Insulation Materials Properties

Insulation materials reduce heat transfer in HVAC systems through low thermal conductivity and resistance to heat flow. Material selection depends on thermal performance requirements, operating temperature range, moisture exposure, mechanical properties, and installation constraints. ASHRAE Fundamentals Chapter 26 provides comprehensive thermal property data for insulation materials.

Thermal Conductivity and R-Value

Thermal conductivity (k) quantifies a material’s ability to conduct heat, expressed in Btu·in/(h·ft²·°F) or W/(m·K). Lower conductivity indicates better insulating performance. Thermal resistance (R-value) is the reciprocal of thermal transmittance:

R = L/k

where L is material thickness in inches. R-value increases linearly with thickness for homogeneous materials. The overall thermal resistance of multi-layer assemblies equals the sum of individual layer resistances plus surface film resistances.

Temperature significantly affects thermal conductivity. Most insulation materials exhibit increasing conductivity at elevated temperatures due to increased molecular activity and radiative heat transfer within the material structure. Mean temperature—the average of hot and cold surface temperatures—determines the appropriate k-value for calculations.

Fibrous Insulation Materials

Fibrous insulation consists of randomly oriented fibers that trap air within the material structure. Air, with k = 0.17 Btu·in/(h·ft²·°F) at 75°F, provides the primary insulating value. Material performance depends on fiber diameter, density, orientation, and binder content.

Fiberglass (Glass Fiber): Manufactured from molten glass spun into fibers with diameters of 3-8 micrometers. Available as blankets, batts, rigid boards, and duct liner. Thermal conductivity ranges from 0.23-0.27 Btu·in/(h·ft²·°F) at 75°F mean temperature depending on density. Density typically 0.6-6.0 lb/ft³ for building insulation, 3-6 lb/ft³ for duct liner. Non-combustible, chemically inert, dimensionally stable. Maximum service temperature 450-850°F depending on binder type. Moisture absorption minimal but wet material loses R-value until dried.

Mineral Wool (Rock Wool, Slag Wool): Produced from molten rock or blast furnace slag. Higher density than fiberglass, typically 4-8 lb/ft³ for board stock. Thermal conductivity 0.24-0.29 Btu·in/(h·ft²·°F) at 75°F. Excellent fire resistance with melting point above 2000°F. Superior sound absorption compared to fiberglass. Hydrophobic treatment available for moisture resistance. Used extensively in pipe insulation and fire-rated assemblies.

Cellulose: Recycled paper treated with fire retardants (boric acid, ammonium sulfate). Density 1.5-3.0 lb/ft³ when blown, 2.5-4.0 lb/ft³ when dense-packed. Thermal conductivity approximately 0.27-0.32 Btu·in/(h·ft²·°F). Settles 10-20% over time reducing effective R-value. Moisture absorption can promote fungal growth and reduce fire retardant effectiveness. Limited to building cavity applications.

Foam Board Insulation

Rigid foam boards provide higher R-value per inch than fibrous materials due to closed-cell structure and low-conductivity gas within cells. Dimensional stability and compressive strength enable structural applications.

Expanded Polystyrene (EPS): Manufactured from polystyrene beads expanded with steam. Open-cell structure containing air. Density 0.7-2.0 lb/ft³. Thermal conductivity 0.26-0.29 Btu·in/(h·ft²·°F) at 75°F, stable over time. R-value approximately 3.8-4.2 per inch. Moisture permeable (2-5 perms for 1-inch thickness), requiring vapor retarders in moisture-prone applications. Compressive strength 10-60 psi. Maximum service temperature 165°F. Degrades under UV exposure.

Extruded Polystyrene (XPS): Manufactured through extrusion process creating fine closed-cell structure. Density 1.3-2.2 lb/ft³. Initial thermal conductivity 0.20-0.22 Btu·in/(h·ft²·°F) with blowing agents, aging to 0.25-0.26 as gases diffuse and air infiltrates cells over 5-10 years. R-value 4.5-5.0 per inch initially, decreasing to approximately 4.2 long-term. Moisture permeability 0.8-1.2 perms per inch. Higher compressive strength than EPS (15-100 psi) enables below-grade applications.

Polyisocyanurate (Polyiso): Closed-cell thermoset polymer with lowest thermal conductivity among common foam boards. Density 2.0-3.0 lb/ft³. Thermal conductivity 0.16-0.19 Btu·in/(h·ft²·°F) at 75°F with hydrochlorofluorocarbon or hydrocarbon blowing agents. R-value approximately 6.0-6.5 per inch initially. Conductivity increases significantly at low temperatures—approximately 0.30 at 0°F mean temperature—reducing effective R-value in cold climates. Foil facings provide vapor barrier (0.02 perms) and enhance fire resistance. Maximum service temperature 250°F. Compressive strength 16-25 psi typical.

Spray Foam Insulation

Spray polyurethane foam (SPF) applied as liquid expands in-place to fill cavities and create air barriers. Two-component systems mix isocyanate and polyol resin at application.

Open-Cell SPF: Low-density foam (0.4-0.6 lb/ft³) with open-cell structure containing air. Thermal conductivity approximately 0.26 Btu·in/(h·ft²·°F), R-value 3.6-4.0 per inch. Moisture permeable (15-20 perms per inch). Expansion ratio 100:1-150:1. Sound absorption coefficient 0.70-0.90 at mid-frequencies. Compressive strength 3-5 psi. Lower material cost than closed-cell but requires greater thickness for equivalent thermal resistance.

Closed-Cell SPF: Medium-density foam (1.8-2.2 lb/ft³) with closed cells containing low-conductivity blowing agents. Thermal conductivity 0.16-0.20 Btu·in/(h·ft²·°F), R-value 6.0-6.8 per inch initially, aging to approximately 5.8 as gases diffuse. Vapor impermeable at thicknesses above 2 inches (0.8 perms per inch). Expansion ratio 25:1-35:1. Compressive strength 20-35 psi. Structural racking strength enhances building performance. Higher cost justified in space-constrained applications.

Both SPF types provide air sealing eliminating infiltration losses. Installation requires proper temperature (60-100°F ambient, 70-90°F substrate), humidity control, and certified applicators. Off-gassing requires ventilation before occupancy.

Reflective Insulation and Radiant Barriers

Reflective insulation reduces radiative heat transfer through low-emittance surfaces. Effectiveness requires facing air space—minimum 0.75 inches—as conductive/convective resistance of material itself is negligible.

Aluminum foil with emittance 0.03-0.05 reflects 95-97% of incident radiation. Effective R-value depends on air space geometry, heat flow direction, and surface orientation. Horizontal air space with heat flow up provides R-2.9 for 0.75-inch space, R-3.5 for 3.5-inch space (reflective surface on bottom). Heat flow down yields higher values: R-4.6 for 0.75-inch space due to reduced convection.

Dust accumulation increases emittance reducing performance. Surface emittance of 0.05 increases to 0.20-0.30 after dust deposition in typical attic environments, reducing reflective effectiveness 40-60%. Radiant barriers in attics reduce ceiling heat gain 25-45% in cooling-dominated climates but provide minimal benefit in heating climates.

Multi-layer reflective insulation systems stack reflective surfaces separated by air spaces. Effective R-values reach 10-15 for assemblies 2-3 inches thick. Applications include metal building insulation and ductwork wraps where space constraints limit fibrous or foam insulation thickness.

Moisture Resistance and Vapor Permeability

Moisture absorption degrades thermal performance and promotes material degradation. Vapor permeance quantifies moisture transmission rate, measured in perms (grains/h·ft²·in.Hg). Materials classified as vapor barriers (<0.1 perm), vapor retarders (0.1-1.0 perm), or vapor permeable (>1.0 perm).

Water absorption increases thermal conductivity—wet fiberglass exhibits k-values 2-3 times higher than dry material. Closed-cell foams resist moisture absorption better than open-cell materials. Facings (kraft paper, foil, polymer films) control vapor transmission. Proper vapor retarder placement prevents condensation within insulation assemblies based on climate zone and construction type.

Material Selection Criteria

Insulation selection involves multiple factors beyond thermal resistance:

• Temperature limits: Service temperature range must accommodate system operating conditions including transients
• Fire performance: Flame spread and smoke development ratings per ASTM E84; non-combustible requirements per building codes
• Dimensional stability: Thermal expansion, creep, and aging characteristics affect long-term performance
• Chemical compatibility: Resistance to solvents, oils, and process fluids in industrial applications
• Mechanical properties: Compressive strength, tensile strength, and flexibility for structural or vibration-prone locations
• Installation requirements: Labor cost, special equipment, weather restrictions, and cure time
• Environmental impact: Embodied energy, recycled content, off-gassing, and end-of-life disposal

Life-cycle cost analysis comparing initial material and installation costs against long-term energy savings determines economic insulation thickness. Incremental cost per added R-value increases with thickness while energy savings provide diminishing returns, defining an economic optimum.

Thermal performance data from manufacturers should reference independent testing per ASTM C177 (guarded hot plate), ASTM C518 (heat flow meter), or ASTM C1114 (pipe insulation) at specified mean temperatures and ages. Published R-values based on time-zero testing overstate long-term performance for gas-filled closed-cell foams.

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