SERVICES

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.

Also calledheat transfermodes of heat transferconductionconvectionradiation heat

Related on InfraLens

CODES

IS 11239 IS 3346

Definition

Heat transfers from hot to cold by three mechanisms: (1) Conduction — through solid materials, governed by k (thermal conductivity). Example: heat through brick wall. (2) Convection — via fluid motion (air or water), governed by h (convective heat transfer coefficient). Example: warm room air losing heat to a cold window. (3) Radiation — electromagnetic waves, governed by emissivity ε and surface temperature⁴. Example: sun heating a roof, or a hot RCC roof radiating into a room below.

In buildings all three combine: solar radiation hits roof surface → conduction through the slab → convection + radiation to room interior. ECBC envelope design controls all three: low-k insulation (conduction), reflective roof coatings (radiation), continuous air barriers (convection/infiltration).

Typical values

Interior convective coefficient h_i7.7 W/m²K

Exterior convective coefficient h_e25 W/m²K

Emissivity — white roof paint0.85 (high emit, good)

Emissivity — bare RCC~0.9

Emissivity — aluminium foil0.05 (low emit, radiant barrier)

Solar absorptance — white paint0.2-0.3

Solar absorptance — bare RCC0.6-0.7

Where used

Building envelope heat-gain calculations
Cool-roof and reflective-coating specification
HVAC load estimation
Radiant-barrier installation

Acceptance / threshold

Per IS 11239 / ECBC 2017: surface resistances combine convection + radiation effects; values per Annexure tables used in U-value calculation.

Site example

On a hot Delhi summer day (45 °C ambient, solar radiation 900 W/m²), an uninsulated RCC roof with bare-concrete surface (absorptance 0.65) absorbs 585 W/m² of solar heat. Surface temperature rises to ~65-70 °C. Heat conducts through 150 mm slab (k=1.74), then convects + radiates into the room. Total heat gain: ~150-180 W/m². With cool-roof paint (absorptance 0.25) + 50 mm rockwool, gain drops to ~25 W/m² — a 6-7× reduction.

Frequently asked

What are the three modes of heat transfer?

(1) Conduction — through solids per Fourier law, governed by k. (2) Convection — via fluid motion, governed by h. (3) Radiation — electromagnetic waves, governed by emissivity ε and T⁴. All three apply to building envelopes.

Which heat-transfer mode dominates roof heat gain in summer?

Solar radiation hits the roof surface (dominant input), then conduction transports it through the slab (slow but steady), then convection + radiation deliver it to the room interior. Cool-roof coatings cut the radiation input; insulation cuts the conduction.

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.

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 Properties of Materials

Thermal conductivity, specific heat, density, and emissivity values that govern heat transfer through building materials

Thermal Conductivity (k-value)

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

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 Insulation