Daylight factor, thermal factors, natural ventilation

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.

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