Thermal Conductivity Calculator

Fourier's Law of heat conduction for a flat slab in steady state is:

$$\frac{Q}{t} = \frac{k \cdot A \cdot \Delta T}{d}$$

where Q/t is the heat flow rate (W), k is the thermal conductivity (W/(m·K)), A is the cross-sectional area perpendicular to heat flow (m²), ΔT is the temperature difference across the slab (K), and d is the thickness in the direction of heat flow (m).

The heat flux (power per unit area) is:

$$q = \frac{k \cdot \Delta T}{d}$$

The R-value (thermal resistance) and U-value (thermal transmittance) are derived as:

$$R = \frac{d}{k} \quad \text{(m²·K/W)}, \qquad U = \frac{k}{d} = \frac{1}{R} \quad \text{(W/(m²·K))}$$

Higher R-values mean better insulation (less heat loss). Fiberglass insulation (k ≈ 0.04) at 15 cm thickness gives R ≈ 3.75, while the same thickness of concrete (k ≈ 1.7) gives only R ≈ 0.088—over 40 times worse at insulating.

The heat flow rate tells you how many watts of thermal power pass through the material under the given conditions. For building walls, this directly translates to heating or cooling energy costs. A wall losing 500 W continuously costs about 12 kWh per day.

The R-value is the industry-standard metric for insulation: higher is better. Building codes typically require wall R-values of 2.0–4.0 m²·K/W (or R-11 to R-21 in imperial units). The U-value, its reciprocal, is used in window and glazing specifications—lower U-values mean better insulation.