University of Wisconsin - Extension

You probably don't think about, but your home is constantly ¿breathing.¿ When it is stuffy or hot, you help the house breathe by opening a window or turning on an exhaust fan. The rest of the time your home is left to ¿breathe¿ on its own.

Air leaks in and out through cracks around windows and doors and elsewhere. Any combustion appliances with chimneys also add to natural ventilation. The problem with this natural ventilation is that when it is windy and very cold outside, there is too much ventilation so you feel drafts and use a lot of energy to keep the house warm. Cold outside air holds little moisture when it leaks into the house. It can make the air in the house uncomfortably dry, which creates a market for humidifiers. In the spring and fall when the inside and outside temperature are nearly the same, there isn't enough air leaking in and out so the house becomes stuffy.

Energy conserving construction techniques used for the last 25 years have greatly reduced the air leaking in and out of homes and thereby, also reduced energy usage. The ¿breathing¿ rate of houses is measured in air changes per hour. Older homes can have air exchange rates as high as one or two per hour. If there are two air changes per hour this means that the equivalent of all of the air in the home is replaced every half hour. New energy efficient homes may have air exchange rates as low as one air change every ten hours.

Our success in tightening homes has created a new set of concerns. There is general agreement among building scientists that homes should have an air exchange rate of about one-third of an air change per hour. Air pollutants can build up in houses with air exchange rate lower than this. The chief concern is water vapor, which we typically refer to as humidity. Homes with high winter relative humidity often experience condensation streaming off windows and mold on outside walls or in closets. In some cases, the moisture finds it way into wall cavities where it can cause decay or the paint on wood siding to peel.

The best way to reduce persistent high winter relative humidity and related condensation problems is to add a whole house ventilation system of some type. For moderate humidity problems, running existing bath and kitchen exhaust fans may help. There are some problems with this approach. First, the noise from these fans can be annoying. Second, many bath fans don¿t move much air. Third, air to replace the exhausted air has to come from someplace. Unless you open a window or install some type of air inlet, the air may come down chimneys, bringing combustion products back into the house, or from the ground, bringing in radon or other soil pollutants.

A second approach is to have a ¿supply-only¿ system. The most common form of this system uses a duct with a motorized damper to bring outside air into the return duct of the furnace. There are a couple of problems with this approach. The first is that the air only comes in when the furnace blower is operating, which is a small percentage of the time even in cold weather. A second problem is that the air that is brought into the house has to find a way out. It may find its way into walls, carrying moisture with it, which can condense in the walls and cause decay and other problems.

Scientists at the Lawrence Berkeley Laboratory in California (LBL) did an extensive analysis of whole house ventilation options. For cold climates, they recommend a balanced ventilation system that has fans to bring in air as well as to exhaust stale air. In fact, they recommend a system that recaptures some of the heat from the exhausted air. This means the supply air does not feel as cold and the exhaust air does not remove as much of the air you have paid to heat. The systems are called heat recovery ventilators (HRVs). These systems are sometimes referred to as air-to-air heat exchangers.

A variation of this system is called an energy recovery ventilator. Energy recovery ventilators (ERVs) will also recovery moisture. They do this by balancing the humidity levels between the incoming and outgoing air. In the winter this would mean adding some moisture to the incoming air stream. In the summer, when outside air is typically more humid, moisture would be transferred to the outgoing air.

When installed in a home under construction, balanced systems usually use separate small diameter ducts to carry supply air to bedrooms and other ducts to pick up air from bathrooms, kitchen and laundry. The system is powered by low-volume fans that are very quiet and operate continuously. Installation during home construction can cost from $1,500 to $3,000. Installation after construction will cost more. Some costs can be saved by using the duct system of the forced air furnace. However, the blower must have controls to assure that it operates regularly and not just when the thermostat calls for heat. The LBL scientists calculated annual operating costs, including the electricity to run the fans and lost heat and cooling in summer, to be $188 for a home in Boston in 1998 dollars. The full LBL study can be found on the Web at http://enduse.lbl.gov/projects/ESVentilation"> http://enduse.lbl.gov/projects/ESVentilation.

If you are considering a heat recovery ventilator, here are some things to think about. First, find a knowledgeable installer who will balance the system to assure that you don't over ventilate or under ventilate. You will want someone you can call back if you are having problems. Second, look for a system that has been rated for performance by an independent agency such as the Home Ventilating Institute. You can compare airflow rates, heat recovery efficiency, warranties and ease of maintenance among different models.

If you are not sure whether your house is tight enough to justify the cost of installing a heat recovery ventilator (HRV), consider having an energy assessment of your house done by a Focus on Energy Consultant. Focus on Energy is a state sponsored energy conservation program. The assessment usually includes an air leakage test. It will give you an idea of how much leakage you have and pinpoint major sources of leakage. For a list of consultants call 1-800-762-7077 or visit the Focus on Energy Web site at http://www.weccusa.org">www.weccusa.org.