---
title: "Heat phenomena in building"
subject: "Building Technology"
chapter: 1
key: "semester_6/building_technology:heat-phenomena"
source: https://www.bctengineer.com/q/semester_6/building_technology/heat-phenomena
site: BCT Engineering
---

# Heat phenomena in building

Building Technology · Chapter 1: Functional Requirements of Buildings

- Asked in: 2071 Magh, 8 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/heat-phenomena
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-1

## Question

What is heat phenomena in Building?

## Short answer

Heat phenomena include transfer by conduction, convection, radiation, and evaporation, causing gains and losses through the envelope. Thermal properties are conductivity, resistance, U-value, thermal mass, time lag, and surface absorptivity. Control uses orientation, insulation, shading, cavity walls, and double glazing, with a steady-state heat balance equation.

## Answer

**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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Radiation heat transfer:
   A. Needs a medium
   B. Is by electromagnetic waves and needs no medium
   C. Needs solid contact
   D. Needs moving air
   Answer: B. It is major through windows and roofs in summer. (Key point: Radiation)

2. Conduction heat flow is q =:
   A. (k/t) dT
   B. k t dT
   C. dT/k
   D. t/k
   Answer: A. q = (k/t) x temperature difference. (Key point: Conduction)

3. Thermal resistance R =:
   A. t/k
   B. k/t
   C. k x t
   D. 1/t
   Answer: A. U = 1/Rtotal. (Key point: Resistance)

4. Thermal mass gives:
   A. Time lag and decrement factor
   B. Immediate response
   C. Higher U
   D. Lower mass
   Answer: A. Heavy mass delays and damps heat flow. (Key point: Time lag)

## Related in Building Technology

- [Heat, ventilation, light, sound, orientation, moisture](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Abuilding-considerations) — Building Technology, chapter 1

## Related in other subjects

- [Polystyrene insulation to cut wall heat loss 55%](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Abrick-plaster-polystyrene-55-percent) — Fundamental of Thermodynamics & Heat Transfer, chapter 7