When the thermometer dips below 20ºF and there is more than a little breeze outside, you can find out a lot about how well your home seals out the cold. You might notice your curtains lightly swaying with each pulse of wind. You might go downstairs into your basement and wonder why it feels 10 degrees colder than the main floor. Perhaps you wonder why the snow has already melted off your roof while your neighbor’s is still totally white. Worse yet, you might have opened your latest utility bill to find you used two or three times more energy than the prior month. All of these situations are often the result of one common problem: infiltration.
Infiltration is the unintentional or accidental introduction of outside air into a home, typically through cracks in the building envelope and through use of entryways. Some people refer to infiltration as air leakage. Infiltration can be caused by wind, negative pressurization of the home, and air buoyancy forces commonly known as the “stack effect.”
The “stack effect” is when warm air moves upward in a building. This happens in summer and winter, but is most pronounced in winter when indoor-outdoor temperature differences are greatest. Warm air rises because it is lighter than cold air. So when indoor air is warmer than outdoor air, it escapes from upper levels of a home and through open windows, ventilation openings or penetrations and cracks in the building envelope. Rising warm air reduces pressure in the base of the home, forcing cold air to infiltrate through open doors, windows or where the house sets on its foundation. In winter, this can result in warm, moist indoor air moving into cold envelope cavities. As that air cools, it condenses, creating unexpected water problems in unforeseen locations.
In most homes, about one-third of the energy used for space conditioning is due to infiltration. As such, reducing infiltration can yield significant energy savings, with rapid payback.
To gain an accurate estimation of how much infiltration your home is subject to, a blower door test can be conducted. What is a blower door? It is a powerful fan that attaches to an external doorway (typically the entrance to the home) and blows air into or out of the house to pressurize or depressurize the home. The house is depressurized to a pressure of 50 pascals less than outside, which is the equivalent of a steady 20 mile-per-hour wind blowing at all sides of the building. Once this pressure is achieved, the device measures airflow needed to produce this pressure, which of course is the same as the airflow leaking into the house through various cracks. The resulting value is measured in Air Changes per Hour, or ACH. This indicates how many times per hour the total volume of air inside a home is replaced by outside air due to infiltration.
The State of Nebraska currently requires new homes be built to meet the 2009 International Energy Conservation Code, which states air changes must be less than 7 ACH at 50 pascals. As Nebraska adopts newer versions of the code, that number will drop below 5 ACH.
If you live in an existing home, there are numerous things you can do to reduce infiltration. Here are a few:
For additional ideas on how you can reduce the effects of infiltration and save energy while heating and cooling your home, contact your local electric utility. You may even find you are eligible for incentives for helping with the cost of other energy-saving home improvements.
If you are one of the unsuspecting owners of the thousands of mercury-vapor lights I still see during my nocturnal travels throughout Nebraska, the following facts may leave you screaming in horror!