Type of external wall of the building and the extent of its ‘permeability’ for air is just one of the factors which decide about the model and effectiveness of natural ventilation. Its course depends also on the pattern of prevailing winds and their range of speed, number of days without wind, shape and spatial structure of the building, its surroundings and the amplitude of internal and external temperatures. One cannot study the impact of double facades on the ventilation of buildings without taking these factors into account. Every building with its specific shape, location and surroundings, is a unique (and variable over time) aerodynamic object and requires separate and detailed analysis.

The implementation of double facade is directly related, if not subordinated, to the general strategy for forcing air circulation within the building. The right choice of parameters should guarantee that this facade functions correctly for the whole building. Factors which have an essential bearing on the intensity with which air circulates in the building include [10]:

use of materials (type of glass, materials from which sun shades and other non-transparent elements of the facade are made),

• width of the void space created between two layers of the facade,

• area of the cross-section and location of the intake and exhaust air ducts in the external layer and the system which regulates the extent of their opening,

• existing vertical and horizontal barriers in the space between the facade walls,

• system of sunshades — horizontal or vertical,

• techniques for opening windows in the internal layer

• additional aerodynamic elements suspended over the roof and increasing the speed of air circulation inside the facade.