PRINCIPLES/THERMAL ENVELOPE
Airflow
Airflow through the building envelope, also referred to as air leakage, infiltration and exfiltration, can
severely degrade the thermal performance of the envelope. Envelope air leakage is discussed
throughout these guidelines, and the existence of poor air leakage performance in office building
envelopes is a major motivation for the development of these guidelines. Air leakage carries heat
and moisture between inside and out, increasing space conditioning loads, degrading the thermal
performance of the insulation system and increasing the potential for condensation problems. The
amount of energy transport due to air leakage through the building depends on the airflow rate and
the temperature difference between inside and out. The airflow rate depends on the physical
leakiness of the building envelope and the magnitude of the pressure differences driving the airflow.
The energy impacts of airflow within the wall on thermal insulation performance are more complex,
depending on the airflow rate, the paths of these flows, the configuration of the envelope elements
and the temperature distribution within the envelope. Air leakage can be controlled by a well-
designed and carefully installed air barrier system that is continuous over the building envelope.
Despite common design intentions and expectations, envelope air leakage is a real problem in
office buildings. While envelope air leakage rates are assumed to be on the order of about 0.1 air
changes per hour, measurements in new office buildings have yielded values of 0.5 air changes per
hour and higher. The results of whole building pressurization tests of envelope airtightness in
modern office buildings also show that these building envelopes are generally quite leaky
(ASHRAE). Some contend that infiltration is not a serious concern because of its relatively minor
contribution to overall energy consumption and even try to take credit for infiltration in meeting
building ventilation requirements. The energy implications of air leakage depend on the particular
building and its infiltration rates, and in leakier buildings the energy impacts can be quite significant.
Also, the detrimental effects of air leakage go beyond energy and include the inability to maintain
thermal comfort due to increased thermal loads and drafts, interference with the proper operation of
mechanical ventilation equipment, degradation of envelope materials due to temperatures, dirt and
condensation, and limitations on the ability to control noise, fire and smoke. Further, it is
undesirable to rely on infiltration air to meet ventilation requirements in office buildings. Infiltrating
air is not filtered or conditioned, and its rate and distribution can not be controlled.
In addition to exterior envelopes, airtightness is also an issue for interior partitions such as the wails
of vertical shafts and the separations between floors. A lack of airtightness in these interior
partitions increases the magnitude of stack pressures across exterior walls and results in significant
vertical airflows through buildings. Such airflows transport significant amounts of pollutants
between the floors of a building and may affect the proper operation of mechanical ventilation and
smoke control systems. Therefore, airtightness is an important design and construction issue for
the walls of stairways, elevator shafts and service chases, intentional openings to these vertical
shafts, and separations between floors.
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