Pascal Calculations & Building Air Infiltration Rate Variation
PASCAL CALCULATIONS - CONTENTS: Calculating building air pressure or air leakage rate in Pascals. Accuracy in measuring air infiltration rates in buildings. Air infiltration rate rules of thumb. Sources of variation in building air infiltration rates & measurements. Measuring air leakage using a blower door. Building air changes per hour and blower door tests. Using a blower door test to improve building energy efficiency & save on heating cost. Explanation of testing building indoor air movement using air handlers and other fans instead of a blower door - data beyond air infiltration or exfiltration.
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Pascal calculations to express building pressure & air infiltration rates:
This article explains methods of measuring building air infiltration rates and the related calculations of pascals. This website discusses how to inspect, diagnose problems in, and install or repair building insulation & ventilation systems including air leaks, air infiltration, heating cost, heat loss, moisture, & interior stains.
Our page top photo, courtesy of Steven Bliss, shows an Infiltec blower door
test being performed at a home.
Accompanying text is reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.
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The question-and-answer article about Pascals and using rules of thumb to estimate building air infiltration rates based on blower door tests found just below paraphrases, quotes-from, updates, and comments an original article from Solar Age Magazine and written by Steven Bliss.
I am confused by a slight lack of consistency in magazine articles about air leakage rates. Solar Age Magazine February 1984 p. 47 said that to get the normal air leakage you divide the building air changes per hour (ACH) at 50 Pascals [of pressure, e.g. by blower door testing] by 7.
But the May 1984 issue of the same magazine said 10 ACH at 50 Pascals equals roughly 0.5 natural ACH (dividing by 20). Which one is correct? -- Joe Bockelman, Dayton OH
Answer: Building air infiltration rate rules of thumb
Definition of Pascals (Pa): Pascals is a unit of measurement of pressure or an SI derived unit of pressure or stress. Pascal measurements are used in evaluating building air infiltration rates, often using blower door tests, where relatively small absolute pressure differences become important, so we need a sensitive unit of measurement.
One Pascal or Pa equals one newton per square meter; one bar of pressure equals 100,000 Pa, one atmosphere equals 101,325 Pa, and one psi equals 6.894 x 103Pa.
A widely accepted rule of thumb for estimating the natural winter air infiltration rate in a building is to divide the ACH at 50 Pascals by 20.
This is most appropriate for a single-story detached house with no unusual wind loads.
This rough rule of thumb on air infiltration rates works pretty well for averages of large samples of homes, yet for a single home, it is not very reliable.
For example, the average air infiltration rate at 50 Pascals of 35 passive solar homes monitored by SERI was 11.1 ACH. The average natural infiltration rate of this sample (as measured by tracer gas) was 0.5.
So dividing by 20 would have worked well for the average.
Variation in Individual Building Air Infiltration Rates
However, for individual homes in this sample, the correct divisor ranged from 10 to 40. The wind speed, shape of the house, location of the cracks and air leaks, temperature differences, and use of combustion equipment all play a role.
A given number tends to work pretty well for a given type of house under similar conditions. The natural ACH in a single house, though, may change by as much as a factor of 10 depending on the time of year and weather variables.
Here we include solar energy, solar heating, solar hot water, and related building energy efficiency improvement articles reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.
Readers should also see AIR LEAK DETECTION TOOLS (including an example of use of the blower door test to measure building air changes per hour)
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