---
title: "Daylight factor, thermal factors, natural ventilation"
subject: "Building Technology"
chapter: 1
key: "semester_6/building_technology:daylight-thermal-natural-vent"
source: https://www.bctengineer.com/q/semester_6/building_technology/daylight-thermal-natural-vent
site: BCT Engineering
---

# Daylight factor, thermal factors, natural ventilation

Building Technology · Chapter 1: Functional Requirements of Buildings

- Asked in: 2079 Jestha, 4+4 marks
- Video: [8.1.1 Daylighting Factor and Window Openings](https://www.youtube.com/watch?v=HPY2KvYtnEM) — SUT Building Physics with Prof. Lv
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/daylight-thermal-natural-vent
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-1

## Question

Define daylight factor. What are the factors affecting thermal performance of the building? Describe types of natural ventilation.

## Short answer

Daylight factor (DF) is the ratio of indoor illumination (from sky, not direct sun) to outdoor unobstructed horizontal illumination, expressed as a percentage. Factors affecting thermal performance: climate, orientation/shape, material properties (conductivity, mass, absorptivity), opening size/type/shading, insulation thickness, ventilation/air leakage, internal gains, surroundings. Natural ventilation types: wind-driven cross ventilation, stack (buoyancy) ventilation, single-sided ventilation, night ventilation.

## Answer

#### Daylight factor
**Daylight factor (DF)** is the ratio of the illumination at a point inside a room (from the sky, not direct sun) to the simultaneous outdoor illumination on an unobstructed horizontal surface, given as a percentage.

$$
DF = \frac{E_i}{E_o} \times 100\%
$$

#### Factors affecting thermal performance
1. Climate (temperature, humidity, radiation, wind).
2. Orientation and building shape.
3. Properties of materials: conductivity, thermal mass, absorptivity.
4. Size, type and shading of openings.
5. Thickness and insulation of walls and roof.
6. Ventilation and air leakage.
7. Internal heat gains and surroundings (trees, buildings).

#### Types of natural ventilation
- **Wind-driven (cross) ventilation**: Wind creates positive pressure on the windward side and negative on the leeward, and air flows through the openings in between. Flow $Q = EAV$.
- **Stack (thermal buoyancy) ventilation**: Warm air, being lighter, rises and goes out at a high opening, while cool air enters low. The stronger the temperature difference and the height between openings, the greater the flow.
- **Single-sided ventilation**: Openings on one wall only; weak flow.
- **Night ventilation**: Cool night air flushes heat out of the thermal mass.

```text
 cool in -> [room] -> warm out (high)
 wind -->  | window     window |  --> 
```

## Check yourself

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

1. Daylight factor is
   A. Outdoor over indoor
   B. Indoor illumination from the sky over simultaneous outdoor horizontal illumination, in percent
   C. Direct sun over sky light
   D. Lux per watt
   Answer: B. DF = Ei/Eo x 100 %, excluding direct sun. (Key point: Daylight factor)

2. In cross ventilation, air flows because
   A. Positive pressure on windward and negative on leeward side
   B. Warm air is lighter
   C. Fans push it
   D. Openings are on one wall
   Answer: A. Pressure difference drives flow through openings. (Key point: Wind-driven)

3. Stack flow increases with
   A. Smaller height
   B. Lower temperature difference
   C. Greater temperature difference and height between openings
   D. Fewer openings
   Answer: C. Buoyancy grows with both. (Key point: Stack flow)

4. Night ventilation
   A. Heats the room
   B. Is single-sided only
   C. Uses DPC
   D. Flushes heat out of thermal mass with cool night air
   Answer: D. Cool air removes stored heat. (Key point: Night ventilation)

## Related in Building Technology

- [Factors affecting thermal performance](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Athermal-perf-factors) — Building Technology, chapter 1
- [Natural/artificial lighting, daylight factor numerical](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Adaylight-numerical-062p) — Building Technology, chapter 1
- [Thermal performance and building geometry](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Athermal-perf-geometry) — Building Technology, chapter 1
- [Mechanical ventilation and building code](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Avent-mech-conditions-nbc) — Building Technology, chapter 1
- [Short note: Daylight factor](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Adaylight-factor) — Building Technology, chapter 1
- [Building types, mechanical ventilation, air flow](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aairflow-078c) — Building Technology, chapter 1

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