Spray Foam Insulation

Spray polyurethane foam (SPF) represents a unique category of insulation materials that provide simultaneous thermal resistance, air sealing, and moisture control through in-situ chemical expansion. SPF systems consist of two-component reactive polymers that expand 30-100 times their liquid volume upon application, creating continuous insulation and air barrier assemblies.

Spray Foam Chemistry

SPF results from the exothermic reaction between polymeric methylene diphenyl diisocyanate (pMDI) and a polyol resin blend containing catalysts, surfactants, and blowing agents.

Chemical Reaction: The fundamental polyurethane formation reaction:

R-N=C=O + R’-OH → R-NH-CO-O-R'

where the isocyanate group reacts with hydroxyl groups to form urethane linkages. Excess isocyanate reacts with water to produce CO₂ gas, contributing to foam expansion and cell formation.

Blowing Agents:

• Open-cell foam: Water as primary blowing agent (produces CO₂)
• Closed-cell foam: Hydrofluorocarbon (HFC) blowing agents (HFC-245fa, HFC-134a) or hydrofluoroolefin (HFO) alternatives (HFO-1233zd)
• Low-GWP formulations: HFO blowing agents with global warming potential <1

Open-Cell vs Closed-Cell Foam Properties

The fundamental difference between open-cell and closed-cell SPF lies in cell structure density and gas retention.

Thermal Performance Characteristics

Open-Cell Thermal Resistance: R-value remains stable over time due to air-filled cell structure. Thermal resistance of R-3.6 to R-3.8 per inch results from conduction through polymer matrix and convection/radiation across cell walls. Performance independent of blowing agent thermal aging.

Closed-Cell Thermal Resistance: Initial R-values of R-6.5 to R-7.0 per inch decrease over time as atmospheric gases diffuse into cells and replace low-conductivity blowing agents.

Aging Effects: Long-term thermal resistance (LTTR) accounts for blowing agent diffusion over 15-year service life:

R_aged = R_initial × (1 - k × √t)

where k represents the diffusion coefficient (0.02-0.04 for HFC foams, 0.01-0.02 for HFO foams) and t is time in years. Typical aged R-values: 5.8-6.2 per inch for HFC formulations, 6.0-6.5 per inch for HFO formulations.

Air Sealing Performance

SPF eliminates air leakage through complete envelope sealing, addressing the primary heat transfer mechanism in building assemblies.

Air Barrier Effectiveness:

• Open-cell SPF: Meets air barrier criteria at 3.5" minimum thickness (air permeance <0.02 L/(s·m²) at 75 Pa per ASTM E2178)
• Closed-cell SPF: Qualifies as air barrier at 1.5" minimum thickness
• Leakage reduction: 25-40% reduction in whole-building air changes per hour compared to fibrous insulation

Pressure Differential Performance: Closed-cell foam withstands pressure differentials exceeding 10 psf without air leakage, suitable for high-wind zones and stack-effect mitigation in tall buildings.

Moisture Vapor Transmission

Moisture permeance varies significantly between foam types, affecting condensation control strategy.

Open-Cell Behavior: Class III vapor retarder (>10 perms at 1" thickness) permits bi-directional vapor drying. Suitable for mixed-humid climates where seasonal vapor drive reverses direction. Requires exterior vapor control in heating-dominated climates when applied to cathedral ceilings or walls without ventilation.

Closed-Cell Behavior: Class II vapor retarder (0.8-1.2 perms at 1") transitions to Class I (<0.1 perms) at thicknesses exceeding 2.5". Creates impermeable vapor barrier preventing inward or outward moisture migration. Application thickness determines hygrothermal assembly behavior:

• 1-2": Permits some drying potential
• 2-3": Borderline impermeable

• 3": Effectively impermeable vapor barrier

Hygrothermal Considerations: Impermeable closed-cell foam on interior or exterior surfaces prevents condensing plane drying. Requires hygrothermal analysis (WUFI, THERM) to verify moisture accumulation remains below critical thresholds in wall assemblies.

Application Requirements

Surface Preparation: Substrate temperature: 40-120°F Surface moisture content: <19% for wood, <5% for concrete Contamination: Remove dust, oils, release agents, and loose particles

Environmental Conditions: Optimal application temperature: 60-90°F Maximum wind speed: 10 mph (open-cell), 15 mph (closed-cell) Relative humidity: 30-80% Substrate temperature differential: Within 10°F of air temperature

Installation Parameters: Open-cell application: 2-3" passes, 10-second intervals between passes Closed-cell application: 1" passes, 5-15 second intervals (formulation-dependent) Maximum single-pass thickness: 2" (open-cell), 1" (closed-cell) Cure time: 30-60 seconds tack-free, 24 hours full cure

Equipment Specifications: Proportioner pressure: 800-1500 psi Temperature set points: 110-140°F (material-dependent) Hose heated length: Required for runs exceeding 100 feet Mix ratio tolerance: ±2% of stoichiometric ratio

Structural Considerations

Load Distribution: Closed-cell foam provides racking resistance in wall assemblies, contributing 10-30% shear strength increase when applied to wood-frame construction. Not considered structural element in design calculations per IBC provisions.

Expansion Forces: Closed-cell foam exerts outward pressure during cure: 5-10 psf on restrained cavities. Requires temporary bracing on forms and potential deflection analysis on thin metal substrates.

Adhesion Strength: Typical bond strength to common substrates:

• Wood: 40-80 psi
• Concrete: 60-100 psi
• Steel: 30-60 psi
• Gypsum board: Limited by board strength (15-25 psi)

Fire Performance

Flame Spread and Smoke Development: SPF requires 15-minute thermal barrier (typically ½" gypsum board) per IRC Section R316.4 and IBC Section 2603.4. Some formulations achieve flame spread index <25, smoke development <450 per ASTM E84.

Ignition Barrier: Closed-cell formulations meeting ignition barrier criteria (NFPA 286, AC377) permit exposure in attics and crawlspaces without thermal barrier when separated from ignition sources.

Application Limitations

Prohibited Applications:

• Direct contact with recessed lighting unless IC-rated
• Concealment of knob-and-tube wiring
• Continuous immersion or hydrostatic pressure exposure
• Substrates exceeding 180°F service temperature
• Direct UV exposure without protective coating

Thickness Restrictions: Maximum single-location thickness without heat management: 4" (open-cell), 3" (closed-cell) Exothermic reaction generates 200-350°F peak temperatures requiring thermal dissipation between passes.

Special Conditions: Electrical wiring embedded in foam experiences ampacity reduction: 10-15% for open-cell, 20-30% for closed-cell per NEC 310.15(A)(3). Requires wire size increase or thermal calculation.

Quality Assurance

Density Verification: Core samples per ASTM D1622 at frequency of 1 per 1,000 ft² application area. Open-cell acceptable range: 0.4-0.7 lb/ft³, closed-cell: 1.7-2.3 lb/ft³.

Thickness Verification: Minimum installed thickness verification at 100 ft² intervals using depth gauge measurements.

Coverage Verification: Infrared thermography 24-48 hours post-application identifies thermal bridging, voids, and insufficient thickness areas.

Economic Considerations

Installed cost comparison (per board foot):

• Open-cell: $0.45-0.65
• Closed-cell: $0.95-1.35

Life-cycle analysis favors closed-cell in applications requiring high R-value per inch due to reduced wall thickness requirements and structural benefits offsetting higher initial cost.

Sections