SERVICES
U-Value (Thermal Transmittance)
Rate of heat transfer through a building element per unit area per °C, in W/m²·K. Lower U-value = better insulation.
Also calledu valueu-valuethermal transmittanceoverall heat transfer coefficientheat transfer coefficient
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Definition
U-value is the overall heat transfer coefficient of a building element (wall, roof, window) — the inverse of total thermal resistance R. Units: W/m²·K. Formula: U = 1 / (R_si + R_layers + R_se), where R_si, R_se are interior and exterior surface resistances (typically 0.13 and 0.04 m²K/W), and R_layers = Σ(thickness / k) for each material layer. ECBC 2017 sets maximum U-value targets per climate zone: roof ≤ 0.33-0.51 W/m²K, wall ≤ 0.40-0.85 W/m²K depending on building type and zone. Lower U-value = better insulator = less HVAC energy needed.
Typical values
ECBC roof — cooling region≤ 0.33-0.51 W/m²K
ECBC wall — cooling region≤ 0.40-0.85 W/m²K
Uninsulated RCC roof~3.5 W/m²K
230 mm brick wall~3.6 W/m²K
230 mm brick + 50 mm EPS~0.58 W/m²K
200 mm AAC wall~0.90 W/m²K
Single-glazed window~5.8 W/m²K
Double-glazed window~2.7 W/m²K
Where used
- ECBC envelope compliance — roof, wall, window targets
- HVAC peak-load and annual-energy estimation
- Green-building rating (GRIHA, IGBC) credit calculation
Acceptance / threshold
Computed per IS 11239 / ECBC 2017 Annexure procedures from layer-by-layer k-values and thicknesses; verified against maximum U-value for the applicable climate zone.
Site example
A 150 mm RCC roof slab alone has U ≈ 3.5 W/m²K — well above ECBC limit. Adding 50 mm rockwool (k = 0.038) on top: R_new = 0.150/1.74 + 0.05/0.038 + 0.17 = 0.086 + 1.32 + 0.17 = 1.58 m²K/W → U = 0.63 W/m²K. Still above ECBC 0.33 limit for cooling regions; need 100 mm rockwool (U = 0.34) for compliance.
Frequently asked
What is U-value in building?
U-value (thermal transmittance) is the rate of heat transfer through a wall/roof/window per unit area per °C, in W/m²·K. Lower = better insulation. ECBC 2017 mandates U-value caps per climate zone (e.g. roof ≤ 0.33-0.51 W/m²K for cooling regions).
How to calculate U-value?
U = 1 / R_total, where R_total = R_si + Σ(t / k) + R_se. R_si = interior surface resistance (0.13 m²K/W), R_se = exterior (0.04). For each layer: thickness / thermal conductivity. Sum, then invert.
What is the difference between U-value and R-value?
R-value = thermal resistance (m²K/W) — how much a layer resists heat flow. U-value = thermal transmittance (W/m²K) — overall heat flow rate. U = 1 / R_total. Higher R = lower U = better insulation.
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