Improving The Energy Performance of Your Home: Air Infiltration

Improving The Energy Performance of Your Home: Air Infiltration

IntroductionTo satisfy the Passive House Standard, a home must be built with an extremely airtight building envelope, among a myriad of other factors. One of the Passive House Standard's primary objectives is to reduce heating energy needs by 90% compared with existing homes. One of the main ways to reduce the heating and cooling load, and one of the Passive House Standard's specific performance requirements, is air tightness. What Is Air Tightness?Air tightness refers to the ability of a building to prevent air infiltration through unsealed joints in the structure and around the windows. A leaky home ventilates poorly, letting too much cold air in during the winter and not enough during the summer. While ventilation is an important factor for indoor air quality, the best approach is to build an airtight home with a whole-house ventilation system that can carefully control how the air is ventilated and conditioned. How Do You Measure Air Tightness?The ability of a window to retain heat by reducing air leakage through gaps in or around the window is described by its air infiltration and is typically measured in cfm/ft (cubic feet of air per minute/square footage of the window). The lower the air infiltration, the less air is leaking in or out. A common replacement window has an air infiltration value of approximately 0.3 cfm/ft2. A high performance window can have an air infiltration value of as low as 0.01 cfm/ft2, allowing approximately 95% less air to pass through and around the window.While air infiltration refers to the window's performance, Air Changes per Hour (ACH) reflects the air tightness of a structure as a whole. ACH measures the amount of air that leaks out of the house (from walls, windows, ceilings, etc.) and is expressed in terms of to the number of times per hour the volume of air in the house is exchanged through leakage - in other words, the rate at which your house breathes. How Do You Test Air Tightness?In practice, air tightness can be measured using a blower-door test. A blower-door is essentially a powerful, calibrated fan that mounts into the frame of an exterior door with a device to measure the pressure inside and outside the door. The fan blows air in or sucks air out of the house and creates a pressure difference between the inside and outside air. This pressure difference forces air through all leaky areas in the building envelope until the inside and outside pressures equalize. The tighter the building envelope, the longer it takes for the pressures to equalize. Air Tightness in the Passive House StandardThe Passive House Standard requires an ACH score of 0.6 or lower on a blower-door test, i.e. a home cannot leak more than 60% of its air volume in an hour. This represents a drastic reduction compared to average air tightness in mainstream construction. According to the Pennsylvania Housing Research and Resource Center, a standard home is considered tight with a score of ACH 5 or below. However, with a combination of high quality, energy-efficient windows and well-sealed building joints, Passive Houses can achieve ACH scores as low as 0.2, as demonstrated by Oregon-based builder Blake Bilyeu. Check out the Bileyu House for a great case study of an air tight Passive House.

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