This article was submitted by Genry Garcia of Comfort Dynamics, Inc. It is the second part of his series on zonal pressure diagnostics. Thanks, Genry!

In Part 1 of this series, we outlined the basic understanding of ZPD—what it means and what it does. Most importantly, we showed how an effort to simplify a fairly complex testing procedure turned it into a wildly inaccurate and almost theatrical part of testing in the building performance community.

In this second part, we will establish a logical starting point for the ZPD testing process and dig a little deeper into its core.

In Part 1, we identified ZPD as primarily the process of diagnosing/estimating how much of the total air leakage for an envelope comes from any given attached or intermediate zone (i.e., attics, crawlspaces, garages, or basements). An attached but unconditioned utility room is another example of such a zone. 

Therefore, in addition to possibly leaky windows and exterior doors’ seals in a conditioned room, when we test how much this room is connected to outside, we are essentially testing how much the attic/crawlspace/garage is connected to the room.

As such, the first logical step to take is to quantify the connection existing between the envelope and the attached or intermediate zones.

Zone Connection to the Envelope

Before diving into this one, we must first understand that with the blower door maintaining the house at -50 Pa, zone pressure measurements indicate a ratio of the cumulative size of the leaks between said zone and the house relative to the cumulative size of the leaks between the zone and outdoor. Read this last sentence again. 

In the not-so-distant past, when I first started getting into ZPD, I would test homes with the blower door running. I’d check the connection between the house and the attic, for example, and I would often find readings like these:

These two readings, though identical, were not taken in the same house. The homes weren’t even similar in construction or age. Heck, these readings weren’t even taken in the same year!

The only two details these two homes have in common are that they both have a well-vented attic and are both VERY leaky. The one on the left is from an 1800-square-foot envelope with a 2,600 CFM50 blower door number or 1.45 Leakage Area Infiltration Ratio (LAIR). The one on the right is from a 3,100-square-foot home with a 5,177 CFM50 blower door number or 1.67 LAIR. 

By looking at these, the YouTube University Alumni might think, “Well, that attic must not be well vented.” But that’s not how ZPD works.  

The ZPD readings from these well-vented attics were so low because the ceiling plane in these two cases is extremely leaky. That, in turn, is a very large contributor to the total home leakage. It explains why the envelope leakage rate is so high on both of them. 

As a side note, however, attic or crawl space ventilation is pretty straightforward:

• Are there vents along the roof overhang (soffits)? 
• Are there vents along the side of the house? 
• Can you see light if you go into any of them?
• Can you feel air movement across these vents? – This last one is largely dependent on indoor vs. outdoor temperature differential, as Stack Effect is its driving force. Wind matters a great deal, too.

In addition to that, vented attics and crawlspaces have a lot less volume than the living space they share their floor area with. Therefore, it doesn’t take an obscene amount of ventilation to be considered properly vented. 

Back to ZPD

Remember from Part 1: “In cases where air must pass through at least two barriers to leak into or out of a house, ZPD is a way to use measured pressures to infer the location and size of air leakage paths.” This snippet is from the Zone Pressure Diagnostics Trainer manual. It is available from The Energy Conservatory.

Suppose we’re testing a house for leakage with the blower door running, keeping the house at -50 Pa. If we then proceed to do ZPD testing and get a reading of 25 Pa for an attached zone (attic/crawlspace/garage), that means that the cumulative size of all the leaks between the house and the zone is the same cumulative size of all the leaks between the zone and outside. 

Kinda ‘sciency’ and not very easy to digest, right? Think of it this way:

If this zone were a vented attic with 1 square foot of ventilation area between it and outside, there would be 1 square foot worth of infiltration area between the attic and the living space.

By the same token, if the zone were a vented attic with 10 square feet worth of ventilation area between it and outside, there would be 10 square feet worth of infiltration area between the attic and the living space. As you can imagine, that’s a nightmare from a temperature and humidity control standpoint in the envelope.

Suppose, on the other hand, the ZPD pressure reading was 45 Pa. Then, according to the table above, the size of the infiltration area between the zone and the house would be 1/4^tr of the ventilation area between the zone and outdoors.

If you have a vented attic with a ZPD reading of 45 Pa and 50 square feet worth of ventilation area, then you’d have more leakage area (50 x 0.25 = 12.5) than if you had an attic with a 25 Pa ZPD reading and 10 square feet worth of ventilation area (10 x 1 = 10). 

Think of ZPD as % of relative humidity. Air at 75 degrees DB and 50% RH contains less moisture than air at 100 degrees DB and 30% humidity. It is the same concept. The ratio of the size of one opening (zone to house) is relative to another (zone to outdoor).

For perspective, here are some photo examples of ZPD readings with different pressure ratios between the zones and the envelope:

As such, with varying degrees of ventilation strategies and net free area for intentionally vented attached zones, establishing the pressure ratio's relationship between the zones and the envelope is the first logical step in the ZPD process. Again, these zones are the attic/crawlspace/basement/garage, not little Johnny’s room that gets hot at night when he closes the door.

Stay tuned for the next part. We will get into how to translate these pressure ratio readings into actual leakage expressed in CFM50.

—Genry Garcia 

Comfort Dynamics, Inc.