Building Diagnostics-Single Point vs Multi-point Blower Door Testing

This Article first appeared in the September/October issue of The Journal of Light Construction.

The main purpose of a blower door test is to confirm the integrity and continuity of the air control layer or air barrier.  I have a few different options on how to perform a test.  Which option I choose is dependent on the reason I am performing the test.

  1. Code compliant testing
  2. Testing as part of an energy audit
  3. Testing before, during and after a renovation or remodeling project
  4. Testing during a new build
  5. Air sealing

In the case of a code compliant test or energy audit, I want the most accurate test I can perform.  Accuracy typically improves when there is more data included in the test.  If we are looking for holes to seal in an existing home, I may not need lots of data, I just need to run the blower door to help find the holes.  These conditions will dictate my choice of using either a single point test, multi-point test or just setting the fan to operate on cruise control to hold a constant pressure to identify air leak locations.

How a Blower Door Operates

Before we get into single and multi-point tests, let’s review how a blower door operates.  A blower door is designed to move air into or out of a structure through a calibrated fan.  In residential construction, we induce a pressure difference of 50 Pascals, or 0.2 inches of water column between inside and outside the building.  A manometer or pressure gauge is used with the fan to measure the amount of air flow moving across the fan.  This air flow matches the air flow of the leaks in the building at the pressure of 50 Pa. (the actual measurement details are little more complicated than this description, but this is basically how it works).  In the United States, the air flow is measured as cubic feet per minute or CFM.  The CFM number can also be used along with the building volume or square footage to calculate other air tightness metrics like air exchanges per hour (ACH), CFM per square foot of surface area (CFM/ft²) or some other metric showing an air leakage rate.  The accuracy of the measurement of cubic feet per minute measured moving across the fan is dependent on several factors, such as equipment accuracy and condition, wind fluctuations, and temperature differences between inside and outside the building.  The accuracy (or measurement uncertainty) can also be improved when more data is collected during the test.

Digital manometers, Retrotec’s DM32 and The Energy Conservatory’s DG-1000.

How data is collected.

To accurately collect data during a blower door test, it is best if the process is automated.  By using software created by the blower door manufacturer, a blower door test can be quickly and precisely performed.  There are a few hardware options for operating the software; smartphones, tablets and laptops are all good choices.  I use a Microsoft Surface Pro but have also used my Samsung phone and a standard laptop to perform testing.  The choice of using a phone, tablet or laptop may be limited by the pressure gauge or manometer used.  The newer DG-1000 from the Energy Conservatory and the DM-32 from Retrotec can communicate with a computer by a hardwire connection or wirelessly.  The older manometers may need additional components to connect to some devices.  Older manometers, such as the one pictured below, are analog and require measurements to be collected manually.

This set of analog Magnehelic pressure gauges were used to perform blower door tests before the creation of the modern digital gauges. This gauge was part of an early Minneapolis Blower Door system, around 1990.

What is a single point test?

A single point test is where we collect the air flow (CFM) data at a single pressure point, typically 50 Pascals.  When testing using the ANSI/RESNET/ICC 380-2019 standard, the cubic feet per minute of air flow moving across the fan is averaged over at least a ten second time period.  Simply put, the fan holds a steady 50 Pascals of pressure difference between inside and outside the structure while measuring the air flow across the fan for 10 seconds.  The data points collected over that ten second time period are then average, giving us the CFM number.

It is possible to adjust the time period for collecting the data, as a matter of fact, there may be times when you want to collect data over a longer time.  Wind has a big effect on accuracy, increasing the time from the minimum 10 seconds to thirty or even 60 seconds will help in these conditions.

There are variations to the single point test.  A repeated single point test will perform several individual single point tests, then average the results.  Another variation is to perform the single point test in both directions, pressurize and depressurize and average the results.  Again, more data equates to a more accurate test.

When performing a single point blower door test to the ANSI/RESNET/ICC 380 standard, (used for both a HERS rating and code compliant testing), an adjustment needs to be factored into the CFM50 result.  Ten percent is added to the final CFM number to meet this requirement.

What is a multi-point test?

A multi-point blower door test is conducted at several different pressure points.  The ANSI/RESNET/ICC 380-2019 standard requires testing at a minimum of five different Pascals of pressure, between 10 Pa and 60 Pa.  The CFM airflow data is collected over a minimum of 10 seconds at each of the pressure intervals.  The manufacturers have software designed to perform these tests to the required standard, for example, The Energy Conservatory’s Auto Test software (currently only operates on phones and tablets) has several multi-point test options.  One of the tests is the RESNET 380 multi-point test that measures five different pressure points from 60 Pa down to 10 Pa.  The pressure points are 60 Pa, 48 Pa, 35 Pa, 23 Pa and 10 Pa.  Another multi-point test option in the software is the ASTM E779 multi-point test (another testing standard allowed per the IRC residential code).  There are eight different pressure points in this automated test, 60 Pa, 54 Pa, 48 Pa, 42 Pa, 36 Pa, 30 Pa, 24 Pa and 18 Pa.

One reason for conducting a multi-point test is accuracy in predicting building leakage at lower pressures.  It is rare that the typical home sees a 50 Pascal of pressure difference between inside and outside during normal operating conditions.  It is more common the pressure will be under 10 Pascals.  To predict the leakage rate at lower pressure, a building leakage curve is produced after a multi-point test is competed.  Though the building leakage curve can be estimated using a single point blower door test, the curve is more reliable when a multi-point test is used.

A building leakage curve created after a multi-point blower door test.

The test performed in the example above was an ASTM E779 multi-point test.  The leakage rate at 60 Pascals was 806 CFM, at 18 Pascals, 339 CFM.  The curve shows us what the predicted leakage rates down to 4 Pascals of pressure.

What happens if you are unable to reach at least 50 Pascals of pressure during multi-point testing?  The 380 standard will allow a multi-point test to be used as long as at least 25 Pascals of pressure can be reached.  Less than 25 Pascals and the test is required to be a single-point test.  Most of the current software options will convert the test results when a “can’t reach 50” test occurs.

Additional information needed to perform a precise and accurate test.

Both single and multi-point blower door test results are only as good as the information provided for the calculations.  If you are conducting a test during the winter in a very cold climate, you will need to account for the difference in temperature between inside and outside the home, or if you live in a mountainous area, elevation also needs to be accounted for.  Another important piece of information is the baseline pressure (difference in pressure between inside and outside the building).  When performing an automated test using software, the software will perform a quick baseline test and automatically adjust the fan speed to account for the pressure difference.  When performing a manual test, you will need to do the calculations by hand and make the adjustments manually.

There are many reasons to perform a blower door test.  Sometimes we want to perform the most accurate test we can, other time we just need to run the blower door to help us find the holes.  Having a basic understanding of the differences between a single point and multi-point test can help in the decision as to which test to perform.  Personally, the only time I perform single point testing is to confirm the leakage rate of a structure before setting the fan on cruise control to find the air leaks.  All other tests are multi-point tests.

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