Thermal Conductivity & Heat Transfer Calculator
Calculate conductive heat transfer through walls, windows, and any material using Fourier's law. Find R-values and U-values.
Heat Transfer Guide
Fourier's Law of Conduction
Q = -kA(dT/dx). For steady-state through a flat wall: Q = kAΔT/L. Where Q = heat flow rate (W), k = thermal conductivity (W/m·K), A = area (m²), ΔT = temperature difference (°C or K), L = thickness (m). Example: brick wall (k=0.72) 100mm thick, 20m² area, 20°C inside vs 5°C outside. Q = 0.72 × 20 × 15 / 0.1 = 2,160 W. That's a 2.16kW heater of heat loss continuously — enormous. This is why uninsulated solid brick walls are so expensive to heat. Adding 100mm mineral wool insulation: total resista
R-Value and U-Value
R-value (thermal resistance): R = L/k. Higher R = better insulator. Units: m²·K/W. R-values add for layers in series. Total wall R = R_brick + R_insulation + R_plasterboard + air films. U-value: heat transfer coefficient = 1/R_total. Lower U = better insulator. Building Regs Part L: walls U-value 0.18 W/m²·K or below for new builds. Roofs 0.13. Windows 1.4. Floors 0.18. These targets require modern insulation thicknesses: 100-150mm mineral wool in walls. 270mm+ in lofts. Triple glazing for windows.
Insulation Materials Compared
Common insulation k-values (lower = better): vacuum insulated panel: 0.007 W/m·K (best — expensive). Aerogel: 0.015. PIR/PUR foam: 0.022. Phenolic foam: 0.020. Mineral wool: 0.035-0.040. EPS polystyrene: 0.035. Sheep's wool: 0.039. Cellulose: 0.040. Hemp: 0.040. To achieve U-value 0.18 (Part L target) using each: PIR: 120mm thickness needed. Mineral wool: 200mm. Cellulose: 220mm. PIR is space-efficient (thinner walls) but expensive and embodied carbon higher. Mineral wool: balance of performance
Beyond Conduction
Total heat transfer through walls involves three mechanisms. Conduction: through solid materials (the dominant mechanism this calculator models). Convection: air movement on both surfaces. Surface heat transfer coefficient ~7-25 W/m²·K. Radiation: longwave IR exchange. Significant especially for low-emissivity surfaces (modern foil-faced insulation, low-E glass coatings). Window U-values: a 'triple glazed' window achieves U-value of approximately 1.0 W/m²·K — mostly due to multiple air/argon gap
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