Heat phenomena in building

Heat phenomena in a building are the processes by which heat enters, leaves or is stored in a building, and which decide the thermal comfort of its occupants.

Modes of heat transfer

1. Conduction: Heat flows through a solid from the hot face to the cold face: $q = \frac{k}{t}\Delta T$, where $k$ is conductivity. This is how heat moves through walls and roofs.
2. Convection: Heat is carried by moving air or fluid. It occurs at wall surfaces, in cavities and in rooms, and may be natural (buoyancy) or forced (fans).
3. Radiation: Heat travels as electromagnetic waves from the sun and from hot surfaces, and needs no medium. It is the major source of heat gain in summer, through windows and roofs.
4. Evaporation: Heat is taken from a surface when water evaporates; it is used for cooling.

Heat gains and losses in a building

• Gains: solar radiation through glass and on roofs and walls, conduction from hot outside air, occupants, lights and appliances, and hot ventilation air.
• Losses: conduction through the envelope in cold weather, ventilation and leakage, evaporation, and radiation to the night sky.

Thermal properties

• Thermal conductivity (k), resistance $R = t/k$, transmittance $U = 1/R_{total}$.
• Specific heat and thermal mass, which give time lag and decrement factor.
• Absorptivity, reflectivity and emissivity of surfaces.

Control

Orientation, shading, insulation, cavity walls, reflective finishes, thermal mass, ventilation and double glazing. For steady state, the heat balance of a building is

$$Q_i + Q_s \pm Q_c \pm Q_v \pm Q_m - Q_e = 0$$

where $Q_i$ is internal heat gain, $Q_s$ is solar gain, $Q_c$ is conduction gain or loss, $Q_v$ is ventilation gain or loss, $Q_m$ is mechanical heating or cooling, and $Q_e$ is evaporative heat loss.

Effect

Heat flow decides the indoor temperature and the heating or cooling energy needed, so it should be studied at the design stage.