Fun with Neutral Pressure Planes (NPP)

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Permalink Reply by Robert Riversong on September 9, 2012 at 12:53pm

An important element of the Klote report is that he uses a simplifying formula that only approximates the typical multiple vertical opening model.

"Analytic equations have been developed for the location of the neutral pressure plane for a few simple cases of leakage openings. However, the combinations of leakage openings in building shafts and compartments can be very complex... Analysis is simpler for horizontal openings, because the pressure difference across the opening does not vary with elevation. The analysis of this section is for horizontal openings…Vertical vents can be approximated by horizontal vents when the height of each vent is small compared to the height of the air column."

In a 2500 SF house with 1 ACH50 (about 350 CFM50), the Effective Leakage Area (ELA @ 4 Pa) is about 8 square inches. Since the blower door test is conducted with deliberate leaks sealed, this envelope leakage area is similar to what we might expect from dampered mechanical openings, making both deliberate and accidental leakage areas equally significant.

I'm still not sure what you're trying to accomplish by "controlling" the height of the NPP, especially given that - as you attest - the actual NPP will vary with both wind and mechanical effects.

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Permalink Reply by Robert Riversong on September 5, 2012 at 5:17pm

Here's another link to an old article, CBD-23. Air Leakage in Buildings, from the National Research Council of Canada:

http://archive.nrc-cnrc.gc.ca/eng/ibp/irc/cbd/building-digest-23.html

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