Thermal resistance

Thermal resistance is a measure of a material's ability to resist the flow of heat. It quantifies how well a material or a system conducts thermal energy and is often used in the context of thermal insulation and heat transfer.

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Codex  0 Created 2026-10-14 Updated 2026-10-18

Thermal resistance measures the temperature drop required to drive a given rate of heat transfer. For the steady one-dimensional bar model in thermal conduction,

R_{th} = \frac{T _{h} - T _{c}}{Q ˙} = \frac{L}{k A} [K W^{- 1}], r = A R_{th} = \frac{L}{k} [m^{2} K W^{- 1}], q = \frac{T _{h} - T _{c}}{r} .

Here

r

relates the temperature drop to heat flux, while

R_{th}

relates it to the rate of heat transfer. The single bar above has

r = 0.050 m^{2} K W^{- 1}

and

R_{th} = 5 K W^{- 1}

, so its

20 K

drop drives

4 W

To see why series thermal resistances add, apply Fourier's law in each layer to obtain

Δ T_{i} = \dot{Q} R_{i}

. With no accumulation or side losses the rate of heat transfer is the same through both, so adding the drops gives

T_{h} - T_{c} = \dot{Q} (R_{1} + R_{2})

. This lets us size layered walls without solving a new differential equation for every layer. The structure matches Ohm's law: temperature difference plays the role of electric potential difference, the rate of heat transfer plays the role of electric current, and thermal resistance plays the role of electrical resistance. ^[ref]