Thermal Resistance Calculator
Calculate thermal resistance for single layers and series composite walls using R = L/(kA).
What Is Thermal Resistance?
Thermal resistance quantifies how much a material or wall opposes steady heat flow. It is the thermal analog of electrical resistance: a larger $R$ means less heat passes for the same temperature difference. Building insulation, electronics heat sinks, and furnace walls are all analyzed with thermal resistance networks.
Conduction Resistance
For one-dimensional conduction through a flat layer:
$$R = \frac{L}{kA}$$$L$ is thickness, $k$ is thermal conductivity, and $A$ is area normal to heat flow. Units are K/W. Heat transfer rate follows $\dot{Q} = \Delta T / R$.
Series Composite Walls
Layers stacked in series add resistance:
$$R_{total} = R_1 + R_2 + R_3 + \cdots = \frac{L_1}{k_1 A} + \frac{L_2}{k_2 A} + \frac{L_3}{k_3 A}$$Brick plus fiberglass insulation plus drywall behaves like resistors in series. Related: Thermal Conductivity Calculator and Heat Transfer Coefficient Calculator.
Frequently Asked Questions
How is thermal resistance related to R-value?
R-value in US building codes is thermal resistance per unit area. In SI, $R$ in K/W equals thickness divided by conductivity times area. Higher resistance means better insulation.
Do series layers share the same heat flow?
Yes. In steady one-dimensional conduction, the same heat rate passes through each layer. Temperature drops across each layer according to $\Delta T_i = \dot{Q} R_i$.
What conductivity should I use for fiberglass?
Loose-fill fiberglass is often near 0.04 W/(m·K). Dense materials like concrete are higher, around 0.8-1.4 W/(m·K). Metals have much higher conductivity and therefore very low thermal resistance.
Can I solve for thickness or conductivity?
Yes. Rearrange $R = L/(kA)$ to find any unknown. Single-layer mode supports solving for $R$, $L$, $k$, or $A$ when the other three values are known.
Does this include convection or radiation?
This tool focuses on conduction through solid layers. Surface convection adds its own resistance, often modeled as $R = 1/(hA)$ where $h$ is the heat transfer coefficient.
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