Open-cell spray foam insulation has a perm rating between 5 and 16, making it vapor semi-permeable to permeable. This permeable nature allows water vapor to pass through the insulation, enabling walls and structural components to dry out if they become wet. This characteristic is crucial in certain climate zones and building applications where moisture management is essential for preventing mold growth and structural damage.
The permeability of insulation directly impacts a building’s moisture control strategy and long-term durability. Unlike closed-cell foam that acts as a vapor barrier with perm ratings below 1.0, open-cell spray foam insulation installation can be advantageous in hot-humid climates or in retrofit applications where allowing walls to dry in multiple directions is beneficial.
Understanding Permeability in Insulation Materials
Permeability measures how easily water vapor passes through a material. For insulation, this property is quantified as a “perm rating”—the higher the number, the more vapor can pass through.
Permeability Measurements Explained
| Permeability Classification | Perm Rating | Examples |
| Vapor Impermeable | Less than 0.1 | Glass, sheet metal |
| Vapor Semi-Impermeable | 0.1 to 1.0 | Closed-cell spray foam |
| Vapor Semi-Permeable | 1.0 to 10.0 | Most open-cell spray foams |
| Vapor Permeable | Greater than 10.0 | Some open-cell foams, unfaced fiberglass |
Testing for permeability typically follows ASTM E96 standards, which measure the rate of water vapor transmission through materials under controlled conditions.
Open-Cell Foam’s Permeability Characteristics
Open-cell spray foam insulation contains millions of tiny cells that are intentionally left open. These interconnected cells allow air and moisture vapor to pass through while still providing excellent thermal resistance.
Technical Specifications of Open-Cell Spray Foam
| Property | Typical Value | Significance |
| Perm Rating | 5.0 to 16.0 | Allows moisture vapor diffusion |
| Density | 0.4 to 0.8 pounds per cubic foot | Lighter than closed-cell foam |
| R-Value (per inch) | 3.6 to 3.9 | Lower than closed-cell but still effective |
| Cell Structure | Open, interconnected cells | Creates permeable matrix |
| Water Absorption | Can absorb 5-70% by volume | More absorbent than closed-cell |
BONUS TIP: When comparing insulation types, remember that higher permeability isn’t inherently better or worse—it simply suits different applications and climate conditions.
Why Permeability Matters in Building Science
Permeability plays a critical role in building science because it directly affects how moisture moves through building assemblies. Controlling moisture is essential for preventing mold, rot, and structural deterioration.
Climate Considerations for Permeability
In hot-humid climates, permeable insulation like open-cell foam can be advantageous when installed on the interior side of walls. This allows any moisture that enters the wall cavity to dry toward the interior, preventing trapped moisture that could lead to mold growth.
In cold climates, however, high permeability in the wrong location can allow warm, moist indoor air to reach cold surfaces within walls, potentially causing condensation. This is why building codes sometimes require vapor retarders on the warm side of insulation in cold regions.
According to recent market data, approximately 65% of spray foam installations in mixed-humid and hot-humid climates utilize open-cell foam specifically for its permeability advantages.
Moisture Management Strategies
The permeable nature of open-cell foam can be strategically used in moisture management:
- It allows for bi-directional drying, meaning wall assemblies can dry to both interior and exterior.
- It can help reveal water leaks earlier (rather than concealing them), allowing for timely repairs.
- When used with proper exterior water barriers, it creates a balanced system that manages both liquid water and water vapor.
BONUS TIP: For optimal moisture performance, pair permeable open-cell foam with an exterior weather-resistant barrier that’s vapor permeable but water-resistant, creating a “breathable” wall system.
Open-Cell vs. Closed-Cell Permeability Comparison
Understanding the permeability difference between open and closed-cell foams helps in selecting the right insulation for specific applications.
| Factor | Open-Cell Foam | Closed-Cell Foam |
| Perm Rating | 5.0 to 16.0 | 0.8 to 1.0 |
| Moisture Control | Allows vapor diffusion | Acts as vapor barrier |
| Water Resistance | Absorbs water | Resists water penetration |
| Air Barrier | Yes (at proper thickness) | Yes |
| Drying Capability | Allows assemblies to dry | Can trap moisture |
| Suitable Locations | Interior walls, unvented attics in most climates | Basements, exterior applications, flood-prone areas |
Things to Consider Before Making a Decision
The permeability of your insulation should factor into your overall moisture management strategy. Consider these important factors:
Climate Zone Analysis
Your location’s climate zone significantly impacts which permeability level is ideal. In mixed or hot-humid climates, permeable open-cell foam often works well for interior applications. In very cold climates, controlling vapor drive from the interior may require additional vapor control measures when using open-cell foam.
Building Assembly Design
The entire wall or roof assembly matters, not just the insulation. Consider:
- Exterior cladding permeability
- Presence of air gaps or rainscreens
- Type of sheathing used
- Interior finishes and their permeability
Renovation vs. New Construction
In renovation projects, existing conditions may limit options. Open-cell foam’s permeability can be advantageous in retrofits where establishing a perfect vapor barrier may be impossible, allowing for some forgiveness in moisture management.
Building Use and Interior Moisture Loads
Buildings with high interior moisture (indoor pools, spas, restaurants) require careful analysis of vapor drive and may need specialized moisture management strategies beyond just insulation choice.
Common Questions About Open-Cell Foam Permeability
Many builders and homeowners have questions about how permeability affects their insulation decisions. Here are key considerations:
- The permeability of open-cell foam can be modified somewhat with specialized formulations or surface treatments.
- Code requirements for vapor retarders vary by climate zone and jurisdiction.
- In most applications, the permeable nature of open-cell foam will not negatively impact energy efficiency if properly installed.
- The cost difference between open and closed-cell foam should be weighed against the specific performance needs of your project.
FAQ About Open-Cell Foam Permeability
Does open-cell spray foam need an additional vapor barrier?
It depends on your climate zone and wall assembly. In cold climates (Zones 5-8), building codes often require a vapor retarder on the interior side. In warmer climates, additional vapor barriers are typically unnecessary and potentially problematic.
Can open-cell spray foam cause moisture problems?
When properly installed in appropriate applications with correct building assembly design, open-cell foam’s permeability is beneficial. Problems typically arise from poor installation, improper assembly design, or using it in unsuitable applications.
How does open-cell foam’s permeability affect its insulating value?
Permeability itself doesn’t directly affect R-value. Open-cell foam has a lower R-value than closed-cell (about 3.7 per inch versus 6.0-7.0), but this is due to its density and cell structure, not its permeability.
Can I use open-cell spray foam in an exterior below-grade application?
This isn’t recommended. Open-cell foam’s permeability and water absorption properties make it unsuitable for areas with direct ground contact or where liquid water exposure is likely. Closed-cell foam is better suited for basement walls and below-grade applications.
How do I know if my climate zone is appropriate for permeable insulation?
Consult local building codes and energy efficiency standards, which typically specify vapor retarder requirements by climate zone. You can also consult with building science professionals familiar with your region’s specific moisture challenges.
Making an Informed Insulation Decision
The permeability of open-cell spray foam insulation represents both an opportunity and a consideration when designing building assemblies. Its breathable nature allows for drying potential that can be beneficial in many applications, particularly in certain climate zones.
Remember that no insulation type is universally “best”—the right choice depends on your specific building, climate, and performance requirements. By understanding permeability and its implications and with a consultation with a professional spray foam insulation contractor, you can make more informed decisions about moisture management in your building envelope.
Reviewer
Reviewer: Lily Johnson offered her feedback after reviewing this post. With 8 years in the spray foam insulation field, her suggestions centered around improving outreach to homeowners looking for quality insulation solutions.
