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Zone Pressure Diagnostics Tool Guide

What this tool does – [Tool]

This tool allows you to easily achieve accurate Zone Pressure Diagnostics (ZPD) testing results. ZPD calculated with this tool will:

  • Determine the amount of air leakage between the house and an intermediate (or attached) zone such as an attic, basement, or garage.
  • Determine the amount of air leakage between an intermediate zone and outdoors.
  • Determine the portion of the whole-house leakage that is attributable to an intermediate zone.
    Attic as a zone with interior and exterior pressure boundaries
  • Determine the effectiveness of air sealing by performing pre- and post-weatherization ZPD testing.
  • Estimate the amount of effective attic venting.
  • Appraise the connection between a zone with potentially bad air, such as an attached garage or a crawl space, and the living area.

Calculated values

  • Zone leakage ratio (zone-to-house: zone-to-outdoor) – the relative leakiness of the interior and exterior pressure boundaries of the zone.
    Note: This ratio only depends on the “House wrt outdoor” and “House wrt zone” ΔP values.
  • Zone-to-house leakage @ 50Pa – the airflow that would pass through the initial interior zone pressure boundary, at 50 pascals, if the exterior pressure boundary were completely removed. The + uncertainty value (95 percent confidence) is listed just after the airflow result.
  • Zone-to-house leakage area – the equivalent leakage area in the initial interior zone pressure boundary. The + uncertainty value (95 percent confidence) is listed just after the leakage area result.
  • Zone-to-outdoor leakage @ 50Pa – the airflow that would pass through the initial exterior zone pressure boundary, at 50 pascals, if the interior pressure boundary were completely removed. The + uncertainty value (95 percent confidence) is listed just after the airflow result.
  • Zone-to-outdoor leakage area – the equivalent leakage area in the initial exterior zone pressure boundary. The + uncertainty value (95 percent confidence) is listed just after the leakage area result.
  • Through-zone leakage at 50Pa – the airflow at 50 pascals through the zone, including both the initial interior and exterior pressure boundaries. This value represents the portion of the whole-house leakage that is attributable to the leakage through the zone. The + uncertainty value (95 percent confidence) is listed just after the airflow result.
  • Percent of whole-house leakage passing through the zone – this value being close to, or above, 100% is a good indication that you have entered some value incorrectly, or that you have made up a physically impossible example.


  • Clicking the label for any input or result will cause a popup help box to appear. This help box includes the allowed and normal values (for inputs).
  • We recommend you reset the tool before performing a new test. This will ensure that you don’t mix inputs from one job with those of another. The Reset button is at the upper right corner of the tool.
  • ZPD zones can include attics, attached garages or sun porches, basements, and knee wall areas.
  • Zone Pressure Diagnostics is sometimes referred to as series-leakage testing. This is because the resistance imposed by the combination of interior and exterior pressure boundaries is analogous to an electrical circuit wired in series.
  • When opening (closing) a hole to reduce (increase) the resistance of a pressure boundary, it is not necessary to create the opening (closure) all in one place; you may open (close) more than one hole to achieve the recommended pressure shift.
  • We recommend the use of two manometers for zone pressure diagnostics testing. If you have two manometers and one has automatic pressure control (Cruise Control), use this manometer to control the blower door.
  • We recommend you use the automatic baseline feature for the house wrt outdoor pressure difference. If you have a second manometer, we recommend you use it for the zone pressure difference using its automatic baseline feature.
  • Be aware that connections between zones can effect single-zone ZPD results.
  • Use of manometers with WiFi capabilities makes zone pressure diagnostics easier to perform.

History and Background

The history and background below are excerpted from “An Investigation into Zone Pressure Diagnostic Protocols for Low Income Weatherization Crews”. 2001. Energy Center of Wisconsin, page 1.

“Zone Pressure Diagnostics (ZPDs) has become an established tool for diagnosing indirect air leakage paths in houses since its introduction to the U.S. weatherization community around 1990. 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.”

“Michael Blasnik developed a set of ZPD techniques that are referred to as the Blasnik methods or “series leakage” measurements. These methods are known as the “Add a Hole”, “Open a Door”, and “Vents” methods or Methods 1, 2, and 3, respectively (Fitzgerald, 1994). These methods have become well established amongst weatherization crews, with hundreds of users across the country. Calculation procedures were developed by Blasnik (Energy Conservatory, 1998) and the uncertainty analysis was included in separate applications produced by Collin Olson based on ANSI/ASME PTC 19.1-1985 Part 1 “Measurement Uncertainty” for computing Method 1 and 2 results. In addition to these PC programs and published nomograms (Energy Conservatory, 1998), handheld calculator programs exist to perform the calculations.”

“The methods are probably most widely used for prioritizing air sealing. By estimating the available air leakage reduction potential of various locations within a house, a crew can make informed decisions of where to best spend their time. Some groups within and outside of the low income weatherization field have also set performance specifications based on ZPDs. For example, the American Lung Association’s Health House program specifies a maximum amount of garage to house leakage, as determined using ZPDs (ALA, 2001). Also, the Metropolitan Airports Commission Minneapolis-St. Paul International Airport Part 150 Residential Sound Insulation Program (SIP) requires contractors to be able to perform Method 3 ZPDs and achieve a minimum level of air leakage in each attic zone (DJR Architecture, 2001).”

“The widespread use of ZPDs is an indication that the methods are generally thought to be a cost effective tool for air sealing houses and useful metrics for building air leakage resistance. However, there are substantial differences in the ways that field personnel use the techniques and there are many questions about how to decide when to use them, which method to use, and exactly how to make the best measurements. Everyone who has used these methods has seen instances where results have not made sense, indicating the need to consider the accuracy of the ZPD estimates.”

Best Practices

  • In order to use the infiltration credit for a new dwelling, you must conduct a blower door test. The appropriate sequence to use for a new dwelling is as follows: First, you must estimate what you think the blower door test value will be when the dwelling is completed. Second, install a dwelling-unit fan with a capacity that exceeds your estimate for the final airflow rate. Third, conduct the blower door test when the dwelling is finished. Finally, adjust the dwelling-unit ventilation fan airflow rate with a variable-speed control to satisfy the actual dwelling-unit airflow rate, Qfan. Or, if you are using intermittent operation of the dwelling-unit ventilation fan, adjust the run time of the fan to correspond with the “Required mechanical ventilation rate, Qfan.
  • Regarding the infiltration credit and the required blower door testing, ASHRAE 62.2-2016 Standard states: “Effective Annual Average Infiltration Rate” shall be calculated using the normalized leakage calculated from measurements of envelope leakage using either ASTM E779 or CGSB 149.10. The authority having jurisdiction may approve other means of calculating effective leakage area (ELA), such as the RESNET Mortgage Industry National Home Energy Systems Standard.” (ASHRAE 62.2-2016, Section 4.1.2). Both ASTM E779 and CGSB 149.10 require multi-point blower door tests. In other words, when using the infiltration credit, you are not in compliance with the Standard unless you do a multi-point blower door test, unless your “authority having jurisdiction” allows you to use a single-point test. So, keep in mind that unless you have approval, a single-point blower door text is not in compliance with the Standard.
  • The ASHRAE 62.2-2016 Standard allows the use of the alternative compliance path (Appendix A of the Standard) for existing dwellings. This path allows compensation for deficits in local ventilation (kitchens and bathrooms) by increasing the flow rate of dwelling-unit ventilation. Generally, the use of this alternative path saves installation time and money, but it might result in lower indoor air quality. Whenever possible, avoid the use of the alternative compliance path and install separate local and dwelling-unit ventilation fans.

RED ZPD Tool Error Checking

Complex error-checking features have been added to this tool to help ensure that your input entries are valid. The items listed below will help you avoid error messages when using this tool.
  • All of the “BD on” (Blower Door on) pressure inputs MUST be the same mathematical sign; negative for a depressurization test and positive for a pressurization blower door test.
  • The house wrt zone ΔP values must be between 0 Pa and the corresponding house wrt outdoor ΔP value. For the initial configuration only, the house wrt zone &DeltaP value must also be at least 1 Pa away from 0 Pa and the initial house wrt outdoor ΔP value.
  • The direction of the shift in the house wrt zone ΔP values, from the initial to the modified zone configurations, must be consistent with the hole location and with whether the hole is being opened or closed.

A warning message will appear at the bottom of the tool if the calculated through-zone leakage @50 Pa plus the reported uncertainty is greater than the initial blower door flow rate (adjusted to 50 Pa).