SERVICES

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.

Also calledr valuer-valuethermal resistanceinsulation r valueresistance to heat flow

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Definition

R-value (thermal resistance) measures how well a material or assembly resists heat flow. Units: m²·K/W. Computed per layer as R = thickness / k. Total R for an assembly = R_si + Σ(R_layers) + R_se. Reciprocal of U-value: U = 1/R_total. Higher R-value = better insulation.

Typical values

Interior surface R_si0.13 m²K/W

Exterior surface R_se0.04 m²K/W

100 mm rockwool2.63 m²K/W

50 mm EPS1.43 m²K/W

230 mm brick wall0.28 m²K/W

150 mm RCC slab0.09 m²K/W

200 mm AAC block1.11 m²K/W

Where used

Insulation specification — required R for target U-value
ECBC compliance checks
Comparison of alternate insulation materials
HVAC sizing

Acceptance / threshold

Per IS 11239 / ECBC 2017: R-values from material k (per IS 3346) and layer thickness; total assembly R must hit climate-zone target.

Site example

To achieve ECBC roof U-value 0.33 W/m²K → R_total = 1/0.33 = 3.03 m²K/W. Subtracting surface resistances (0.13 + 0.04) and RCC slab (0.09), insulation must provide R = 2.77 m²K/W. With rockwool k=0.038: thickness = 2.77 × 0.038 = 105 mm. Specify 110 mm rockwool.

Frequently asked

What is R-value of insulation?

R-value is thermal resistance per unit area, in m²·K/W. Higher R = better insulation. Computed as thickness ÷ thermal conductivity (k). 100 mm rockwool with k = 0.038 has R = 2.63 m²K/W.

How is R-value different from U-value?

R is resistance (m²K/W), U is transmittance (W/m²K). U = 1/R_total. Higher R = lower U = better insulation. R adds linearly across layers; U does not.

Related terms

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.

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.

Thermal Conductivity (k-value)

Heat conducted through a unit thickness of material per unit area per °C temperature difference, in W/m·K

Thermal Insulation