Ventilation Considerations. As the wind blows onto and around
buildings it creates regions in which the static pressure is above or below
that of the undisturbed airstream. (Refer to Appendix A, Section 3.)
Positive pressure on the windward side forces air into the building, and
negative pressure on the leeward side pulls it out of the building. Pressures
on the other sides are negative or positive depending on the wind incidence
angle and the building shape. The rate of interior airflow is determined by
the magnitude of the pressure difference across the building and the
resistance to airflow of the openings. The size, shape, type and location of
the openings, especially the inlets, determine the velocity and pattern of
When designing and placing windows and openings for ventilation the
following factors must be considered:
a) Predominant external wind and directions when the winds occur.
b) Construction of the building envelope and landscaping may
hinder or facilitate natural ventilation of the interior spaces.
c) Location and type of inlets has the largest effect on the
airflow pattern through the space.
d) Location and outlets type has little effect on airflow pattern.
e) Number of airchanges per hour has little to do with body
cooling; the airflow velocity and distribution pattern are more important.
f) Changes in indoor airflow direction tend to retard airspeed.
Cross Ventilation. Cross ventilation provides the greatest
interior velocities and the best overall air distribution pattern. Openings
in both positive and negative pressure zones are required for cross
ventilation (see Figure 27). For windows on adjacent walls, the overall room
air distribution is best (10 to 20 percent higher average velocities) when the
wind incidence angle is perpendicular to the building face. For windows on
opposite walls, oblique wind incidence angles give 20 to 30 percent higher
average velocities than perpendicular winds. See Figure 17 and para. 220.127.116.11.
Windows on One Wall. When windows are restricted to only one
surface, ventilation will usually be weak, and is independent of the wind
direction. Average internal wind speed will not change significantly with
increasing window size. One-sided ventilation can be made effective when two
openings are placed on the windward face, the wind angle is oblique (20 to 70
degrees), the windows are as far apart as possible and if deflectors such as
wingwalls are used (see Figures 26B and 28).
Expected Interior Airspeeds. Indoor airspeeds, even under the most
favorable conditions, are only 30 to 40 percent of the free exterior wind
speed in cross-ventilated spaces, 5 to 15 percent of the free exterior wind
speed in rooms with openings in one wall only and only 3 to 5 percent in rooms
with one opening.