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.
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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!

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