Thermal Resistance Calculator

Calculate thermal resistance, heat flow, and temperature profiles through walls and composite structures.

Insulation≈0.04, Brick≈0.7, Steel≈50
Indoor≈10, Wind≈25
Please check your inputs and try again.

Thermal Resistance Turns Heat Flow into a Circuit-Like Balance

Thermal resistance expresses how strongly a layer or interface opposes steady heat transfer, allowing many conduction and convection problems to be combined like resistors. For one-dimensional conduction through a plane layer, Rcond=L/(kA). For convection at a surface, Rconv=1/(hA). The steady heat-transfer rate is Q̇=ΔT/Rtotal when resistances are arranged in series.

Parallel heat-flow paths combine using reciprocal resistance, analogous to electrical circuits. Contact resistance, radiation, cylindrical geometry, and changing area can require additional or different resistance expressions.

Rcond=L/(kA),   Rconv=1/(hA),   Q̇=ΔT/Rtotal
SymbolMeaningWhy it appears / units
RthThermal resistanceK/W.
LLayer thicknessm.
kThermal conductivityW/(m·K).
hConvective coefficientW/(m²·K).

The largest resistance in a series path often controls most of the temperature drop. Insulation works by increasing thermal resistance, while high-conductivity materials reduce it.

The largest series thermal resistance should dominate the temperature drop. For steady one-dimensional heat flow, Q=ΔT/Rtotal; increasing any series resistance lowers Q. If adding insulation makes the predicted heat flow larger under the same boundary temperatures, the network has been assembled incorrectly.

Worked Examples

Example 1: Brick wall: L=0.1m, k=0.7, A=10m², ΔT=20°C
R=0.1/(0.7×10)=0.0143 K/W
Result: Q=20/0.0143=1400 W — very high!
Poorly insulated — need insulation
Example 2: Insulation: L=0.1m, k=0.04, A=10m²
R=0.1/(0.04×10)=0.25 K/W
Result: Q=20/0.25=80 W
25× better than brick
Example 3: Wall layer
L=0.10m, k=0.05W/mK, A=10m²
Result: R=0.20K/W
Low conductivity gives substantial resistance even with a moderate thickness.
Example 4: Heat rate through series path
Rtotal=0.50K/W, ΔT=20K
Result: Q̇=40W
Once total resistance is known, the overall heat rate follows directly.

Common Mistakes

⚠️
Adding parallel resistances directly

Parallel paths add conductances: 1/Rtotal=Σ1/Ri.

⚠️
Forgetting area in conduction or convection resistance

Both L/(kA) and 1/(hA) depend on the heat-transfer area.

⚠️
Using plane-wall resistance for cylinders without checking geometry

Radial conduction through cylinders and spheres has logarithmic or reciprocal-radius resistance forms because area changes with radius.

Frequently Asked Questions

R-value in buildings?
US R-value (ft²·°F·h/BTU) = 5.678 × SI R-value (m²·K/W). Walls: R-13 to R-25. Attics: R-30 to R-60. Higher R = less heat loss = lower heating bills.
Thermal bridges?
Metal studs, window frames, and fasteners conduct heat much better than insulation — thermal bridges. They can account for 30-50% of total heat loss even in well-insulated walls. Continuous exterior insulation eliminates bridges.
Why is thermal resistance measured in K/W?
A temperature difference divided by thermal resistance gives watts of heat flow, so K/W is the natural unit.
How do series thermal resistances combine?
Add them directly because the same steady heat rate passes through each layer and the individual temperature drops sum.
What is thermal conductance?
Conductance is the reciprocal 1/Rth with units W/K. Parallel heat paths are often easier to combine using conductances.
Where does most temperature drop occur?
In steady one-dimensional series heat flow, each drop is Q̇Ri, so the largest resistance produces the largest temperature difference.
When can thermal resistances be added in series?
Thermal resistances add directly when the same steady heat rate passes sequentially through each layer or interface. Examples include conduction through stacked wall layers plus convection films. Parallel heat-flow paths require conductances to be combined instead, analogous to parallel electrical resistors. Contact resistance and radiation may add further paths or series terms depending on the physical arrangement in practice.

Formula Explorer connections

Interpretation: This formula tracks heat, temperature, work, entropy or transport in a thermodynamic system. Assumption: Use absolute temperature where required and consistent energy units. Constant properties, equilibrium, ideal gases or negligible losses may be assumed.

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