The Better Blower Door Test for Multifamily Energy Code

Hello Ladies and Gentlemen, 

I've been thinking about the topic of multifamily blower door tests for a while, and I want to get your opinion. This is an article that I wanted to publish in Home Energy Magazine but I thought it would be better to get up sooner. It's about energy code language regarding blower door testing. When I was at Steven Winter Associates, the NY State Codes Council asked for advice about blower door testing language, so that's where this originates. I hope you find this helpful or at least interesting. Let me know if there's a better place for me to post this. Thanks. 

The Better Blower Door Test for Multifamily Energy Code

Now is an exciting time for energy conservation. Several states have recently adopted newer versions of the International Energy Conservation Code, including provisions for blower door air leakage testing of most low-rise residential buildings. This is great news, and it is a big step toward lower-energy, healthy homes. But New York State may soon take a significant step further by requiring testing of individual apartments in multifamily buildings, and ruling out testing of whole buildings at once. This article will explain why this distinction is significant and useful, and why you should press your state code representatives to adopt similar language. We have a distinct opportunity to guide the building industry in the right direction, so read on.


Testing Methods


You may already know that there are a lot of valid ways to test a multifamily building’s air leakage with a blower door. Below a few are discussed, but first let's lay out some terminology. The simplest blower door test on a single family home measures only the leakage of the exterior envelope, which can be referred to as “exterior leakage.” In multifamily buildings, there are lots of other air leakage pathways besides from the exterior, and a blower door test on a single unit will measure some leakage from outside and some “inter-unit” leakage from other spaces. Multifamily buildings typically have lots of inter-unit leakage, but it’s exterior leakage from the outside that relates most directly to energy consumption.


Similar to a single-family home test, whole-building leakage testing uses one or more blower door systems to pressurize or depressurize the entire building at once, measuring all the exterior air leakage in one shot. Because all spaces in the building are under the same pressure, inter-unit leakage is negated. An excellent description of the process was given by Don Hynek in Home Energy Magazine back in September 2011 (see link at the end of the article). For a larger building this method often requires multiple blower doors and experienced technicians to conduct properly. Factors such as building height, design, stage of construction, and especially weather on the testing day can make a whole-building test a challenge. Still, this test is quite popular, particularly in Europe.


Another method is guarded blower door testing, which also requires multiple blower doors and experienced technicians to perform. It also aims to measure exterior envelope leakage by manipulating inter-building pressures with multiple fans. Figure 1 illustrates this technique in more detail. It's called guarded testing because it uses secondary “guard” blower doors placed around the target unit. These are maintained at the same test pressure as the target unit, which neutralizes any inter-unit leakage. The result is that only exterior leakage is recorded from the target unit. By moving the doors around a building like a tic-tac-toe board, the exterior leakage of all the spaces can be isolated and recorded. The time-consuming process requires experienced operators, a good plan, and several blower doors even for a small multifamily building. This method is often used for research, but it is also a practical way to test some townhouse building configurations.

Figure 1. Comparison of different blower door testing approaches


By comparison, single-unit or compartmentalization testing (the terms are used interchangeably here) is simpler than other methods. A single technician with a single blower door moves around the building and tests each unit individually. Because no steps are taken to nullify leakage from other apartments as in whole-building or guarded testing, this test quantifies leakage from the exterior walls as well as the interior demising walls, floors, and ceilings of a unit. Experienced technicians will tell you that it is common for this inter-unit leakage to rival or even exceed that of the exterior leakage. Because exterior leakage is really what matters for energy use, this test is not very useful for energy cost calculations.


Which test is best?


Each test has its merits, but when Steven Winter Associates (SWA) was asked to help guide New York State’s codes council on testing language in the building code, they specifically recommended single-unit tests. In their opinion, guarded blower door testing is most useful as a research tool, and whole-building testing can be too complicated and expensive to require on a state level. But if single-unit test results are not useful for calculating energy savings, why use them for an energy-focused building code? The reasons are both practical and forward-thinking.


Above all, for a code provision to be beneficial, it needs to be enforceable. More than the other methods, single-unit testing is easiest to introduce on a statewide basis. It requires less training, experience, and equipment than whole-building tests. Many of our nation's network of HERS raters and BPI professionals are all already qualified for it. As described below, the additional cost to builders in many cases will be less for single-unit tests than for other methods when sampling protocols are used.


There are several complications with conducting whole-building tests on a wide scale that make it more troublesome to require by code. First, these large tests have high mobilization costs, and there are fewer companies that own either high-powered fans or multiple blower doors for bigger buildings. Second, the entire building has to be prepared (central exhaust registers taped, windows locked, central fans switched off, etc.) before the test can be conducted – for a large building this can take several hours. Workers also cannot enter or leave the building during the test; it must be “locked down.” For this reason it is virtually impossible to do a big blower door test early in construction because work on the building cannot stop. Punch list time is ideal because fewer workers are present, but fixing big problems at that stage is much more expensive.


A single-unit test can be more useful to builders – it can be used to check progress and identify problems very early in construction when they are cheaper to fix. A tester can leap-frog and work around other trades in the building, reducing the disruption to regular workflow. If a unit passes the blower door test early on, the job is done; if not, fixes can be applied and a retest can be done in minutes. The possibility of demonstrating compliance early is very attractive because it reduces uncertainty and potential cost at the end of the project. In fact, a higher-volume builder may decide to purchase a blower door kit and self-check periodically. Wouldn’t it be a very positive side-effect if the blower door became a common tool of the builders of tomorrow?


One might argue that a compartmentalization test yields a number that is essentially useless for energy models because it measures mostly interior leakage. That is largely true. But the goal of an energy code requirement is to save energy, and requiring and testing compartmentalization will do that. By paying attention to leaks of all types – interior and exterior – the goals of energy conservation will be met.


As for utility incentive programs that reward tight building exteriors, the builder may have to do a whole-building test at the end, but this is a much less worrisome prospect if many smaller tests have been done all along. Alternatively, some attempts have been made to find links between single-unit, guarded, and whole-building blower door test data, and to arrive at “factors” for converting results from single-unit tests to exterior leakage figures that are more useful to energy models. This is as complicated as it sounds, and the best summary of that research is that more research is needed (Faakye, Arena, and Griffiths 2013). One might argue for an alternative compliance option in the code that allows whole-building testing. But compartmentalization is a valuable goal in itself, and whole-building and guarded blower door tests do not take it into account.


Voluminous research indicates the benefits of more airtight apartments. Compartmentalized units are safer in a fire because they reduce transfer of smoke and hot gasses between units. They are healthier because they reduce the transfer of second-hand smoke, odors, and other pollutants between neighbors and attached garages. They are more comfortable because they help to reduce drafts and cold complaints, and reduce sound transfer between units. They offer better control of heating, cooling, and ventilation because uncontrolled air movement is minimized. They also reduce pathways for bugs and vermin to travel between units. Guarded blower door testing and whole-building leakage testing, while they are better at quantifying energy benefits of a tight exterior, turn a blind eye to these benefits.


Part of SWA’s impetus for moving in this direction was years of research into ventilation, airflows, and compartmentalization. Because multifamily buildings have complex networks of inter-space airflow, controlling the many pathways becomes very important to ensuring that the air that enters a unit is from a clean, healthy source. It turns out to be extremely hard to get fresh air into apartments when and where you want it if you don’t have substantially airtight apartments. According to research from SWA and others, exhaust-only ventilation, the most common design in multifamily buildings, often draws more air from other apartments than it does from fresh sources such as vents in the window or HVAC.


Compartmentalization testing leads the building industry in the right direction. It fosters a natural alliance between fire safety, health, and energy conservation professionals that other test methods may not. It is easy to explain and understand even for someone who has never seen a blower door, and the immediate benefits are apparent to builders, residents, and landlords alike. It also takes the building industry as a whole in a progressive direction by actually quantifying compliance with requirements for unit separation. Currently, enforcement of unit fire-stopping requirements currently relies on visual inspections by code officials, so compliance is in practice somewhat subjective. Testing with a blower door backs that assessment up with a real number that is easy to verify with another test.


It is easy to imagine next steps from here. One obvious step is to require blower door testing of apartments in larger residentially-classified commercial buildings. It is also possible to look forward a decade to when compartmentalization of other major spaces – boiler rooms, trash rooms, and even separate leases in commercial buildings – is verified with a blower door. Who would not like to verify that a boiler room is isolated from the rest of a large building? A quick test with a blower door will do that. This is the direction the industry should go.


Cost Comparison


A main concern of a new code provision is the cost imposed on the building industry. In many cases, single-unit tests are less expensive than whole-building tests because they require significantly less equipment and fewer personnel. The above mentioned article by Don Hynek, a very experienced multifamily professional, gives an example of a whole-building test of a 50-unit building that used five blower doors and six technicians at a cost of about $6,000, or $120 per apartment. Single-unit testing may only cost less than whole-building testing if sampling protocols are used. Defined and carried out correctly, these can be effective and cost-saving. Based on concepts from RESNET's Sampling Standard, SWA recommended a minimum sampling rate of one in seven units after an initial round of successful tests.


Figure 2 below illustrate cost estimates for testing buildings in a mature market with experienced technicians. A cost comparison between whole-building testing and single-unit testing is made, for multifamily buildings with interior-entry (shared hallway) layout and for townhome configuration. The number of man-hours is estimated for a range of building sizes, and a typical cost for a blower door technician of $90 per hour is assumed. A mobilization cost of $200 per blower door per day to account for transportation and setup of equipment is included as well. By these estimates, the cost for testing is lower when using single-unit tests, possibly less than half the cost of whole-building tests for buildings of equivalent size. Of course these are only theoretical figures and vary greatly from project to project, and you are encouraged to make your own estimates.


Figure 2. Theoretical costs for whole-building vs. single-unit tests. Mobilization cost is estimated at $200 per blower door per day is required.


Defining an appropriate building code requirement for compartmentalization testing


After reviewing the advantages of compartmentalization testing over other methods, let us review some reasonable language to be considered by codes councils considering its adoption. The language that SWA recommended to the NY Codes Council is largely aligned with language from ASHRAE 62.2-2013, LEED BD+C: Multifamily Midrise, and the EPA's ENERGY STAR® Multifamily High Rise program, which call for a maximum leakage rate of 0.30 cubic feet of leakage per minute at 50 Pascals per square foot of apartment envelope area (0.3 CFM50/SF), which includes the floors, ceilings, and interior and exterior walls of an apartment.


How appropriate is this threshold? Steven Winter Associates maintains a database of multifamily blower door tests that it has conducted over the past five years in the course of certifying thousands of units of green and high-performance housing in New York State, and it shows that this threshold is generally achievable. The following chart shows graphically the results of over 600 of these tests. In the database, 88% of units tested in the SWA portfolio meet 0.30 CFM50/SF. While most of the projects participated in some sort of utility program that required compartmentalization, it clearly shows that the threshold is within reach for builders that make an effort.


Figure 3. Histogram showing the most common compartmentalization test results from multifamily apartments tested by SWA.



Involvement in the code adoption process was not a typical SWA role, but it was a chance to offer experience and expertise in testing multifamily buildings. Along the way, SWA garnered the support of experts in the building science consulting, health, and fire safety industries. The idea was that the code language in the IECC is good, but that it needs to be modified to reflect the unique challenges and opportunities of testing multifamily buildings.


I suggest that you put this article, as well as other materials available from SWA's website, in front of code officials in your state if they are considering adopting the IECC, so that they can modify the language so that it is more appropriate and useful for multifamily. Most code committee meetings are open to the public, so you can attend one of the meetings and get yourself on the speaking list. It’s easy to do, and it will help us all move in a good direction.


For more information, and to see a description and explanation of the language that SWA recommended to the New York State Codes Council, follow this link: (SWA link to be provided)





This website allows you to see the status of your state’s latest energy code.


ASHRAE (2013). ANSI/ASHRAE Standard 62.2-2013 addendum e: Ventilation for Acceptable Indoor Air Quality in Low-Rise Residential Buildings. Atlanta, GA: ASHRAE. Compliance. Page 5.


ENERGY STAR MFHR Testing and Verification Protocols Version 1.0. Page 75.


Hynek, Don (2011). Blower Door Testing in Multifamily Buildings. Home Energy Magazine. September 2011.


LEED: Homes Design and Construction. Version 4. Prerequisite 7. Compartmentalization. 


O. Faakye, L. Arena, D. Griffiths (2013). “Predicting Envelope Leakage in Attached Dwellings.” July 2013. Washington, DC; Building America Building Technologies Program, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy.


RESNET Sampling Standard.







This is additional information


The following highlights the language in the 2015 International Energy Conservation Code and offers suggested alternate language.


2015 IECC Language

R402.4.1.2 Testing. The building or dwelling unit shall be tested and verified as having an air leakage rate not exceeding five air changes per hour in Climate Zones 1 and 2, and three air changes per hour in Climate Zones 3 through 8 Testing shall be conducted in accordance with ASTM E 779 or ASTM E 1827 and reported at a pressure of 0.2 inch w.g. (50 Pascals). Where required by the code official, testing shall be conducted by an approved third party. A written report of the results of the test shall be signed by the party conducting the test and provided to the code official. Testing shall be performed at any time after creation of all penetrations of the building thermal envelope.

Suggested Changes


Suggested Language



R402.4.1.2 Testing.  Each dwelling unit shall be tested and verified for air leakage with a blower door at a pressure of 0.2 inches w.g. (50 Pascals) and in accordance with ASTM E779 or ASTM E 1827. Where required by the code official, testing shall be conducted by an approved third party. A written report of the results of the test shall be signed by the party conducting the test and provided to the code official. Testing shall be performed at any time after creation of all penetrations of the building thermal envelope.

This calls for single-unit tests specifically, rather than allowing a whole-building test. Whole building tests ignore inter-unit leakage, which can cause performance, comfort, and indoor air quality problems.

For detached one- and two-family dwellings, not more than three stories above grade in height, dwelling units shall be tested and verified as having an air leakage rate not exceeding 5 air changes per hour in Climate Zones 4 through 6.

This uses different metrics for single-family and multifamily, as preferred by the Codes Council. The metric for multifamily is the same one referenced in LEED for Homes Mid-Rise, ENERGY STAR® for Multifamily High-Rise, ASHRAE 62.2-2013, and NYSERDA’s Multifamily Performance Program.

For other Residential groups (R2, R3, R4) or townhouses, dwelling units shall be tested and verified as having an air leakage rate not exceeding 0.3 CFM per square foot of dwelling unit envelope area (i.e., the sum of the area of walls between dwelling units, exterior walls, ceiling, and floor) in Climate Zones 4 through 6. 

For buildings with more than seven units, a sampling protocol may be allowed by the code official. The sampling protocol requires that units be grouped into sample sets of seven units, representative of all unit types. All units of the first sample set shall be tested without any failures. Upon successful testing of the initial sample set, remaining units may be tested at the rate of one per sample set. If any tested unit fails compliance with the maximum allowable air leakage rate, two additional units in the same sample set must be tested. If additional failures occur, all units in that sample set must be tested. In addition, all units in the next sample set must be tested for compliance before sampling of further units can be continued.

This language is very similar to that recently adopted by Connecticut for its building code. It is based on established protocols from RESNET, but it is simplified here. This would allow the code official to accept sampling for larger buildings to reduce costs.


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Tags: air, apartment, blower, code, door, energy, infiltration, multifamily, residential, sealing, More…testing


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Comment by Paul Morin on September 23, 2015 at 11:15am

Hi Sean,

Wow, 20 ACH50 is normal!  I have gotten calls for parts of Flordia where they have to use Ring A for the first time and are confused.  I thought that was bad.  

We have had a distributer, Pro Clima, in New Zealand for a while and they have now expanded into Australia (  So there quite a few Minneapolis blower doors down there.  Keep up the good work.

Comment by Sean Maxwell on September 23, 2015 at 4:46am

Hi Paul,

I don't disagree with the importance of exterior envelope sealing, but I think that compartmentalization testing would typically reveal critical flaws in the exterior envelope as well. After all, the exterior envelope should be easier in concept to seal, given that the air and water barriers are often coincident or at least near each other. Interior leaks by comparison may be more difficult to get your (and the builder's) head around. I think if you are able to achieve a compartmentalization standard, you likely have done an okay job with the exterior. 

In the end, the most compelling reason for me for pushing compartmentalization testing in code is that it's really the easiest and cheapest to require on a statewide basis, especially for markets that aren't used to it. This is becoming even more clear to me as I just moved to Australia, where the buildings truly suck (like 20 ACH50 house = normal). There's no way that they should allow whole-building or guarded tests for multifamily, because there just aren't enough people here who know what they're doing. Smaller tests are the way to go.

That being said, I think a natural division should be made between multifamily and commercial/industrial buildings. Sure, most bigger buildings would need a multi-fan blower door test anyway. And sure, the exterior envelope really is of utmost concern. These should be tested along the lines of some of the existing big-building standards such as the USACE and UK's ATTMA.

I'd love to get you guys to ship more blower doors to Australia! The company I've been working with has a mix of Retrotec fans, TEC manometers, and even some home-made equipment. It's crazy! I want what Terry Brennan has, which is a truckload of TEC equipment so that we can start kicking some ass down here. 

Comment by Paul Morin on September 1, 2015 at 11:12am

Thanks Sean for a very will stated article.  I agree with your assessment of the importance of compartmentalizing apartment units.  If the only focus is on tightening the exterior enclosure and leakage to adjacent spaces is not addressed, the ventilation systems will not be effective and air quality will suffer.  Also as you mentioned, you need to start the inspection and testing process during construction and not wait until the end.  Testing unit by unit is the most practical way to make sure details are being dealt with during the construction process.  Testing the first units that are dry walled on the lower level is a great way to catch problems before they are duplicated many times.  I agree that for code compliance, it is probably best if blower door tests are done unit by unit.

But, let’s not forget about the building’s exterior enclosure.  The US Army Corp of Engineers (USACE) has been requiring airtightness testing of new military buildings for years to an airtightness standard of 0.25 CFM75 per sq, ft, of surface area.  As contractors have gotten experience, they have found that if they work with air barrier consultants, on the average they are able to achieve 0.11 CFM75/SF of surface area.  Key details that have been an issue for years are finally being dealt with and air barriers are much more effective.  It is moving the industry forward.  Not only are air barrier consultants and contractors getting better, but a network of testers that test large buildings is growing.

Blower door testing is now required in the 2012 and 2015 IECC for multifamily buildings that are 3 stories or less.  It is the Wild West out there on how it is being enforced.  We get calls all the time at the Energy Conservatory for advice on how testing should be done to meet the code.  It is up to the testers to convince the building official on the proper way to do a test.  I am fine with single unit testing being the preferred way, but It also makes sense to me that above code programs like EnergyStar, LEED and utility funded programs should require whole building air tightness testing.  This will help builders learn how to get the air barrier right and help build capacity for the code to eventually require a performance testing of the exterior enclosure.  

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