• wind (or wind effect);
• difference in air density due to temperature difference between indoor and outdoor air (stack or chimney effect); or
• combination of both wind and stack effects.

When air flow is due to wind, air enters through openings in the windward walls, and leaves through openings in the leeward walls. The pressure distribution patterns due to wind in a number of cases are illustrated in Figure 1.

Figure 1  Air flow around a building

Wind pressures are generally high/positive on the windward side of a building and low/negative on the leeward side. The occurrence and change of wind pressures on building surfaces depend on:

• wind speed and wind direction relative to the building;
• the location and surrounding environment of the building; and
• shape of the building.

(1)

where	Pw	= mean pressure on the building surface (N/m2 or Pa)
	Po	= static pressure in undistributed wind (N/m2 or Pa)
	vw	= mean wind velocity (m/s)
		= density of air (kg/m3)
	Cp	= surface pressure coefficient

Few data exist on pressure coefficients for buildings of different form and degree of shelter. For buildings of simple form which stand alone by itself, or are much higher than surrounding buildings and obstruction, the British Standards BS5925 gives average surface pressure coefficients.

For a building with numerous partitions and openings, it is under various pressures depending on the relative sizes of the openings and the wind direction. With large openings on the windward face, the building tends to be under positive pressure. The reverse is true if the openings are smaller than those downstream.

2.2 Stack effect

When air movement is due to temperature difference between the indoor and outdoor, the flow of air is in the vertical direction and is along the path of least resistance. The temperature difference causes density differentials, and therefore pressure differences, that drive the air to move. During the winter season (see Figure 2a), the following stack effect occurs:

• indoor temperature is higher than outdoor temperature;
• the warmer air in building then rises up;
• the upward air movement produces negative indoor pressure at the bottom;
• positive indoor pressure is created on the top;
• warmer air flows out of the building near the top; and
• the air is replaces by colder outside air that enters the building near its base.

Figure 2  Stack effect

During the summer season (see Figure 2b), the reverse occurs when indoor temperature is lower than outdoor temperature. Figure 3 shows stack effect that may occur in different forms of buildings, including a building with no internal partition, a building with airtight separation of each storey, and an ideal building with vertical shafts and horizontal openings.

When thermal force is acting alone, a neutral pressure level (NPL) exists, where the interior and exterior pressures are equal. At all other levels, the pressure difference between the interior and exterior depends on the distance from the neutral pressure level and the difference between the densities of inside and outside air.
 

(2)

where	Ps	= pressure difference due to stack effect (N/m2 or Pa)
		= density of air (kg/m3)
	g	= gravitational constant = 9.81 m/s2
	h	= height of observation (m)
	hneutral	= height of neutral pressure level (m)
	T	= absolute temperature (K) (subscripts i = inside and o = outside)

2.3  Combined effect of wind and temperature difference

In most cases, natural ventilation depends on a combined force of wind and stack effects. The pressure patterns for actual buildings continually change with the relative magnitude of thermal and wind forces. Figure 4 shows the combined effect of wind and thermal forces. The pressures due to each effect are added together to determine the total pressure difference across the building envelope.

The relative importance of the wind and stack pressures in a building depends on building height, internal resistance to vertical air flow, location and flow resistance characteristics of envelope openings, local terrain, and the immediate shielding of the building structure.