Reviewed and Revised on 10/15/2013
In days past, energy was cheap and energy efficiency was not a concern. As energy prices increased, homeowners sought to reduce costs by insulating attics, walls, and basements, which reduced large-scale heat transfer and energy loss. Recently, due to high energy costs, better materials, and better information, homeowners and builders are reducing the air leaks to reduce energy loss and and costs. In some homes, the home natural air exchange (replacement of indoor air with outdoor air) may happen every four to 10 hours, compared with every 30 minutes 40 years ago.
Although an airtight home saves energy, this reduction of outside air entering the structure can cause problems with indoor air quality. Two of the most common quality issues are excess of humidity and pollutants.
Relative humidity is the ratio of the amount of water vapor in air compared with the maximum amount of water vapor the air may hold at a particular temperature. Dew point is the temperature at which the relative humidity is 100% and condensation forms. Warm air has the capacity to hold more water vapor than cold air. As the air cools, the temperature gets closer to the dew point, or the point where the water vapor begins to settle out of the air in the form of condensation. Roughly, a 20°F drop in temperature cuts the water-holding capacity in half and doubles the relative humidity.
In tight homes, human activities such as showers, drying clothes, and cooking raise the relative humidity to problematic levels, leading to condensation on windows and high humidity that may lead to mold growth.
The recommended relative humidity for people is around 40-60% for health and comfort. When warm, moist air comes in contact with cool surfaces, moisture condenses on the surface if it is below the dew point. Just as water condenses on a glass of ice water, condensation will form on cold surfaces in a home. This can happen on windows, doors, floors, and even inside walls. Sustained wet conditions may cause structural damage and associated problems with rot and mold.
Different pollutants exist in different levels in different homes. Examples include carbon monoxide (CO) and other pollutants from combustion appliances and back drafting of chimneys or flues, radon gas from the soil under foundations, formaldehyde and other fumes from building materials, mold spores, dust mites, insect residue, smoke, and particulates.
Controlling pollutant sources is the most important strategy for good indoor air quality, but some fresh air ventilation is still needed to dilute what cannot be avoided. Filtration systems are the third and last strategy for good air quality. For additional information see Pollutants in Homes.
As mentioned above having a leaky house is not a solution to having ventilation. Instead, the ideal is to "build tight, and vent right" with controlled, mechanical fresh air ventilation that is ducted in from a planned, clean location and filtered. More energy can be saved with special equipment such as a heat or energy recovery ventilator to exchange heat between the outgoing and incoming air.
ASHRAE has set the standard for minimum ventilation standards in homes. Care must be taken when reducing the natural ventilation in your home, so the indoor air quality is not reduced below acceptable levels.