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Thermal Conductivity

General

Thermal Conductivity (Fourier's Law)

A material property describing how efficiently heat conducts through a substance, measured as the heat flow rate per unit area for a given temperature gradient.

Definition

Thermal conductivity, denoted k, is a material property that describes how efficiently heat conducts through a substance. Materials with high thermal conductivity, like copper and aluminum, transfer heat quickly and are used in applications like cookware and heat sinks; materials with low thermal conductivity, like fiberglass and foam, resist heat transfer and are used for insulation. It is measured in watts per meter-kelvin (W/m·K), representing how much heat flows through a 1-meter thickness of material per square meter of area for each degree of temperature difference across it.

Thermal conductivity underlies nearly every calculation involving building heat loss, insulation performance, and HVAC sizing. The Heat Loss Calculator uses a material's thermal conductivity, together with its thickness and the indoor-outdoor temperature difference, to estimate how much heat escapes through walls, roofs, and windows — directly informing heating system sizing and energy cost estimates. The Thermal Conductivity Converter converts between the various unit systems used internationally, including W/m·K, BTU·in/(hr·ft²·°F), and cal/(s·cm·°C).

Thermal conductivity is one of several transport properties engineers consider alongside viscosity when designing systems that move both heat and fluid, such as radiators, heat exchangers, and cooling loops — a fluid's viscosity affects how it flows and mixes, while its thermal conductivity affects how efficiently it carries heat away.

Formula

Fourier's Law of Heat Conduction:

Q = −k × A × (dT/dx)

Where:

  • Q = rate of heat transfer (watts)
  • k = thermal conductivity of the material (W/m·K)
  • A = cross-sectional area through which heat flows (square meters)
  • dT/dx = temperature gradient across the material's thickness (kelvin per meter)

The negative sign indicates heat flows from higher to lower temperature. For a simple flat wall of thickness L with temperature difference ΔT across it, this simplifies to:

Q = k × A × ΔT / L

Worked Example

A wall section measures 10 m² in area, is made of fiberglass insulation with k = 0.04 W/m·K, has a thickness of 0.15 m (15 cm), and separates an indoor temperature of 21°C from an outdoor temperature of 1°C (ΔT = 20°C).

Q = k × A × ΔT / L Q = 0.04 × 10 × 20 / 0.15 Q = 8 / 0.15 Q ≈ 53.3 watts

This means roughly 53 watts of heat continuously escape through this wall section under these conditions — a relatively small loss thanks to the low conductivity of fiberglass; the same wall built from uninsulated concrete (k ≈ 1.7 W/m·K) would lose over 40 times more heat under identical conditions. Use the Heat Loss Calculator to estimate total building heat loss across all surfaces and the resulting heating energy cost.

Key Things to Know

  • Lower conductivity means better insulation: Materials engineered for insulation, such as fiberglass (≈0.04 W/m·K) or aerogel (≈0.01–0.02 W/m·K), work by trapping still air or gas in small pockets, since air itself has very low thermal conductivity around 0.024 W/m·K.
  • R-value accounts for thickness, conductivity alone does not: Thermal conductivity is an intrinsic property independent of material thickness, while R-value (thickness divided by conductivity) reflects the actual insulating performance of an installed layer — always check which measure a specification uses.
  • Metals conduct heat far faster than most non-metals: Copper (≈400 W/m·K) and aluminum (≈205 W/m·K) sit at the opposite end of the spectrum from insulation materials, which is why they're used for heat sinks and cookware rather than building envelopes.
  • Thermal conductivity and viscosity jointly govern heat exchanger and cooling system design: Engineers must balance a fluid's ability to carry heat (thermal conductivity) against its resistance to flow (viscosity) when sizing pumps, radiators, and coolant loops.
  • Values are typically referenced at a standard temperature: Published thermal conductivity figures are usually given at approximately 20°C (293 K), and while most materials show only minor conductivity changes across normal building temperature ranges, precision engineering applications may require temperature-adjusted values.

Related Calculators

Heat Loss Calculator →Calculate heat loss through a wall or surface in BTU per hour. Enter area, U-value, and indoor/outdoor temperatures to estimate energy loss for insulation planning.

Related Terms

ViscosityDynamic and Kinematic Viscosity

Frequently Asked Questions

Metal has a much higher thermal conductivity than wood — steel conducts heat at roughly 45 to 60 watts per meter-kelvin compared to wood's 0.1 to 0.2 — so metal draws heat away from your hand far faster even though both objects are at the identical room temperature. This rapid heat transfer is perceived by your skin as coldness, even though no actual temperature difference between the materials exists. The Thermal Conductivity Converter lets you compare these values directly across materials and unit systems.
A material's thermal conductivity, combined with its thickness and surface area, determines how much heat escapes through a wall, roof, or window for a given indoor-outdoor temperature difference, following Fourier's law of heat conduction. Lower conductivity materials, like fiberglass insulation at roughly 0.04 W/m·K, dramatically slow heat loss compared to higher conductivity materials like uninsulated concrete at roughly 1.7 W/m·K. The Heat Loss Calculator uses these conductivity values, along with building dimensions and temperature difference, to estimate total heat loss and heating energy requirements.
Thermal conductivity (k) is an intrinsic material property, independent of thickness, measured in watts per meter-kelvin, while R-value measures a specific material's resistance to heat flow for its actual installed thickness, calculated as thickness divided by conductivity. A thicker layer of the same insulation material has a higher R-value even though its thermal conductivity stays the same, which is why insulation is typically rated and compared by R-value rather than raw conductivity. The Thermal Conductivity Converter and Heat Loss Calculator work together to translate between these related but distinct measures.
Aerogel has one of the lowest known thermal conductivities at around 0.01 to 0.02 W/m·K, followed by materials like polyurethane foam and fiberglass batts at roughly 0.02 to 0.04 W/m·K, all of which trap air or gas in tiny pockets that resist heat conduction. Still air itself has a thermal conductivity of about 0.024 W/m·K, which is why most effective insulation materials work by trapping air rather than relying on the solid material itself to block heat. The Heat Loss Calculator can model how swapping insulation materials changes total building heat loss.
Yes, most materials show some variation in thermal conductivity with temperature, though the effect is usually small over typical building temperature ranges. Metals generally show a slight decrease in conductivity as temperature rises, while some insulation materials and gases show a slight increase, and published conductivity values are typically given at a reference temperature such as 20°C or 300 Kelvin. For most everyday engineering calculations, using the standard reference value from the Thermal Conductivity Converter is accurate enough without adjusting for temperature.