier. However, in mild climates where vapor drive is less of a concern, open cell foam may be adequate and more economical.

Soundproofing Interior Walls

Open cell foam is the clear winner for sound attenuation, making it ideal for media rooms, bedrooms, or home offices where noise reduction is a priority.

Frequently Overlooked Considerations

Building Codes and Permits

Local building codes may dictate the allowable type, thickness, and location of spray foam insulation. Some jurisdictions require ignition barriers or thermal barriers (such as drywall) over spray foam for fire safety. Always consult your local authority before starting a project.

Potential for Trapped Moisture

Improper application can trap moisture within wall assemblies, leading to hidden mold or rot. This risk is highest when closed cell foam is installed on the wrong side of a vapor drive in cold climates. Professional assessment and correct placement are critical.

Professional vs. DIY Installation

While DIY spray foam kits are available, achieving uniform coverage, correct chemical ratios, and safe installation requires experience and specialized equipment. Poor installation can result in off-gassing, voids, or even structural damage. For most homeowners, hiring a certified spray foam contractor is the best option.

Off-Gassing and Indoor Air Quality

Spray foam releases chemicals during installation (off-gassing), which can linger if not properly cured. Occupants should vacate the premises for at least 24 hours after application. Professional installers use ventilation and ensure safe re-entry times.

Energy Efficiency and Long-Term Value

Utility Bill Savings

Both types of spray foam significantly reduce air leakage, minimizing heating and cooling costs. Closed cell foam’s higher R-value per inch provides the greatest savings in extreme climates, while open cell offers substantial benefits in moderate regions.

Durability and Longevity

Properly installed spray foam insulation can last 50 years or more without sagging or settling, outperforming fiberglass and cellulose. Closed cell foam is especially durable, resisting water and pests.

Resale Value and Appraisals

Homes with advanced insulation and air sealing are increasingly attractive to buyers. Quality spray foam installation can increase appraised value and marketability, especially in energy-conscious markets.

Cost Breakdown: What to Expect

• Open Cell Spray Foam: $0.45–$0.65 per board foot (installed)
• Closed Cell Spray Foam: $1.00–$1.50 per board foot (installed)

Additional costs may include removal of old insulation, installation of thermal barriers, and professional labor. Comparing estimates from multiple installers and verifying credentials is essential for quality results.

Maintenance and Troubleshooting

• Inspect annually for signs of moisture intrusion, cracks, or gaps—especially after severe weather events.
• Avoid drilling or cutting into foam-insulated walls without confirming the location of electrical or plumbing lines.
• If you detect odors, off-gassing, or persistent moisture, contact a professional for assessment.

Conclusion

The debate between open cell and closed cell spray foam insulation is often complicated by persistent myths, confusing marketing claims, and technical complexities. As we’ve explored, the right choice depends on your specific project needs, climate, and budget—not on one-size-fits-all advice. Open cell foam shines in interior applications where soundproofing and breathability matter, while closed cell foam is ideal for moisture-prone or space-limited areas demanding maximum R-value and structural strength.

Don’t let misconceptions steer you toward the wrong solution. Consult with insulation professionals, understand local building code requirements, and weigh the long-term benefits against initial investment. With a fact-based approach and careful planning, spray foam insulation can deliver outstanding comfort, energy savings, and value for your home. By debunking these common myths and understanding the true capabilities of both open and closed cell spray foam, you’ll be empowered to make confident, cost-effective choices for your next renovation.

When in doubt, prioritize quality installation and ongoing maintenance. The right insulation upgrade, installed in the right place, will continue to pay dividends for decades—improving not just your home’s efficiency, but its health, durability, and peace of mind.