Wood Materials

Wood materials constitute a significant component of building envelopes and structural systems. Their thermal properties vary considerably based on species, grain orientation, density, moisture content, and temperature. Accurate characterization of wood thermal behavior is essential for precise heat transfer calculations in HVAC design.

Fundamental Thermal Properties

Thermal Conductivity Range

Wood thermal conductivity varies with multiple factors:

Density Effects:

• Low density woods (< 400 kg/m³): k = 0.10-0.13 W/(m·K)
• Medium density woods (400-600 kg/m³): k = 0.13-0.17 W/(m·K)
• High density woods (> 600 kg/m³): k = 0.17-0.23 W/(m·K)

Grain Orientation:

• Parallel to grain: k‖ = 2.0 to 2.8 times perpendicular conductivity
• Perpendicular to grain: k⊥ (baseline reference)
• Radial direction: typically 5-10% higher than tangential
• Most heat transfer calculations use perpendicular values (conservative)

Specific Heat Capacity

Wood specific heat varies with moisture content and temperature:

Dry Wood (0% moisture content):

• cp,dry = 1200-1400 J/(kg·K) at 20°C
• Temperature dependence: cp = 1030 + 3.9T (T in °C)

Moist Wood:

• cp,moist = cp,dry + mc × cp,water
• Where mc = moisture content (decimal)
• cp,water = 4186 J/(kg·K)

Practical Values:

• Oven-dry wood: 1300 J/(kg·K)
• 6% MC (typical interior): 1380 J/(kg·K)
• 12% MC (typical exterior): 1500 J/(kg·K)
• 20% MC (high humidity): 1750 J/(kg·K)

Density Classifications

Oven-Dry Density (ρ₀):

• Mass of wood substance / volume at given moisture content
• Fundamental property for thermal calculations

Moisture Content Effects:

• ρ = ρ₀(1 + mc)
• Below fiber saturation point (FSP ≈ 28-30%)
• Dimensional changes accompany moisture changes

Softwood Species

Softwoods (coniferous species) generally exhibit lower density and thermal conductivity compared to hardwoods.

Common Softwood Properties

Douglas Fir (Pseudotsuga menziesii):

• Density: 480-530 kg/m³ (oven-dry)
• k⊥: 0.12-0.14 W/(m·K) at 12% MC
• k‖: 0.27-0.34 W/(m·K)
• Specific heat: 1380 J/(kg·K)
• Widely used in structural framing

Southern Yellow Pine (Pinus spp.):

• Density: 510-590 kg/m³
• k⊥: 0.14-0.15 W/(m·K) at 12% MC
• k‖: 0.32-0.38 W/(m·K)
• High resin content affects moisture behavior
• Common in dimensional lumber

Spruce-Pine-Fir (SPF) Group:

• Density: 360-450 kg/m³
• k⊥: 0.10-0.12 W/(m·K) at 12% MC
• k‖: 0.22-0.28 W/(m·K)
• Lower thermal conductivity due to lower density
• Standard framing lumber in cold climates

Western Red Cedar (Thuja plicata):

• Density: 310-370 kg/m³
• k⊥: 0.09-0.11 W/(m·K)
• Excellent insulating properties
• Naturally durable, used in exterior applications

Hemlock (Tsuga spp.):

• Density: 400-480 kg/m³
• k⊥: 0.11-0.13 W/(m·K)
• Moderate thermal resistance
• Used in framing and sheathing

Softwood Design Values

For HVAC calculations, use these conservative values:

Hardwood Species

Hardwoods (deciduous species) generally have higher density and thermal conductivity, affecting thermal mass and heat transfer rates.

Common Hardwood Properties

Oak (Quercus spp.):

• Red Oak density: 630-720 kg/m³
• White Oak density: 690-770 kg/m³
• k⊥: 0.16-0.18 W/(m·K)
• k‖: 0.35-0.42 W/(m·K)
• High thermal mass, used in flooring

Maple (Acer spp.):

• Hard Maple density: 630-710 kg/m³
• k⊥: 0.16-0.17 W/(m·K)
• Common in flooring and interior finish

Ash (Fraxinus spp.):

• Density: 600-680 kg/m³
• k⊥: 0.15-0.17 W/(m·K)
• Similar properties to oak

Birch (Betula spp.):

• Yellow Birch density: 620-680 kg/m³
• k⊥: 0.15-0.16 W/(m·K)
• Used in cabinets and millwork

Walnut (Juglans nigra):

• Density: 550-660 kg/m³
• k⊥: 0.14-0.16 W/(m·K)
• Premium interior applications

Hardwood Design Values

Engineered Wood Products

Engineered wood products consist of wood elements bonded with adhesives. Thermal properties depend on composition, density, and manufacturing process.

Plywood

Construction:

• Cross-laminated veneer layers
• Alternating grain orientation
• Adhesive bonds between layers

Thermal Properties:

• Density: 450-650 kg/m³ (varies by species and voids)
• k: 0.12-0.14 W/(m·K) perpendicular to face
• Effective isotropic behavior due to cross-lamination
• Specific heat: 1380 J/(kg·K)

HVAC Applications:

• Sheathing: use k = 0.12 W/(m·K)
• Subfloors: use k = 0.13 W/(m·K)
• Interior paneling: use k = 0.11 W/(m·K)

Common Types:

• CDX plywood (sheathing): ρ = 500 kg/m³, k = 0.12 W/(m·K)
• Marine plywood: ρ = 600 kg/m³, k = 0.13 W/(m·K)
• Hardwood plywood: ρ = 550 kg/m³, k = 0.13 W/(m·K)

Oriented Strand Board (OSB)

Construction:

• Wood strands oriented in layers
• Phenolic resin binder
• Compressed under heat and pressure

Thermal Properties:

• Density: 600-680 kg/m³
• k: 0.13-0.14 W/(m·K)
• Higher density than plywood of equal thickness
• Specific heat: 1400 J/(kg·K)

Design Values:

• Wall sheathing: k = 0.13 W/(m·K), ρ = 640 kg/m³
• Roof sheathing: k = 0.13 W/(m·K), ρ = 650 kg/m³
• Subfloor: k = 0.14 W/(m·K), ρ = 660 kg/m³

Moisture Sensitivity:

• More susceptible to edge swelling than plywood
• Affects dimensional stability and thermal bridging
• Critical in moisture-control designs

Particleboard

Construction:

• Wood particles bonded with resin
• Uniform density distribution
• Lower strength than OSB or plywood

Thermal Properties:

• Density: 600-750 kg/m³
• k: 0.17-0.18 W/(m·K)
• Higher conductivity due to compression
• Specific heat: 1420 J/(kg·K)

Applications:

• Underlayment: k = 0.17 W/(m·K)
• Core stock: k = 0.18 W/(m·K)
• Shelving: k = 0.17 W/(m·K)

Medium Density Fiberboard (MDF)

Construction:

• Fine wood fibers bonded with resin
• Uniform, dense structure
• Smooth surface finish

Thermal Properties:

• Density: 700-850 kg/m³
• k: 0.18-0.20 W/(m·K)
• Highest conductivity of common engineered woods
• Specific heat: 1450 J/(kg·K)

Design Considerations:

• Thermal mass higher than solid wood of equal volume
• Used in interior applications (trim, cabinetry)
• Not suitable for direct exterior exposure
• Standard value: k = 0.19 W/(m·K), ρ = 750 kg/m³

Laminated Veneer Lumber (LVL)

Construction:

• Thin wood veneers laminated with grain parallel
• Used in structural applications (beams, headers)

Thermal Properties:

• Density: 550-650 kg/m³
• k‖: 0.28-0.35 W/(m·K) (along length)
• k⊥: 0.13-0.15 W/(m·K) (through thickness)
• Acts as thermal bridge in wall assemblies

HVAC Impact:

• Headers over openings create thermal bridges
• Parallel grain orientation increases heat flow along length
• Consider thermal break strategies in high-performance assemblies

Glulam Beams

Construction:

• Laminated dimensional lumber
• Grain parallel in all layers

Thermal Properties:

• Density: 500-550 kg/m³
• k‖: 0.30-0.38 W/(m·K)
• k⊥: 0.12-0.14 W/(m·K)
• Similar to solid wood of same species

Moisture Content Effects

Moisture profoundly affects wood thermal properties through multiple mechanisms.

Thermal Conductivity and Moisture

Relationship:

• k = k₀[1 + (mc/100)(α)]
• Where α = 2.5 to 3.5 for most species
• k₀ = conductivity at 0% MC

Physical Mechanism:

• Water conductivity: kwater = 0.60 W/(m·K) at 20°C
• Water replaces air in cell structure (kair = 0.026 W/(m·K))
• Net increase in effective conductivity

Practical Impact:

• 12% MC increases k by 25-35% vs oven-dry
• 20% MC increases k by 50-70% vs oven-dry
• Critical for exterior applications and high-humidity spaces

Moisture Content Guidelines

Equilibrium Moisture Content (EMC):

• Interior conditioned space: 6-9% MC
• Exterior in temperate climate: 12-15% MC
• High humidity environments: 16-20% MC

Design Values by Application:

Fiber Saturation Point

Definition:

• Moisture content where cell walls are saturated but no free water exists
• Typically 28-30% MC for most species
• Above FSP, dimensional changes cease

Thermal Implications:

• Conductivity increases continue above FSP
• Latent heat effects become significant
• Condensation risk in assemblies

Temperature Effects

Wood thermal properties vary with temperature, though effects are less pronounced than moisture.

Conductivity Temperature Dependence

Relationship:

• k(T) = k₂₀[1 + β(T - 20)]
• Where β = 0.002 to 0.003 K⁻¹
• T = temperature (°C)

Practical Range:

• -20°C to +50°C: conductivity increases 15-20%
• Effect is secondary to moisture content changes
• Most design standards use 20°C reference

Specific Heat Temperature Dependence

Dry Wood:

• cp(T) = 1030 + 3.9T [J/(kg·K)]
• Linear relationship over HVAC temperature range
• At -20°C: cp ≈ 952 J/(kg·K)
• At +50°C: cp ≈ 1225 J/(kg·K)

Combined Temperature-Moisture Effects

Interactive Effects:

• Higher temperatures increase EMC at given RH
• Thermal conductivity increases with both T and MC
• Design calculations should account for seasonal extremes

Example Calculation:

• SPF framing, interior winter: T = 20°C, MC = 7%
  • k = 0.12 × 1.18 = 0.14 W/(m·K)
• Same framing, summer exterior face: T = 50°C, MC = 14%
  • k = 0.12 × 1.35 × 1.05 = 0.17 W/(m·K)

Density-Property Relationships

Density serves as the primary predictor of wood thermal properties.

Empirical Correlations

Thermal Conductivity:

• k = 0.04 + 0.00026ρ [W/(m·K)]
• Valid for ρ in kg/m³, MC = 12%
• Perpendicular to grain orientation

Alternative Form:

• k = 0.01w + 0.1 [W/(m·K)]
• Where w = specific gravity (relative to water)

Thermal Diffusivity:

• α = k/(ρcp)
• Decreases with increasing density
• Typical range: (0.8-1.5) × 10⁻⁷ m²/s

Species Comparison

ASHRAE References

ASHRAE Handbook - Fundamentals

Chapter 26: Heat, Air, and Moisture Control in Building Assemblies:

• Table 4: Thermal properties of building materials
• Wood species thermal conductivity data
• Moisture effects on conductivity

Chapter 33: Energy Resources:

• Wood as fuel source properties
• Heating values and combustion characteristics

Standard Values (ASHRAE Table 26.4):

• Softwood lumber (80 lb/ft³): k = 0.12 W/(m·K), R = 1.25 per inch
• Hardwood (45 lb/ft³): k = 0.16 W/(m·K), R = 0.91 per inch
• Plywood (34 lb/ft³): k = 0.12 W/(m·K), R = 1.25 per inch
• Particleboard (50 lb/ft³): k = 0.17 W/(m·K), R = 0.85 per inch

ASHRAE Standard 90.1

Envelope Requirements:

• Wood framing thermal bridging calculations
• Parallel path method for framed assemblies
• Clear wall vs whole wall R-values

Framing Factors:

• 2×4 @ 16" O.C.: 15-20% framing
• 2×6 @ 16" O.C.: 15-20% framing
• 2×4 @ 24" O.C.: 10-15% framing

Code References

International Energy Conservation Code (IECC)

Thermal Performance:

• Wood framing assumed at R-1.25 per inch (softwood)
• Advanced framing techniques reduce thermal bridging
• Continuous insulation strategies

International Residential Code (IRC)

Prescriptive Requirements:

• Framing member thermal properties
• Wall assembly calculations
• Foundation wood sill plates

Building Codes

Fire Resistance:

• Wood charring rates affect thermal barrier
• Typical char rate: 0.6-0.8 mm/min
• Char layer provides insulation

HVAC Design Considerations

Heat Transfer Calculations

Framed Wall Assemblies:

• Parallel path method accounts for wood framing
• Effective R-value lower than cavity insulation alone
• Framing fraction typically 15-25%

Calculation Method:

1. Identify framing fraction (Ff) and cavity fraction (Fc)
2. Calculate U-value through framing path: Uframe = Σ(1/R)
3. Calculate U-value through cavity path: Ucavity = Σ(1/R)
4. Effective U = FfUframe + FcUcavity
5. Effective R = 1/U

Example:

• 2×6 wall, R-21 cavity insulation, 20% framing
• Framing path: R = 1.0 (exterior) + 6.9 (stud) + 0.68 (interior) = 8.58
  • Uframe = 0.117 W/(m²·K)
• Cavity path: R = 1.0 + 21 + 0.68 = 22.68
  • Ucavity = 0.044 W/(m²·K)
• Ueff = 0.20(0.117) + 0.80(0.044) = 0.0584 W/(m²·K)
• Reff = 17.1 (compared to R-22.68 nominal)

Thermal Bridging

Common Bridge Locations:

• Studs and joists penetrating insulation
• Headers over windows and doors
• Rim joists and band boards
• Sill plates and bottom plates

Mitigation Strategies:

• Continuous exterior insulation
• Advanced framing (24" O.C., single top plate)
• Insulated headers
• Thermal breaks at critical junctions

Moisture Management

Condensation Risk:

• Wood framing members as condensing surfaces
• Dew point analysis required in cold climates
• Vapor retarder placement critical

Design Approach:

1. Calculate temperature profile through assembly
2. Determine dew point temperature at each interface
3. Compare actual temperature to dew point
4. If Tactual < Tdew, condensation occurs

Hygrothermal Modeling:

• WUFI or similar software for dynamic analysis
• Accounts for moisture storage in wood
• Validates assembly performance over annual cycle

Thermal Mass Effects

Lightweight Wood Frame:

• Low thermal mass compared to masonry or concrete
• Minimal heat storage capacity
• Faster temperature response

Quantification:

• Thermal mass per unit area: M = ρcpL
• 2×6 stud wall: M ≈ 3.5 kJ/(m²·K) of framing area
• Negligible compared to conditioned space loads

Design Implications:

• Less effective for passive solar thermal storage
• Reduced peak load shifting capability
• Lower temperature swing damping

Load Calculations

Envelope Transmission:

• Use effective U-value accounting for framing
• Apply area-weighted average for assemblies
• Consider orientation-specific solar gains

Thermal Bridging Impact:

• Increases heating loads 10-30% vs clear wall
• Greater impact in high-performance envelopes
• Critical for net-zero and passive house designs

Dynamic Effects:

• Wood structures respond quickly to setback/setup
• Lower recovery loads compared to mass construction
• Shorter time constants for transient analysis

Flooring Systems

Wood Floor Over Conditioned Space:

• Minimal heat transfer impact
• Thermal comfort driven by surface temperature
• Radiant floor heating compatibility

Wood Floor Over Unconditioned Space:

• Significant heat loss pathway
• Requires insulation below
• Cantilever conditions create thermal bridges

Design Values:

• 3/4" hardwood flooring: R = 0.67
• 3/4" softwood flooring: R = 0.90
• Subfloor typically adds R = 1.0 to 1.3

Ceiling and Roof Assemblies

Cathedral Ceilings:

• Wood rafter thermal bridging
• Ventilation channel requirements
• Insulation depth limitations

Attic Floor:

• Wood joists bridge insulation
• Lower impact due to full cavity insulation
• Effective R-value reduction typically 2-5%

Advanced Considerations

Anisotropic Heat Flow

Grain Orientation Effects:

• Parallel grain conductivity 2-3× perpendicular
• Important in heavy timber construction
• Glulam beams act as thermal highways

Finite Element Analysis:

• Required for complex geometries
• Orthotropic material properties
• Transient heat transfer in mass timber

Mass Timber Systems

Cross-Laminated Timber (CLT):

• Alternating layer orientation
• Effective isotropic behavior in plane
• Through-thickness: k ≈ 0.13 W/(m·K)

Nail-Laminated Timber (NLT):

• All layers same orientation
• Highly anisotropic thermal properties
• Requires directional analysis

Thermal Mass:

• Significant compared to light frame
• CLT panel: M = 40-80 kJ/(m²·K) for 175mm thickness
• Affects peak load timing and magnitude

Fire Effects

Charring:

• Surface char forms insulating layer
• Reduces heat penetration to interior wood
• Char conductivity: k ≈ 0.05 W/(m·K)

Temperature-Dependent Properties:

• Wood decomposes above 200-300°C
• Thermal properties change significantly
• Specialized analysis required for fire scenarios

Hygrothermal Coupling

Moisture Transport:

• Vapor diffusion through wood
• Capillary action in wood structure
• Liquid water movement

Latent Heat Effects:

• Evaporation/condensation within assembly
• Affects effective thermal properties
• Dynamic heat and moisture transfer analysis

Summary Design Values

Quick Reference Table

Standard Conditions

All values at:

• Temperature: 20°C (68°F)
• Moisture content: 12% (typical building interior)
• Perpendicular to grain (except where noted)

Adjust for actual site conditions using correction factors provided in moisture and temperature sections.