Blower Door Testing in Multifamily Buildings

September 01, 2011
September/October 2011
A version of this article appears in the September/October 2011 issue of Home Energy Magazine.
Click here to read more articles about Multifamily

The owner knew he had a problem. During the previous Wisconsin winter, ice dams had pulled a 20-foot x 5-foot section of eave and roof right off the building!

"The ice on the roof edge was 18 inches thick," he told us. "Even with the whole mess lying there on the lawn, we couldn't bust it up. We finally had to bring a crane right into the yard and lift the thing out in one big piece, so we could truck it away."

The weatherization crews working for La Casa de Esperanza (based in Waukesha, Wisconsin) knew that those ice dams meant lots of air leaks, lots of energy waste, and lots of opportunities to improve this building. They had years of experience fixing infiltration and ice dam problems. They knew what to look for, and pretty much found it all. The vented crawl space was well connected to the living space above. The windows were pretty loose, and the door weatherstripping was long gone. The attic was a mess; multiple open soffits, oversized plumbing and chimney penetrations, a huge attic hatch with no weatherstripping. Air sealing was clearly in order. Blower door testing would be imperative to ensure that the job was done well.

The problem? This weatherization project was a 50-unit apartment building.

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Any number of experienced weatherization specialists "know" there are a number of reasons why we don't test these buildings: (1) it is hard to do, (2) no one has proven that sealing apartment buildings saves energy or improves the building, and (3) it is just too costly to implement. LEED Multifamily requires blower door tests only on a sample of individual units. The Energy Star program for multifamily buildings uses the same standard. The only programs that explicitly require a whole-building blower door test for apartment buildings are the Alaska Weatherization Assistance Program and the U.S. Army Corps of Engineers.

When it comes to apartment buildings, it seems that the universal—and wrong—answer is to try to pretend that everything we know about infiltration doesn't apply.

Over the last ten years, various accomplices and I have completed close to 100 blower door tests on more than 60 apartment buildings. The buildings have ranged in size from 5,000 to 280,000 square feet, and from two to nine stories. We have used every blower door system available in the United States. Based on all this work, we have concluded that blower door testing and air sealing in multifamily buildings is not only possible, but important. The evidence is developing that, just as in single-family buildings, air sealing is the single most cost-effective measure in most apartment buildings.


In single-family homes, we know that blower door testing is the fundamental, indispensable tool for residential energy improvements. We know that in single-family homes, air sealing is frequently the single most cost-effective measure completed. We also understand that infiltration flaws cause more performance problems than any other building defect. (Although we do recognize that bad flashing jobs may be a very close second!)

And we have learned the hard way that blower door testing is crucial. I think every crew has tried, at one time or another, air sealing by inspection. But air is too slippery and buildings are too complicated—it just doesn't work. So we have learned: If you don't test, you don't get infiltration improvements or energy savings.

When it comes to apartment buildings, it seems that the universal answer is to try to pretend that everything we know about infiltration doesn't apply. We've concluded that blower door testing is difficult, it's expensive, and it's not clear that it saves energy. For most apartment buildings in the United States, none of these reasons applies anymore.

With improvements in systems, especially in software control systems, blower door tests can now be completed efficiently and accurately in almost any low- or midrise building. The most flexible systems are built around the same residential-size blower doors that are used by every energy auditor in the country. I believe it is likely that within five years every program intending to complete real and meaningful energy improvements in apartment buildings will routinely perform in-and-out blower door testing of wood-frame buildings up to 100 units.

If we intend (especially in weatherization) to provide the benefits of our skill and experience to those people who need it most, apartments buildings are a good place for us to be. In a recent DOE weatherization guidance document, WPN 11-4 notes that fully 70% of all apartment dwellers qualify for low-income weatherization. And the authors of the guidance document further conclude that 30% of all U.S. households that qualify for weatherization rent their residence.


For our 50-unit project in the Milwaukee area, we assembled a team and made our plan. La Casa auditors had already been through the building, but we checked it out again to note critical features: the layout of doors and stairs, the location of power outlets, and the connections between utility spaces (boiler room, crawl spaces, and so on) and the core of the building. The test was completed in April 2010. I am not sure about the exact age of the building, but I'd guess it dates from the mid-1970s.



Testing multifamily buildings is typically done with multiple standard blower doors working in tandem. Examples are Minneapolis DG-700 systems from The Energy Conservatory or Retrotec's Q4E or Q5E systems. Both can be set up with two fans in a single pedestrian entry, so one technician can manage two fans at a time. In most multifamily buildings we test, two fans can be mounted at the fire exit at each end of the building, leaving the main center entry open for the inevitable tenant traffic in and out of the building. This four-fan setup will generally achieve useful (greater than 60 Pa) test pressures in buildings of up to about 40 units.

We have also conducted tests using the Infiltec G54 blower door system. This trailer-mounted, 60,000 CFM unit, with its 54-inch fan driven by a 25 HP gasoline engine, puts a lot of power in the hands of one technician. (A new upgrade package uses a 35 HP motor to develop 75,000 CFM50!) The G54 is admirably suited to testing open-plan commercial buildings, such as warehouses and factory buildings. In a number of tests with the G54, we have found that apartment buildings often do not have an exterior door large enough to accept the unit's 60-inch tunnel connector. Only some multifamily buildings have enough interior doors to distribute pressure and airflow evenly through the building. But if "big toys for big boys" is your mantra, the G54 certainly fills the bill!


Because so many technicians had never seen a large-building blower door test, we had more people than we needed. A typical crew would have been composed of six or seven blower doors, four technicians, and three management staff. In this case, we could only fit five blower doors easily, but we had six technicians, and those two additional techs saved us a lot of time. We started at 8 a.m. For our 50-unit, two-story apartment building, with 22,000 square feet of second-floor ceiling and 16,000 square feet of above-grade wall, 30,000 CFM of blower door capacity seemed about right. We could only find good placements for five Minneapolis 3 fans, each with its own digital gauge. If we could have found five higher-capacity Retrotec 3000 fans, the added capacity would have been useful. (For more on the technology, see "The Equipment.") Our building had four walk doors; we mounted a pair of fans, each with its own digital gauge (in double-hole capes) in the fire escape doors at each end of the building and a single-fan blower door in the back door. This left the main entrance open for traffic in and out of the building. One operator can control two fans at once so three fan locations required three operators. In the interest of getting the best precision (though with a slight risk of less accuracy) we connected all five digital gauges to a common outdoor pressure reference. We wanted to complete a multipoint test at a variety of pressures to get the most accurate results, and that requires careful coordination of everyone's work. So we distributed handheld radios to each operator and to the building manager at the front door. With all radios set on the same frequency, all operators were on a common communications system.


The blower door operators assembled and secured the blower doors. Two ¾ HP fans make a lot of noise and generate a lot of torque. Popping two fans out of the door, both running at full power—that's not a mistake you make twice! The techs also ran outdoor reference tubes to every digital gauge, keeping them near the hallway walls to minimize the possibility of pressure spikes caused by people stepping on the tubes.

A ¾ HP TEC fan running at full power pulls 5-7 amps. Retrotec 3000 fans can pull 15-20 amps. So whenever possible, our crews plugged every fan into a different building circuit. Using heavy-duty (10-gauge) power cords reduces the voltage drop to fan motors, keeping them running cooler and further reducing the likelihood of popping a fuse or breaker. If the auditor notes that the building wiring is on fuses rather than breakers, it is useful to note their size, and to have spare fuses on hand. These precautions reduce the delays from tripped circuit breakers or blown fuses in the middle of a test.

A team of two techs got onto the roof and sealed the air intake for the hallway supply furnace. Then they put the boilers and water heaters on standby (and left my car keys on the water heater!). They wedged the boiler room door closed, and secured the doors leading into the two crawl spaces under the building. The building managers opened every unit door and checked to see that all windows were closed and latched. They then dropped a door wedge at every apartment, leaving the door cracked just an inch or two, to be sure that it didn't lock when they left.

Once we had everything set, we reviewed our strategy and emphasized important details. Every technician had to be clear on what signal would be used in the event of a fire emergency, so they could rip their hardware out of the fire doors to allow for a speedy evacuation. Managers needed to be clear on how to manage tenant traffic in and out of the building.

With these last details reviewed, we went to work. Two building managers opened all the unit doors, and then took their stations—one in each hall—to maintain security in the building. The site manager took control of the front-door traffic. The test manager personally inspected every digital gauge, to be sure they were all set up the same (pressure and flow, five-second averaging, no baseline correction). We took three baseline pressures to gauge the strength of the wind, and then threw the juice to the fans and started testing.

The distributed-fan setup allowed us to get clean airflow through this large building. Air moves only to low pressure, but it is important to minimize pressure drops from the core of the building to the fans. A standard pedestrian door, with 20 square feet of area, allows about 10,000 CFM50 to flow with minimal pressure drop. So the open doors from the fire stairs into the halls on both floors allowed plenty of open area to move the air volumes needed. Retrotec 3000 fans move up to 8,000 CFM50, so mounting two fans in a single walk door may cause undesirable pressure drops unless there are two exit doors connecting the fans to the rest of the building.

We pulled the building to maximum pressure and then did a hasty survey. One tech walked the outside of the building, looking for open windows. (I don't believe that tenants do everything they can to screw up a blower door test, but it sure seems that way some days!) Two other techs surveyed the halls, looking for unusually large airflows that might indicate open windows, or doors to service areas that had pulled open.

Once we were sure we had proper control of the intended pressure boundary, we started testing. With all the fans balanced, we sent a call out on the radio, and the three technicians simultaneously pressed the hold buttons on all five gauges. Then they called in their results by radio to the test manager. With the gauges taken off hold, the test manager called out information to adjust fan speeds as needed to reduce the building pressure slightly, while keeping the building pressures the same throughout the entire building. We waited for a gust of wind to pass, and then captured another data point. In between data points, tenants were allowed in and out of the front door as needed.

Since we had two techs to spare, they spent their time in the attic and in the crawl spaces, with an IR camera and smoke pencils. They confirmed the leakage locations we knew about and found some other construction flaws, too.

On a calm day, accurate and precise results can be attained with 5 or 6 data points. TECTite will accept a maximum of 10 points, so a typical test will take 11 or 12 points. This makes it possible to delete 1 or 2 points badly affected by wind, while still maximizing the data analysis capabilities of the software. The leakage in this particular building was far greater than we expected, but we still got useful data at three pressures.

By 11 am, we had a number: 85,000 CFM50. This gave us an air-sealing goal of getting the building down to 20,000-25,000 CFM50. By noon, we were all packed and on the road.

Evaluating our effort, I estimated that our morning's work cost around $6,000, or $120 per unit. On a per-unit basis, testing those 50 units cost significantly less than testing the same number of houses. Doing that test again today (using new software and fan systems now available) would probably cost 10-25% less. (See "Software Improvements Are the Game Changer.")


Usually, it is best to test a large apartment building after the rest of the audit has been completed. The auditor should take special care to note the location of every exterior door and to identify multiple sheltered locations for outdoor references suitable for different wind directions. It works best if enough fan power can be fitted into doors in different parts of the building, leaving the main entrance open for the inevitable tenant traffic in and out.

It's important to have enough fan capacity. Based on the dozens of tests that we have completed to date, it appears that wood-frame low-rise apartment buildings in Wisconsin are pretty leaky. The infiltration rate averages 0.75 CFM50 per square foot of above-grade envelope, so it's best to have fan capacity of 1 CFM50 per square foot of envelope. (Curiously enough, preliminary—unpublished—data from a Focus On Energy program suggest that new-construction apartment buildings in Wisconsin are about as leaky as 40-year-old buildings in our weatherization program.) For testing a wood-frame apartment building, I usually try to have fan capacity sufficient to achieve pressures of at least 60 Pa. More is better, if the building has enough walk doors to hold more fans. (Apartments with patio doors make it easy to get plenty of fans distributed around a building.)

In large buildings, it is possible to get reasonably accurate (+/- 2-7%) data from a single-point test at 50 Pa on a calm day. However, even a moderate breeze can have significant effects on a large building. In those conditions, higher pressures (more fan power) can help overcome wind effects. However, the real secret to getting good results on windy days is to complete a multipoint test. Given the work involved to secure the building and set up the systems, it takes only a small amount of extra time to test at several different pressures. The best data for large buildings usually come from a multipoint test at 6 to 12 pressure levels.

With a test completed at 12 pressure points, multipoint test software simplifies the data analysis. Two good choices are The Energy Conservatory's TECTite software and Retrotec's MultiFanTestic program. Graphing the raw data in one of these tools helps to determine the effects of wind gusts, so that bad data can be removed from the analysis. (I frequently complete this preliminary inspection with an Excel spreadsheet.) Computer analysis also simplifies the chore of correcting for baseline pressures and stack effects. This greatly improves the accuracy and precision of a large-building test. The result is that careful tests can yield precision of better than +/- 1% at 50 Pa. Some new-construction programs require higher pressures and utilize 12-point tests in both directions (depressurize and pressurize) to achieve incredible precision (+/- 0.25% flow error at 50 Pa).


As in any energy efficiency project, the most important thing about testing multifamily buildings is to decide first what you want to accomplish. That will guide your testing and air-sealing work and will indicate what test method will be most useful.

In existing buildings, the most important goal is to align the pressure boundary of the building with the thermal boundary. This means that the most obvious way to test and treat these buildings is to close all the exterior doors and windows, open all the interior doors to create a single large zone, and test the building as a whole. The air-sealing work is then completed at that pressure boundary. Given that the goal is to reduce air leakage as much as possible, the goal of testing is to get accurate infiltration measurements, and to identify as many bypasses as possible.

How much can apartment buildings be improved? Teams that I have worked with have run six buildings (ranging from 30 to 50 units) through a full cycle of test/air seal/test again. From these pilot buildings, we have come to conclude that the average wood-frame apartment building in Wisconsin can be air sealed to reduce its infiltration rate by roughly 18%. The range was 11-33% reduction in CFM50. (Looking at the data from 3,600 single-family homes weatherized in Wisconsin in 2007, I found that the CFM50 reduction averaged 19.05%.)


Most of the infiltration sites we find in wood-frame buildings will be familiar to single-family technicians. They include plumbing and wiring penetrations, open soffits, open can lights, and vented crawl spaces.

However, air-sealing crews will have to learn a few new tricks to treat apartment buildings. Multifamily auditors learn to be wary of "tiger traps" in apartment attics, concealed under the insulation. In some wood-frame buildings (maybe 25% of them) some weird design irregularity will leave behind large openings in the top-floor ceiling, 1 foot or more square. They are often found near masonry boiler flue chases and next to elevator shafts. (A colleague of mine found a 6-foot x 10-foot tiger trap next to the elevator shaft in a four-year-old luxury condo building. It extended from the attic above the third floor all the way to the ceiling above the parking garage in the basement!) If you find any odd-shaped garbage chute closets or utility closets, it's very likely that the little triangular space beside them never got ceiling drywall when the building was built. The insulator frequently just throws a fiberglass batt over the top, to keep the rest of the insulation from falling into the chase.

Since these spaces already have the appropriate fire separation from the living space, they can often be treated just like balloon-frame house walls—filled in with expanded polystyrene (EPS) board foamed into place. However, it's wise to check with your local fire marshal before going to work. He or she may have a different opinion about the need for air-sealing materials that also serve as a fire barrier.

Skilled auditors will always find and assess the party walls in wood-frame apartment buildings. One common feature is what we know in Wisconsin as the split top plate party wall. (It is not unique to Wisconsin or the Midwest; I have talked to colleagues in the southeastern United States and in the Northwest who report identical construction in similar buildings.) Starting in the early 1970s, architects succeeded in greatly reducing noise transmission from unit to unit by building two separate walls to divide units, leaving an air gap between the two top plates. This bypass can be ½ inch to 2 inches wide, and can extend the entire length of every party wall.

In some buildings, the party wall is also a "wet wall." Apartment buildings are frequently built with the kitchens and bathrooms against the party wall and back-to-back from each other, so that two units can share electrical feeders, sewer lines, and supply plumbing. These wet walls can be 1 foot across. And yes, they often have no ceiling, except for the insulator's batt thrown over the top. The 1-foot x 8-foot bypass just doesn't do the building a whole lot of good!


Finally, it is important to note that multifamily buildings usually have service spaces that need some special thought. Boiler and mechanical rooms are required to have large combustion air grilles, and are also generally required by code to have a robust fire separation from the rest of the building. They belong outside the pressure boundary of the building. The fire door, fire walls, and mechanical room ceiling are supposed to be the fire barrier and the air barrier. And most codes assume (or pretend) that they are, without ever testing them! In fact, this air barrier is rarely well detailed.



For years, multipoint testing with multiple blower doors has required careful coordination and a walkie-talkie or cell phone communication system, since test personnel and blower doors are distributed around the building. It can take a lot of practice to achieve clear communication and perform tests efficiently.

It is only very recently that the final piece of the puzzle has been put in place. Now both The Energy Conservatory (with TECLog 2) and Retrotec (with MultiFanTestic) have developed stable, reliable software systems designed to use a laptop computer to control multiple blower door fans and capture test data in real time. Every door and gauge system communicates to the laptop via CAT5 computer cable and USB active hub connections. These "fly by wire" systems give one operator complete control of a dozen or more individual fans. They automate all data capture, reducing communications problems.

Using these new systems dramatically simplifies the mechanics of testing large buildings. They don't simplify building setup, nor do they make tenants more tractable. But the brute force testing method described above requires that every technician be knowledgeable about every aspect of blower door testing, and demands slow, careful communication to retrieve accurate data. The automated testing systems eliminate the most problematic chore—communicating around the building.

Most important of all, these laptop-driven systems can collect dozens of data points per minute, and calculate results instantly (while the fans are still set up). It is not clear that the actual accuracy of tests is improved with these systems, but the precision of the test (and thus our confidence in the data) is improved substantially.

Thus, the appropriate testing strategy is to leave the combustion air opening unsealed and wedge the boiler room door closed so that the fan pressure won't force it open. This way, the blower door fans "feel" the fire wall and measure its leakage as part of the building leakage, and the combustion air opening is isolated from the fans and from the building. (This is why you don't need two extra blower doors just to neutralize the combustion air opening.) The air-sealing strategy will then be to fill gross openings with drywall or cement board (not EPS board), and seal with fire mortar or high-temp caulk in place of foam. In some jurisdictions, this sealing work can be performed only by technicians licensed to perform fire and smoke stopping.


The best thing about properly testing and air sealing the mechanical room is that it essentially eliminates the need for CAZ and worst-case draft testing in these buildings. After all, once we verify by testing that it is impossible for the building to pull combustion hardware to negative pressures, the whole issue of backdrafting appliances becomes moot. If the boiler room is measured to be at -1 Pa with respect to outside when the building is at -50 Pa with respect to outside, then we know that the boiler room and the building are, for all practical purposes, completely independent of each other. How good a separation is good enough? One pascal at -50? Three pascals? Five? The truth is, that standard has not been written. The science has not yet been done, but it will be.

Garbage rooms need to be isolated from the building to control odors and pests. Workshop areas may not be separated, but if the staff use them for painting, welding, or soldering, the building and residents may benefit greatly by isolating them. In the best of all worlds, workshop spaces used for these purposes will also get their own ventilation system and space conditioning. In that case, make sure they get their own thermostat! (How do you get a super on your side? Help convince the owners and managers that their staff need the well-planned, well-ventilated workshop they've always wanted!)


In short, the evidence is becoming clear that testing and air sealing apartment buildings is feasible, practical, and valuable. Just as in single-family homes, good air sealing, verified by accurate testing, saves energy, improves resident health and comfort, makes buildings more durable, and reduces energy costs.


learn more

The Energy Center of Wisconsin offers a three-day class in large-building blower door testing twice per year (usually in the spring and fall.) Go to and search the site for multifamily blower door to find information on the next class.

For the recent DOE guidance document mentioned above, see Weatherization Program Notice 11-4 at

Completing accurate and precise blower door tests on these buildings is likewise feasible, practical, and valuable. Like every other aspect of working in apartment buildings, the work requires significant knowledge and careful planning. But it carries substantial economies of scale. An experienced crew can test a sizable building in less time and at less expense than it would take to test the same number of single-family homes. It's about time we started!


Don Hynek is the field coordinator for Wisconsin's large-building Multifamily Weatherization program. The program is on track to provide comprehensive weatherization service in 80 buildings of 20 units or larger, serving more than 5,500 households, by 2012.

The opinions, views and ideas expressed within this article are those of the author and do not necessarily reflect the official policy or position of any agency of the U.S. government.

This article is part of a series sponsored by Home Performance with Energy Star, jointly managed by the U.S. Department of Energy and Environmental Protection Agency.

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