APPENDIX/DIAGNOSTICS
Airtightness
Component Pressurization Testing
The airtightness of building envelope components, e.g. windows, doors or larger wall sections, can
be measured with pressurization testing. ASTM E 283 describes the test procedure as applied to
an envelope mock-up. ASTM E783 describes the use of the test procedure in the field. In both
cases a chamber is sealed around the test specimen and an air-moving device is used to establish
and maintain an air pressure difference across the specimen. Both the pressure difference and
airflow rate are then measured at a series of pressure differences to determine the airtightness of
the component being tested. Specific requirements of the test equipment are described in detail in
the standards including requirements for the chamber, the air-moving system, and equipment for
measuring the pressure difference, airflow rate and other parameters. The results of these tests are
generally reported as the airflow rate per unit length of specimen perimeter or per unit area of
specimen at some specific pressure difference, usually 75 Pa (0.3 in. water, 1.57 psf).
Whole Building Pressurization Testing
The overall airtightness of an entire building envelope can be measured using whole building
pressurization testing. This technique is described in detail in ASTM E 779. In this procedure a
large fan induces a large and uniform pressure difference across the building envelope, and the
airflow rate required to induce and maintain this pressure difference is measured. The airflow rate
required to induce a specific reference pressure difference then serves as a measure of the
envelope airtightness. Although the test conditions differ considerably from those that normally
induce envelope air leakage or infiltration, pressurization testing provides a repeatable and
relatively quick measurement of building air-tightness. The technique has been applied to a number
of commercial buildings using either a large fan brought to the site, or more often, the existing air
handling equipment. When using the building air handlers to conduct a pressurization test, one
modulates the airflow through them to obtain a series of inside-outside pressure differences and
measures the airflow rate through the air handlers at each pressure difference. If the building is
being subjected to a positive pressure difference, one uses the supply fans with 100% outdoor air
while sealing all recirculation and exhaust dampers. If the building is being depressurized, one
uses the exhaust fans and seals all intake dampers. A detailed description of the technique as
applied to large commercial buildings is contained in Persily (1986).
Tracer Gas Measurements of Air Exchange
Building air change rates can be measured with the tracer gas decay technique as described in
ASTM E 741. These measurements determine the air change rate caused by weather-induced
pressure differences, which serves as a measure of the envelope airtightness. The technique can
also be used to determine the air change rate when mechanical ventilation equipment is in
operation, though envelope airtightness is not the primary determinant of the air change rate under
these conditions. In the tracer gas decay technique, a volume of tracer gas is released in a building
and allowed to mix with the interior air until a uniform tracer gas concentration is achieved within the
building. The tracer gas concentration decay is then monitored and the rate of decay is related to
the air exchange rate of the building during the test. the measurement technique is based on the
assumption that the tracer gas concentration is uniform throughout the entire building, and if this
assumption is valid then the measurement determines the air exchange rate for the entire building.
These measurements can be conducted as soon as the exterior envelope is complete, though it is
preferable if they are conducted when the building is being space conditioned so there is a
temperature difference to induce infiltration.
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