Ventilation
Prior to the 1970s relatively little attention was paid to the need to cut air infiltration and prevent heat loss in homes. But with greater emphasis now placed on providing tighter, energy-efficient homes, we have also created problems with indoor air quality. Low air change rates contribute to the build-up of moisture and pollutants that can affect occupant heath and cause damage to the structure. To prevent these concerns, the house structure and systems must allow for adequate air changes, as well as adequate air movement through unconditioned areas such as attics and crawlspaces.
A moderate indoor humidity level is desirable for occupant comfort and to prevent damage to the structure. A considerable amount of moisture is generated in most homes in the course of a typical day from cooking and bathing, as well as respiration. Indoor humidity is also affected by water penetration problems in crawlspaces and basements. Excessive humidity can manifest itself in the form of condensation on windows or cold surfaces, and eventually lead to mold or mildew problems.
On the other hand, chronic low humidity is also uncomfortable and can damage furnishings and finishes as they dry out and crack. Low indoor humidity levels induce many people to use humidifiers during the heating season, especially if they have forced warm air heating. Humidifiers, however, must be maintained on a regular basis to ensure that proper humidification takes place. It is not unusual for humidifiers to either pump in excessive amounts of moisture or not operate at all. Whole-house mechanical ventilation systems are now available to help control indoor air quality. Heat recovery ventilators incorporate a heat exchange process for energy efficiency.
In addition to providing sufficient air changes within the living areas, adequate ventilation of attics and crawlspaces is also essential to prevent excessive heat build-up in the summer or moisture build-up at any time of the year. In warm weather, excessive attic heat can contribute to premature roof wear and increased indoor temperatures. In many areas, summer heat also means high humidity. Just as high humidity causes problems within the living areas of a house, excessive moisture in an attic or crawlspace can lead to fungal growth, odors, and in chronic situations, decay of structural wood. Excessive moisture also serves to attract insects and other pests.
Infiltration of moisture-laden air into an attic or crawlspace from the living area is also a concern. During the winter, if warm moist air from the living areas infiltrates into the attic and contacts the colder underside of the roof sheathing or the joists in a crawlspace, the water vapor will condense. Chronic condensation may eventually cause structural decay and fungi growth, including mold.
To help prevent these concerns, it is generally recommended that there be at least one square foot of net-free vent area for each 150 square feet of attic floor area. Additional provisions are required if there is no vapor retarder below the attic insulation or due to the peculiarity of the house style or location. Cold weather condensation concerns are generally greatest with cathedral-style ceilings, insulated roofs, and/or whenever there is an inadequate vapor retarder or an exhaust vent that discharges into the attic. A warm unvented attic is also often the cause of ice dam formation during the winter.
Actual conditions, not design values, dictate whether additional ventilation measures may be required. If the attic temperature is significantly more than outside air temperature on a moderately warm day, or if there are any signs of condensation, odors, or mold growth, remediation work and additional ventilation will be required. The ideal venting arrangement has intake vents at the lower eave area and exhaust vents near the roof peak (preferably ridge vents). These vents generally must be left open year round; with proper insulation there should not be a concern for energy loss.
In some cases, forced venting may also help. A wind- assisted or electric-powered rooftop exhaust fan can help improve air flow through an attic. These units are thermostatically controlled; in some situations a humidistat may also be required. If an exhaust fan is present, it is important to ensure that adequate intake air provisions exist, otherwise the unit will not operate effectively. Whole house fans are also used for air-exchange purposes in the summer. They remove warm air from the living area by introducing cooler, fresh air into the house.
Below-house crawlspaces also need year-round ventilation. As a minimum, at least one square foot of free vent area is typically needed for every 1500 square feet; even more vent areas are required in many cases. The ideal vent placement is near the foundation corners for optimum cross ventilation. In addition to vents, adequate foundation grading and drainage should be provided and a moisture barrier should be placed over any exposed dirt floor areas. In cold climates, there should be insulation between floor joists or on the crawlspace walls, depending on the method of venting used. Of course, care must also be taken to prevent any pipe freeze-up.
The improper venting of fuel-burning appliances (i.e. furnaces, boilers and water heaters) can also contribute to humidity and indoor air quality problems. All fuel-burning appliances must be vented according to manufacturer recommendations and codes. These appliances should also be serviced periodically, including a check of the venting systems to reduce possible air quality concerns, including the hazardous buildup of carbon monoxide.
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