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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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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