The Benefits of Zone Pressure Diagnostics

March 22, 2018
Spring 2018
A version of this article appears in the Spring 2018 issue of Home Energy Magazine.
Click here to read more articles about Codes and Standards

Weatherization and home performance contractors have been using zone pressure diagnostic (ZPD) testing since the 1990s to better diagnose, and ultimately improve, the airtightness of homes. Though our understanding of ZPD testing has come a long way, the dynamics of ZPD testing can still be challenging or even intimidating for some field staff. Interpreting test results and knowing how to use the results are two common challenges. The good news is that if you can operate a blower door, you can conduct a ZPD test. Calculating the amount of air leakage through zones like an attic or an attached garage can involve daunting mathematics. Luckily, online tools are available to complete these calculations for us. With the right tools and a plan in place, pressure and flow readings can be collected quickly and accurately, making ZPD testing a valuable tool to facilitate high-quality retrofit upgrades.

The Benefits of ZPD

The benefits of ZPD are many. Let’s look at a few of them.

Improved Indoor Air Quality

In general, we tend to see the potential for air leakage reduction as the primary reason to test intermediate zones like attics, crawl spaces, and attached garages. Reducing excessive air leakage is a valid reason to include ZPD testing in your assessments and inspections. But even more important are the energy and indoor air quality (IAQ) consequences of not effectively sealing these zones—and especially of not sealing the garage from the living space.

Evaluators record pressure differential and flow readings to estimate through zone air leakage. This technician is using one gauge to measure the readings and the other to maintain a 50-pascal pressure difference using the cruise control feature. (Courtesy of Wisconsin Energy Conservation Corporation)

Tuck Under Garage: It is not hard to visualize how an unsealed zone like an attached garage could worsen the IAQ of this house. (Courtesy of Wisconsin Energy Conservation Corporation)

In addition to parking our cars in the garage, we often use that garage to store paint, gasoline, and other volatile organic compounds that we really don’t want in the air we breathe. Bedrooms are often above the garage, which makes reducing or eliminating the air leakage pathways from the garage into the house a priority. Conducting ZPD testing to confirm that no significant pathways remain after the work has been completed is critical to the health and safety of the building occupants. In Wisconsin’s Weatherization Assistance Program, the target total path leakage (defined as leakage through the zone into the house) for attached garages is limited to 50 CFM50 or less. The target for unconditioned attics and crawl spaces is 10% of the whole-house CFM50 measurement. For example, a house with a blower test of 1,500 CFM50 should allow no more than 150 CFM50 in total path leakage.

Improved Air Leakage Reductions and Energy Savings

My personal experience, both working on a weatherization crew and performing inspections of completed projects, suggests that when ZPD protocols are used consistently, the overall air leakage results tend to be better than when ZPD is not used consistently. If overall air leakage reductions improve, so will the energy savings for each project. Here are just a few benefits you might see by regularly performing ZPD testing:

  1. Enhanced ability to prioritize air-sealing work
    • Zones with higher estimated leakage offer more opportunity for effective air leakage reductions.
    • Not spending time in zones where little to no leakage exists allows you to focus on other areas of the home.
    • Energy auditors can identify primary pressure boundaries (align thermal and pressure boundaries).
  2. More-effective work orders can be created when zone leakage flows are estimated and evaluated.
  3. More-robust verification of completed air-sealing work
    • Crew leaders and inspectors can use ZPD as a tool to check work quality.
    • ZPD helps identify effective progress or lack of progress, guiding field staff to redirect work if necessary.
    • More-complete sealing reduces the number of undiagnosed moisture problems. This is especially true in cold climates.
  4. Potential for average air leakage reduction improvements
    • Simply implementing a reasonable ZPD protocol or encouraging crews to test can improve reductions.

Everyone wants to do a good job. Having a tool to show your progress and document your work can be motivating.

Teamwork can improve with everyone working toward a common goal.

Interpreting Zone Pressure Readings

The key to effectively using ZPD as a tool starts with a fundamental understanding of the relationship between the pressure boundaries and the zone pressure readings.

Using a blower door to measure pressure differences from the inside of a house to an unconditioned part of the building enclosure is the first step in conducting zone pressure diagnostics. The results, when accompanied by CFM50 flow readings, both before and after opening the zone access (door or hatch), are used to quantify the amount of air leakage from the outdoors through the zone and into the house.

It is extremely helpful to understand the relationship between a building’s pressure boundaries and the overall air leakage of the home. For an attic, think of the two boundaries as the interior ceiling and the roof (see Figure 1). The first of these two boundaries is the attic lower boundary. An example would be a drywall ceiling immediately below the attic. The pressure across this boundary is stated as house pressure (or boundary) with reference to zone (HwrtZ).

Figure 1. The boundary with the greatest measured pressure difference is the primary (most restrictive) pressure boundary. But is it where you thought it was, and is it restrictive enough?

The second of these two boundaries is the roof deck. It consists of the shingles and the decking. The pressure across this boundary is stated as zone pressure (or boundary) with reference to outside (ZwrtO).

Attic Pressure Boundaries

If you measure the pressure differences across each of the pressure boundaries shown in Figure 1, the sum should be equal to +/– 50 Pa. A technician in the field can measure both of these values simultaneously while using a jumper tee connector on the digital pressure gauge (see Figures 2a, 2b, 3, and Table 1). If channel A reads –5 Pa, then channel B should read –45 Pa, without significant baseline pressures. Technicians should always adjust for baselines on very hot, cold, or windy days to improve the accuracy of the test. Zone baseline pressures are subtracted from the actual zone pressure reading. If HwrtZ pressure reading was –45 Pa, we know from previous research that the attic bypasses (the holes between the house and the attic) are about one-quarter the size of the holes between the attic and outside (attic ventilation). In this example, if the attic ventilation was estimated to equal 100 square inches, then the attic bypasses to the house would be about 25 square inches, or roughly 250 CFM50. When interpreting pressure readings, it is important to take into account the size of the openings on each of the two boundaries. It would not be appropriate to make decisions based on a pressure reading alone, which is why it is helpful to survey the interior and exterior of the building to gain a better sense of how much ventilation there appears to be in each respective zone.

Figure 2a. The first step is to connect a jumper tee from the channel A input tap to the channel B reference tap. Note that when no hoses are connected, there will be no impact on the pressure read by the gauge. The tee connector can be left on during blower door testing, provided that a hose is not connected to the tee.

Figure 2b. After connecting the green hose to the outside, a technician would depressurize or pressurize the house to create a 50Pa pressure difference between the house and the outdoors. After the 50Pa difference is created between the house and the outdoors, the clear hose (zone) is connected to the tee and the two pressures can be read simultaneously.

Figure 3. When using the tee connector to measure the zone pressures, a technician can quickly determine which pressure boundary is more restrictive to airflow, and if the two readings are close to 50 Pa when added together. In this example the HwrtZ pressure boundary is more restrictive to airflow, and the holes or bypasses are about one-quarter the size of the holes in the ZwrtO pressure boundary. This may or may not be a major problem, depending on the size of the holes in the ZwrtO boundary, which is why it is so important to evaluate the integrity of both boundaries.

Table 1. Hose Configuration for Measuring Zone Pressure Differentials

Gauge Channel

Gauge Input Channel

Gauge Reference Channel

Channel A (ZwrtO)

Input to jumper tee

Reference green hose to outdoors

Channel B (HwrtZ)

Input open to house

Reference to jumper tee (which references to the zone via the clear hose*)

*The clear zone hose is not connected until HwrtO pressure difference is 50 Pa.

Each of these two pressure boundaries provides some resistance to airflow. The number of holes in each boundary can either increase or decrease the amount of air leakage you measure with the blower door. For the roof, think of the holes as roof vents, gable vents, or soffit venting. For the ceiling, think of the holes as attic bypasses (such as leaky access panels, wall plate leaks, plumbing, lighting, and electrical penetrations). This is a concept I overlooked as an inexperienced technician who thought ZPD results were just not accurate. I had conducted two zone tests on my attic, one with the roof vents taped off and the other with them open. To my surprise, I ended up with different results. I thought this meant the testing was invalid and pointless. However, what I was missing was the relationship between the two pressure boundaries and how they work together within the building enclosure. The term for this is series leakage, defined as air leaking through two or more boundaries before entering the house (see Figure 4). In this example, the roof is pressure boundary number two and the ceiling is pressure boundary number one.

Figure 4. The physical size of the holes in pressure boundary number two will affect the amount of air leakage passing through pressure boundary number one.

What I failed to see was that I had attic bypasses that were not as well sealed as I thought, as the air leakage I measured with the roof vents open was significantly higher than the air leakage when the vents were temporarily sealed. This is because with the vents open, pressure boundary number two (the roof) is made less restrictive to airflow, which equates to more airflow available to pass through the remaining unsealed or partially sealed bypasses in pressure boundary number one (the drywall ceiling). The same would hold true if you had known bypasses and installed roof vents without performing air sealing.

Our predecessors learned this early on, but even today this is not fully understood in all industries. For example, imagine a roofing contractor selling a homeowner on adding more roof vents to comply with code or eliminate excess moisture that has been collecting in the attic (probably through attic bypasses). If the ventilation is added without attention to the proper air sealing between the attic and the living space, this home will be leakier. The blower door or whole-house air leakage rate would increase, and the house could actually become less energy efficient.

Fortunately, this is not a common practice among good weatherization and home performance contractors, who treat the house as a system.

Improving Accuracy and Test Efficiency

One of the common concerns with completing ZPD tests is the additional time and effort it takes to test. The good news is that with a little practice, a complete zone test can be completed in five to ten minutes. Testing generally requires the following information:

  • Baseline pressures (HwrtO, ZwrtO, and HwrtZ)
  • Pressure difference before and after modifying the zone access (ZwrtO, HwrtZ)
  • CFM50 flow before and after modifying the zone access

A number of free software tools are available that can be used to calculate actual zone leakage after obtaining the data points listed above. See “learn more.”

ZPD Accessories

Some simple things that can be picked up at most hardware stores can make your life easier and help keep you organized. Longer pressure probes, connector tees, and extension cord reels for tubing can speed up the process.

The longer probes allow you to extend above existing insulation or penetrate through floored attics when you cannot tap the zone via an access panel. You can also add 100 feet of tubing to an extension cord reel so that you can quickly tap a zone and return the end of the hose back to your blower door, making zone baseline measurements easier and faster. A connector tee is a great way to measure two pressures simultaneously. When both zonal readings are visible at the same time, you can quickly determine if the readings are within a desirable range.

ZPD accessories. (Courtesy of Wisconsin Energy Conservation Corporation)

Other things to consider for improved accuracy:

  • End the zone hose about 10 feet from the zone access.
  • Adjust test results for temperature when extremes exist between indoors and outdoors, or between zones.
  • Include baseline adjustments on very hot, cold, or windy days.
  • Use two gauges. Use one gauge to cruise at 50 Pa while you measure the zone pressures with the other.
  • Attain at least a 6 Pa pressure difference between your first and second zone pressure readings. This may require opening the access completely.

learn more

The following free software tools can be used to calculate actual zone leakage:

Residential Energy Dynamics (RED)

Zone Pressure Diagnostics Quick Training Aid

ZPD Calculation Utility

Helping the Industry Move Forward

While there is no substitute for your personal experience and the dedication of hardworking field staff, ZPD is a valuable tool that offers a number of significant benefits. As we learn more about how buildings interact, I believe that ZPD testing has a place in moving the industry forward. The connection between health and homes is beginning to gain more recognition nationally, and it is increasingly important that zones be assessed by qualified technicians like you. I encourage those who are testing to continue to do so, and to share your experiences with others. If you are not already testing, I encourage you to consider adding this skill set to your toolbox.

Cory Chovanec, Building Science Specialist, is a hands-on energy professional with more than 20 years of experience in various facets of residential building performance. In addition to developing and delivering a number of Wisconsin Energy Conservation Corporation (WECC)’s building science training courses, he performs in-field quality assurance inspections. Cory has acquired multiple certifications from BPI and is a BPI proctor for field and online exams. Additionally, he participated in the development of WECC’s first training program to be accredited by the Interstate Renewable Energy Council.

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