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
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)
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.