Recent subzero weather has kept everyone inside more than usual. Yet people rarely think about the quality of the air in the home or work space. Today’s tighter houses have raised the question of indoor air quality. The general assumption is that it is better (healthier) to have a little air leaking through the house, to keep the air fresh.
Fresh air controls moisture build-up, which can contribute to mold growth. It also helps reduce indoor odors, as well as pollutants such as radon, formaldehyde and volatile organic compounds (VOCs).
Green building practices encourage the use of sustainable and non-toxic materials and formaldehyde-free and low-VOC paints, fabrics and carpeting. These certainly contribute to healthy indoor air quality.
But proper ventilation should not be confused with air leakage. It may seem counter-intuitive, but, except in mild weather, reliance on fresh air to “ventilate” a house is not an energy-efficient approach. Air leakage, determined by how leaky a house is or how hard the wind is blowing, is the unintentional, uncontrolled movement of air. Ventilation is the intentional movement of air, exchanging indoor air with outdoor air.
It is much more energy-efficient to insulate a house well, prevent air infiltration, and mechanically control how fresh air enters a house than to have a leaky house. This also applies in hot weather, when air conditioning is used. The building industry’s motto in this regard is “build tight – ventilate right.” The City of Evanston’s energy code mandates higher levels of insulation than previously. There is also increased emphasis on sealing ductwork to minimize air infiltration even more.
Some preferred ventilation strategies for tight houses are as follows:
-ERV (Energy Recovery Ventilator) or HRV (Heat Recovery Ventilator):
These are heat exchangers that make it possible to bring in fresh air while reclaiming energy (60-80 percent of the conditioned space temperature) from the air that is exhausted (in winter use, heat is regained, and in summer use, the opposite). That way the incoming fresh air is tempered before it reaches the living spaces.
-Spot ventilation, in which exhaust fans in kitchens and baths are used to remove pollutants and moisture at their source. These can be designed to operate at preset intervals.
Determining how tight a house is and whether additional ventilation is needed falls to a group of professionals who do energy performance tests (also known as energy audits, energy surveys or home energy ratings.) Using specialized diagnostic methods, they assess the energy performance of a house. These include “blower door tests” (calibrated air leakage and duct air leakage tests) that push air out of the house to create enough of a pressure difference that smoke puffers can locate sources of air infiltration. Infrared scanners determine the location and amount of wall insulation, both observing bad insulation placement and determining poor insulating value. Laser thermometers measure wall temperature. One or more of these can be used to determine the cause of frozen pipes.
These tests are especially appropriate when purchasing a new house, so that utility bills can be anticipated. Best practices for air sealing can also improve fire safety by blocking air flow in building cavities. Energy audits reveal that 80 percent of home air leakage occurs in locations that are not windows or doors. They can also determine whether there is leakage between a garage and a house.
When mild weather makes it possible to have windows open, natural ventilation is the most efficient way to ventilate. Even then, building design can improve the way the air is brought in, using cross ventilation or by using a chimney or stack effect at a stairwell. Neither uses electricity. Window and whole house fans, although they use electricity, are far more energy efficient that air conditioners.
Home energy audits are an excellent way to learn what else should reasonably be done. If a house is quite tight, an energy audit can help determine the need to introduce fresh air through mechanical means.
www.eere.energy.gov (do-it-yourself energy audit)
www.commerce.state.mn.us, click on “Energy Info Center”.
hes.lbl.gov (do-it-yourself home energy audit from the Lawrence Berkeley
www.energydetectives.com (Informed Energy Decisions, local provider of energy audits)