O&M Advisor: Building Shell--Air Leakage Control

O&M Advisor
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Building Shell: Air Leakage Control

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Air leakage is a major source of heating load, but controlling air leakage is important for other reasons as well. Certain types of uncontrolled leakage result in moisture deposits in the building envelope. When warm, moisture-laden air works its way out through the wall system, the moisture condenses on the cold outer wall panels, providing an opportunity for mold and mildew to grow, degrading insulation performance, and even causing structural damage. Uncontrolled ventilation can also contribute to the influx of hazardous gases such as radon. And the comfort advantages of living in a draft-free house (one that is comfortable even near windows and exterior walls) are obvious to anyone who has experienced a drafty environment.

Understanding the Basics

Even if the building envelope is full of holes, air can’t move through the holes without the key driving force: a pressure difference between inside and outside. The strength of the driving force depends on the temperature differential between indoors and out, wind speed and direction, and mechanical air movement. Indoor/outdoor temperature differentials cause a force called the stack effect (see Figure 1). The stack effect is the force that causes hot-air balloons to rise—if a house were light enough, the warm air inside it would cause it to float away on a cold day. Instead, all that warm air just collects at the top of the house and pushes against the envelope until it finds a way out. Any warm air that leaks out is replaced by cold air that tends to come in at the bottom of the house—where negative pressure results from all the warm air trying to go up.

Bottom Line

Envelope leaks in the attic and basement have the largest effect on air leakage and energy consumption.
Sealing air leaks also helps reduce potential problems with moisture migration and condensation in the building envelope.
Efforts to tighten building envelopes should consider the impacts on air quality, as most residential buildings depend on leakage for fresh air. Mechanical ventilation systems are available to provide this function.
Controlling air leakage can save energy in both residential and commercial buildings, but the savings are greatest in small buildings, where leakage can be a significant fraction of the building’s volume.
High-rise residential buildings can be dramatically affected by air leakage because of the stack effect.

Figure 1: Stack effect
The buoyancy of warmer air creates high pressure in the top of a building and low pressure at the bottom.

Source: Platts

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