If too little outdoor air enters a home, pollutants can sometimes accumulate to levels that can pose health and comfort problems. Likewise, one approach to lowering the concentrations of indoor air pollutants in your home is to increase the amount of outdoor air coming in.
Outdoor air enters and leaves a house by: infiltration, natural ventilation,
and mechanical ventilation. In a process known as infiltration, outdoor air
flows into the house through openings, joints, and cracks in walls, floors, and
ceilings, and around windows and doors (air may also move out of the house in
this manner — this is called exfiltration). In natural ventilation, air moves
through opened windows and doors. Air movement associated with infiltration and
natural ventilation is caused by air temperature differences between indoors and
outdoors and by wind. Finally, there are a number of mechanical ventilation
devices, from exhaust (vented outdoors) fans that intermittently remove air from
a single room, such as bathrooms and the kitchen, to air handling systems that
use fans and duct work to continuously remove indoor air and distribute filtered
and conditioned outdoor air to strategic points throughout the house. The rate
at which outdoor air replaces indoor air is described as the air exchange rate.
When there is little infiltration, natural ventilation, or mechanical
ventilation, the air exchange rate is low and pollutant levels can increase.
Unless they are built with means of mechanical ventilation, homes that are
designed and constructed to minimize the amount of outdoor air that can "leak"
into and out of the home may have higher pollutant levels than other homes.
However, because some weather conditions can drastically reduce the amount of
outdoor air that enters a home, pollutants can build up even in homes that are
normally considered "leaky."
Most home heating and cooling systems, including forced air heating systems,
do not mechanically bring fresh air into the house. Opening windows and doors,
operating window or attic fans, when the weather permits, or running a window
air-conditioner with the vent control open increases the ventilation rate. Local
bathroom or kitchen fans that exhaust outdoors remove contaminants, including moisture, directly from the room where the fan is
located and also increase the outdoor air ventilation rate.
Ideally, new homes will be built to minimize leakage to control energy loss,
improve comfort, and minimize the transport of moisture and pollutants through
the building shell. These homes should then also have mechanical ventilation to
remove pollutants generated in the home and provide outdoor air in a controlled
manner. Whether a mechanical ventilation system makes sense in your existing
homes depends on the house, your existing heating, ventilation, and
air-conditioning (HVAC) system, and the changes you have planned. You should
discuss this with your HVAC contractor. A local Weatherization office, or
building performance contractor, might also be able to help you with this
decision or point you to local experts.
For a detailed analysis of ventilation system options for new homes, see the
Lawrence Berkeley National Laboratory report Recommended Ventilation Strategies for
Energy-Efficient Production Homes. Copies of ASHRAE Standard 62 are
available from ASHRAE.