How air enters a building
The controlled way to bring air inside is through the ventilation systems’ supply air vents.
However, air also enters via so-called passive ventilation or infiltration. This occurs when
we open doors and windows, and through minor leaks around doors and windows when
they are closed and via gaps around pipes, vents and chimneys. The air exchange is
additionally affected by the outside temperature and how much the wind blows.
The effects of the Stack Effect
The Stack Effect is how air moves (stacks) inside. It occurs due to warm air being lighter
than cold air. When the outside temperature is cold, the warm indoor air rises and creates
a difference in pressure that makes colder air come in (infiltrate) via doors, windows and
other leakage points at the bottom of the building. When the temperature outside is warm,
the Stack Effect is reversed and the warmer air enters via the upper levels creating a
downward draft.
Rates vary between countries
Recommended ventilation rates often vary quite significantly between countries. A study
of a number of European countries showed that one group of countries had ventilation rates
of around 10 l/s and another group of countries had 4 l/s.
Another study (in the United States) of how indoor air quality impacts people indicated that
when the ventilation rate was increased from 20 to 40 cfm (10-20 l/s) of outdoor air per person,
it led to an 8 percent increase in employee decision making. Or about $6,500 in increased
productivity per person per year.
Ventilation ensures the air is safe
Ensuring a satisfactory ventilation rate, is not just about people’s general well-being and
productivity. Without proper ventilation, the indoor air can become dangerous. It can lead
to a buildup of deadly levels of carbon monoxide and harmful concentrations of radon.
Radon is a naturally occurring, radioactive, odourless and invisible gas that is harmless
outdoors but has the potential to accumulate to hazardous levels indoors. Exposure to
radon can cause cancer.
New buildings are more energy efficent
Newer buildings are usually tighter and therefore need a ventilation system with greater
capacity for air exchange compared to older houses. The upside is that newer buildings
are more energy efficient. Older constructions have more leakage through passive ventilation
(infiltration), which leads to higher energy costs.
When a building functions, for example, as an office, a school, a library or a hospital, it is
wise to invest in air purifiers to help the ventilation system take care of air pollution from
indoor sources.
In industrial indoor environments, the ventilation system is often supplemented by hoods
and extractors that capture air pollution at the source. It can be welding dust, oil mist, wood
dust, etc. Industrial operations also benefit from purifying the indoor air using products such as
Camfil’s industrial air cleaners and Camfil’s dust, fume and mist collectors.
New Zealand