MATERIALS
Thermal Conductivity (k-value)
Heat conducted through a unit thickness of material per unit area per °C temperature difference, in W/m·K
Also calledthermal conductivityk valuek-valuelambdalambda value
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Definition
Thermal conductivity (symbol k or λ) measures how fast heat flows through a material. Units: W/m·K. Low k = good insulator (rockwool 0.038), high k = good conductor (aluminium 211, steel 50). The fundamental input to U-value and R-value calculations. Determined per IS 3346:1980 (guarded hot-plate method).
Typical values
PUR foam0.022-0.028 W/m·K
EPS / XPS0.030-0.040 W/m·K
Rockwool0.035-0.045 W/m·K
AAC block0.16-0.22 W/m·K
Brick masonry0.81 W/m·K
Concrete (dense)1.74 W/m·K
Steel50 W/m·K
Aluminium211 W/m·K
Where used
- U-value and R-value computation
- Insulation thickness sizing
- HVAC load calculation
- ECBC envelope compliance
Acceptance / threshold
Per IS 3346: k determined by guarded hot-plate apparatus on representative samples; manufacturer test report or third-party lab cert accepted.
Site example
For 100 mm rockwool (k = 0.038), thermal resistance R = thickness / k = 0.1 / 0.038 = 2.63 m²K/W. To hit ECBC roof U-value 0.4 W/m²K (R = 2.5), 100 mm rockwool is sufficient.
Frequently asked
What is thermal conductivity (k-value)?
Thermal conductivity k measures how easily heat flows through a material per unit thickness per °C, in W/m·K. Low k = good insulator. Determined per IS 3346 by guarded hot-plate method.
What is a good k-value for insulation?
Below 0.05 W/m·K is considered insulation-grade. Rockwool 0.038, EPS 0.035, XPS 0.032, PUR foam 0.025 — all well below the threshold. Compare to brick (0.81) — over 20× worse.
Related terms
Thermal Properties of Materials
Thermal conductivity, specific heat, density, and emissivity values that govern heat transfer through building materials
Specific Heat Capacity (c)
Heat required to raise 1 kg of a material by 1 °C, in J/kg·K. High c × density = high thermal mass = slow temperature swings indoors.
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.
Heat Transfer Modes (Conduction, Convection, Radiation)
Three modes of heat transfer in buildings — conduction (through solids), convection (via fluid/air), radiation (via electromagnetic waves). All three apply to roof, wall, and window heat gain.
R-Value (Thermal Resistance)
Resistance of a material layer or assembly to heat flow, in m²·K/W. Higher R = better insulator. Reciprocal of U-value.
Thermal Insulation
Materials and techniques to reduce heat transfer
Fly Ash (Pulverized Fuel Ash)
Coal-combustion byproduct used as supplementary cementitious material in cement mortar and concrete. Covered by IS 3812 (Part 1 — pozzolana; Part 2 — admixture). Also used in PPC (IS 1489) and fly-ash bricks (IS 12894/IS 13757).
GGBS (Ground Granulated Blast-furnace Slag)
Cementitious by-product of iron production. Replaces 25-70% of OPC for sulphate resistance and lower hydration heat.
uPVC Windows
Unplasticised polyvinyl chloride frames — corrosion-free, thermal-insulating, replacing aluminum in residential.
Bamboo Construction
Bamboo as a sustainable building material per IS 6874