Thermal Conductivity Converter

To convert thermal conductivity from Watts per meter-Kelvin (W/(m·K)) to imperial British Thermal Units per hour-foot-Fahrenheit (BTU/(h·ft·°F)), multiply by 0.577789 (or divide by 1.730735): BTU/(h·ft·°F) = W/(m·K) × 0.577789.

HVAC Insulation Calculation Example:

• Fiberglass building insulation has a k-value of 0.040 W/(m·K). In imperial units: 0.040 × 0.5778 = 0.0231 BTU/(h·ft·°F).
• Reverse Conversion: To convert BTU/(h·ft·°F) back to W/(m·K), multiply by 1.730735 (e.g., 10 BTU/(h·ft·°F) × 1.7307 = 17.307 W/(m·K)).

Thermal conductivity (denoted by the symbol k, λ, or κ) is an intrinsic property of a material measuring its ability to conduct thermal heat energy through solid matter without macroscopic mass transport.

• Fourier's Law of Conduction: q = -k × (dT / dx), where q is the heat flux density (W/m²) and dT/dx is the temperature gradient across the thickness of the material.
• SI Unit: Watts per meter-Kelvin (W/(m·K)) or W/(m·°C). Because temperature difference Δ1 K is exactly identical to Δ1 °C, W/(m·K) and W/(m·°C) are numerically identical.
• Low vs High Conductors: Materials with low thermal conductivities (k < 0.1 W/(m·K)) are effective thermal insulators (polyurethane, aerogel, fiberglass), while materials with high k (copper > 390 W/(m·K)) are thermal heat sinks.

Thermal conductivity (k) is an intrinsic material property independent of thickness or size, whereas thermal resistance (R-value) measures the total insulating effectiveness of a specific material thickness:

• Mathematical Relationship: R = d ÷ k, where d is thickness in meters and k is thermal conductivity in W/(m·K).
• Thermal Transmittance (U-factor): U = 1 ÷ R = k ÷ d.
• Lower k means Higher R: For a given wall thickness, a lower thermal conductivity material yields a significantly higher R-value insulation performance.

In solid materials, heat conducts via two mechanisms: lattice acoustic vibrations (phonons) and free electron motion.

Metals possess a high density of delocalized valence electrons (the electron sea). When heated, these mobile electrons transport kinetic energy through the metal crystal lattice at high velocities, yielding conductivities of 50 to 400 W/(m·K). In non-metals and insulators, free electrons are absent, and heat relies solely on slower phonon lattice dispersion.

At room temperature (25°C), synthetic Diamond (CVD) and natural diamond have the highest thermal conductivity of any known bulk solid material, reaching 2,000 to 2,200 W/(m·K) (over 5 times higher than pure copper).

Diamond's exceptional conductivity stems from its rigid, ultra-strong tetrahedral covalent carbon lattice structure and low atomic mass, allowing phonons to propagate with minimal lattice scattering.

The thermal conductivity of substances varies significantly with operating temperature:

• Gases: Thermal conductivity increases with temperature (k ∝ √T) because increased kinetic temperature accelerates molecular speeds and collision frequencies.
• Pure Metals: In most pure metals (such as copper and aluminum), thermal conductivity decreases as temperature rises because thermal lattice vibrations scatter the conduction electrons.
• Insulators & Dielectrics: Varies depending on molecular structure; porous insulators often exhibit increased conductivity at elevated temperatures due to internal radiative heat transfer across pore voids.