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Building Science 4 Dummies: Pascals & Pressure

The pascal aka “Pa” is simply a unit used to measure pressure on a manometer. In this context, pressure (p) is the measurement of force acting on a unit area with respect to (WRT) another. One example is when we are testing an HVAC’s system ductwork; we pressurize the entire system to 25 pascals with respect to the house.

Pressure measurements generally vary based on geographic locations & what one is measuring. For example a tire manufacturer will list the “PSI” to inflate your tires in the US, whereas your tyre pressure might be listed in “bar” across the pond. Atmospheric or barometric pressure is not measured in PSI or bars but rather in millibars or inHg (aka inches of mercury). The HVAC industry on the other hand likes to deal with inches of water column.

No matter where you are it in the world, the home performance & building science fields generally deal in pascals. This is because it not only works well with imperial & metric measurements but it also scales well for the testing needing to be done. (see ACH article for 1 example)

Unit to pascal 25 Pa 50 Pa 75 Pa
1 atmosphere = 101,325 Pa 0.000 247 0.000 49 0.000 74
1 bar = 100,000 Pa 0.000 25 0.000 50 0.000 75
1 PSI = 6,894.76 Pa 0.003 63 0.007 25 0.0108
1 in Hg = 3,386 Pa 0.007 0.015 0.022
1 in H2O = 249 Pa 0.10 0.20 0.30
1 mm Hg = 133 Pa 0.19 0.38 0.56
1 millibar = 100 Pa 0.25 0.50 0.75
1 PSF = 47.88 Pa 0.52 1.05 1.56

The chart above was split into 4 distinctive fields as an example of why some of the other well-known measurements don’t scale as well. The first column shows a single unit & how many pascals it contains while the three on the right show the most common testing units.

One of the most common ways to equate the pressure a blower door exerts is by saying it is the equivalent to a 15 to 20 mph wind blowing against all sides of the house equally (don’t forget there are 6 sides). While that is a nice analogy, it isn’t fully accurate as converting wind speed to actual pressure has quite a few variables.

With that said, can you imagine what would happen if a 15 mph wind gust came blasting down your chimney? That is one reason why we make sure if you do have a fireplace that it isn’t operational & has been cleaned out. Along those lines, while a tester should check for any obvious problems, you should make sure he knows about any cracked windows, ignition sources (furnace, water heater, etc…), E/HRV’s, asbestos, or other IAQ issues as there are special steps that apply to them.

To pressurize or depressurize that is the question… While there are many schools of thought on this the most popular are as follows with the reasoning;

  • 25 Pa is generally used for duct testing where we pressurize the whole system – this helps prevent contaminates from getting into the system & prevents false readings from tape or something similar from being sucked down & covering the leakage area

  • 50 Pa is the generally used for performing air leakage testing on houses and  buildings
  • The most common method is to depressurize the house allowing for one to test how leaky the house is when it is windy.
  • Pressurizing the house is generally only used if it is raining outside (and we wouldn’t be pulling moisture in) or there might be an IAQ issue & we would not want to pull any of those containment’s into the building. In a perfect world those issues would be addressed first but that is not always possible & testing might be required to get them remediated

  • 75 Pa is mainly a military / specialty specification that generally requires both pressurizing & depressurizing the building (though that is not always required). For more on these procedures
The French are it again… The International System of Units aka Le Système International d’Unités or SI is responsible for how items are displayed. The SIwas established back in 1960 during the by the 11th Conférence Générale des Poids et Mesures. 

For example the unit “pascal” is to be in lower case unless it is the first word in a sentence. When you are abbreviating it though, the P is to be capitalized because the term “pascal” was derived from a real individual’s name, in this case Blaise Pascal.

Another interesting item is how the numbers are displayed in the table above. As you can see any time a number exceeds 4 digits to the left or right of the decimal point, a space should be added after the third digit. For more on this, you might want to check out the NIST pages on this.  NIST & SI Unit rules

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  • Eisenhower_Republican

    Wow! I didn’t know about the space with the digits over 4. Something new everyday!

    • http://blog.SLS-Construction.com/ SLS Construction

      Thanks for chiming in & shoot, I actually stumbled on that yesterday when double checking my facts