Air Leaks: How They Rot Houses and Waste Energy
One third of the energy you buy probably leaks through holes in your house.
Synopsis: When it comes to conserving energy in your home, one of the best things you can do is identify and seal air leaks. In this article, building-science specialist John Straube outlines the various ways in which a house can leak conditioned air, wasting resources and ultimately your money. Wind pushes drafts through a leaky house, a definite problem in the cold winter months. The stack effect can be problematic in both summer and winter. In winter, rising warm air leaks through the roof and is replaced by cold air sucked in through the bottom floor. In summer, warm air is pulled in through the roof while cool conditioned air is forced out of lower floors. In a house, mechanicals also contribute. Devices such as gas fireplaces, dryers, furnaces, water heaters, range hoods, and bath fans remove conditioned air. The best solution is to build tight and ventilate right.
Stopping air is the second-most-important job of a building enclosure. Next to rain, air leaks through walls, roofs, and floors can have the most damaging effect on the durability of a house. Uncontrolled airflow through the shell not only carries moisture into framing cavities, causing mold and rot, but it also can account for a huge portion of a home’s energy use and can cause indoor-air-quality problems.
A tight house is better than a leaky house, with a caveat: A tight house without a ventilation system is just as bad as a leaky house with no ventilation system — maybe worse. Energy efficiency requires a tight shell; good indoor-air quality requires fresh outdoor air. Ideally, the fresh air should come not from random accidental leaks of unknown size and quantity, but from a known source at a known rate. For this to happen, the house needs an adequate air barrier and a controlled ventilation path.
In a leaky house, large volumes of air — driven by exhaust fans, the stack effect, and wind — can blow through the floor, walls, and ceiling. Because air usually contains water vapor, these uncontrolled air leaks can cause condensation, mold, and rot.
The only way you can know for sure that the air coming into a house is clean is to know where it’s coming from. People who say “I want my house to breathe” are really saying “I want to rely on the mistakes that were made by the plumber and the electrician to provide me with fresh air.” That’s exceptionally dangerous. Any air that enters a house through leaks in the building envelope may be loaded with pollutants. The dead squirrel in your attic and the SUV idling in your garage are not going to provide you and your family with fresh indoor air.
Many indoor-air-quality problems are related to poor control of air flowing through an enclosure that has been damaged by exposure to moisture, heat, or UV-rays. Good indoor-air quality comes from having a good air barrier. Only with a good air barrier can we know where the air is coming from and have a chance that air quality (and quantity) can be controlled.
The importance of an air barrier is recognized in Canada, where the national building code has required one for 25 years. In the United States, it’s absent from state energy codes and has just recently been added to the 2009 version of ASHRAE’s Energy Efficiency Standard (ASHRAE 90.1). In 2006, the International Residential Code tightened up the language to require walls to be sealed, and as of 2009, the IECC requires airtightness testing.
Wind can push drafts through a house; air barriers push back
Many houses built to code are leaky. Air leaks can be responsible for a third or more of the energy loss in typical houses.
What pushes and pulls air through a house? Three things: wind, fans, and the stack effect. Wind is somewhat predictable, or at least its average speed and direction are. Fans include kitchen and bath exhaust fans, HVAC equipment fans, and clothes dryers. The stack effect generates pressure because warm air rises, pushing up and out on the ceiling in cold weather.
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