Heat Transfer Coefficient
Calculate overall heat transfer coefficient U and thermal resistance R for walls with up to 10 layers. Free online heat transfer calculator with conduction and convection modes, material presets, and interactive charts.
Layers
About This Calculator
The Heat Transfer Coefficient Calculator determines the overall thermal transmittance (U-value) and thermal resistance of building walls, heat exchanger surfaces, and multi-layer structures. Engineers, architects, and HVAC professionals use this tool to evaluate insulation performance, comply with building energy codes, and optimize heat transfer in thermal systems.
This calculator uses the standard thermal resistance network method. In conduction-only mode, the total resistance is the sum of each layer's conductive resistance: R = Σ(Lᵢ/kᵢ). In convection mode, the inner and outer surface convective resistances (1/hᵢ and 1/hₒ) are added. The overall heat transfer coefficient is U = 1/R_per_unit_area. If a temperature difference ΔT is supplied, the calculator also computes the heat transfer rate Q = U × A × ΔT.
Regional Notes
India: Energy Conservation Building Code (ECBC) specifies maximum U-values: walls 0.33 W/m²·K (composite climate), roofs 0.20, windows 2.2. Common wall materials include brick (k=0.6), concrete (k=1.8), with PU foam or EPS insulation.
United States: IECC 2024 recommends wall U-values ≤0.089 BTU/h·ft²·°F (≈0.51 W/m²·K) in Zone 4. R-values are commonly used (e.g., R-13 for walls). Convective coefficients typically use ASHRAE standards: indoor 8.3 W/m²·K, outdoor 34.1 (winter) based on 24 km/h wind.
United Kingdom: Building Regulations Part L 2021 require wall U-values ≤0.18 W/m²·K, roof ≤0.11, floor ≤0.13, windows ≤1.2. Standards reference BR 443 for calculation methods with indoor hᵢ = 7.7 W/m²·K and outdoor hₒ = 25 W/m²·K.
Frequently Asked Questions
What is heat transfer coefficient?
The heat transfer coefficient (U-value) measures how well a building element conducts heat. It is the rate of heat transfer through a unit area per unit temperature difference, expressed in W/m²·K. A lower U-value indicates better insulation.
How do I calculate the overall heat transfer coefficient?
For a multi-layer wall with convection, use: 1/U = 1/hᵢ + Σ(Lᵢ/kᵢ) + 1/hₒ, where hᵢ and hₒ are the inner and outer convective coefficients, Lᵢ is each layer thickness in meters, and kᵢ is each layer's thermal conductivity in W/m·K. Then U = 1 / (1/U).
What is thermal resistance in heat transfer?
Thermal resistance R is the opposition to heat flow through a material, analogous to electrical resistance. It is calculated as R = L/k for a single layer (in m²·K/W), where L is thickness and k is thermal conductivity. The total resistance R_total = R_per_unit_area / A gives the actual resistance in K/W.
What are typical convective heat transfer coefficients?
Typical values: natural convection in air (still air) 5-25 W/m²·K, forced convection with moderate air flow 10-100 W/m²·K, boiling water 1000-5000 W/m²·K, and condensing steam 5000-10000 W/m²·K. For building walls, typical design values are 8 W/m²·K for indoor still air and 23 W/m²·K for outdoor with moderate wind.
What materials have the best thermal conductivity for heat transfer?
Diamond (2000 W/m·K), silver (429), copper (401), and gold (317) have the highest thermal conductivities, making them excellent heat conductors. For insulation, air (0.024), urethane foam (0.022), and glass fibre (0.04) have very low conductivities, making them effective thermal insulators.
How do building regulations define U-value requirements?
Building energy codes specify maximum U-values: walls (0.15-0.30 W/m²·K), roofs (0.10-0.20), and windows (1.0-2.0) depending on climate zone and country. India's ECBC recommends walls ≤0.33, roofs ≤0.20, and windows ≤2.2 W/m²·K for composite climate zones. US IECC and UK Building Regulations Part L have similar requirements.
What is the relationship between U-value and R-value?
U-value (W/m²·K) and R-value (m²·K/W) are reciprocals: U = 1/R. A material with R-value 2 has U-value 0.5. The total R-value of a multi-layer assembly is the sum of each layer's R-value plus surface resistances. In the US, R-values are often given in h·ft²·°F/BTU, while SI units are m²·K/W.
How do I improve the heat transfer coefficient of a wall?
To lower the U-value (improve insulation): add insulation layers with low thermal conductivity, increase wall thickness, use reflective barriers or double-glazed windows, eliminate thermal bridges, and ensure proper air sealing. Each additional layer of insulation increases the total R-value and decreases the overall heat transfer coefficient.