Home / Blog / May 2019 / Research sidesteps obstacles measuring air tightness in multifamily buildings

Research sidesteps obstacles measuring air tightness in multifamily buildings

Posted by Dave Bohac, P.E. | Date May 17, 2019

In October 2016, a research team began collecting data on low-rise multifamily buildings as part of a study led by Ecotope with support from CEE and Slipstream, and funded by the U.S. Department of Energy. Interpretation and enforcement of energy codes often differs across jurisdictions, and because of those differences, complying with energy code is generally more difficult than it needs to be.

This study will increase understanding of current practices and opportunities for improving energy efficiency in multifamily buildings — a crucial first step in standardizing the code to benefit building owners, designers, contractors, and occupants.

As part of the ongoing study, CEE researchers are examining air tightness in two types of low-rise multifamily buildings in Minnesota. (Similar tests are being performed in Illinois, Washington, and Oregon.) Understanding air tightness is important because leakage impacts how much you need to heat and cool each apartment. Residences with too much air leakage will experience higher energy bills and a less comfortable home than those with proper air tightness.

air-leakage-chart-(3).png In a single-family home, we can measure the air flow needed to depressurize the building with just one blower door, since we only need to measure leakage from the inside to the outside.

In multifamily buildings, however, there are several more places leakage can occur — including between units and common areas. And when it comes to energy efficiency, exterior leakage is more relevant than inter-unit leakage, because heating and cooling energy is lost to the outside rather than redistributed between units.

The simplest way to test air tightness in multifamily buildings is to perform a compartmentalization blower door test, measuring one unit’s air leakage at a time. This method measures exterior leakage of individual apartments, as well as all leakage between units and to hallways — which is problematic if your goal is calculating energy waste to the outside.

Another option for buildings with common areas is to open all units to the hallways and attempt to replicate the conditions of a single-family home, essentially creating one large internal space and using blower doors to depressurize the entire space at once. This whole building approach is a better guide for measuring energy waste, but requires a lot of equipment and coordination.

To address such challenges, CEE researchers recently used a guarded whole building approach to measure the total building exterior leakage of a 16-unit apartment complex in Minnesota, meaning they used blower doors in every unit and building exit at once, to neutralize air flow between units. While this approach requires additional equipment, coordination, and expertise, it also isolates the exterior leakage measurement that is most relevant to energy efficiency and code requirements.

CEE’s test included the largest number of “garden style” building units ever tested with this approach in the Midwest. In a garden style building, each unit opens to the outside, presenting more opportunities for exterior leakage than “common entry” apartments where interior hallways connect all units to a single entrance. In addition to measuring each unit’s exterior leakage, researchers also recorded each unit’s total leakage (including inter-unit), which should provide greater detail about the relationship between multifamily building leakage types than has previously been achieved in the U.S. Specific to the 16-unit Minnesota building, exterior leakage of first floor units averaged 34 percent of total leakage, and the average was 70 percent for the second floor units. (See chart above.)

CEE Senior Mechanical Engineer Russ Landry will present recent findings from the project at the 2019 National Energy Codes Conference the last week in May. At the end of the study, researchers will provide their assessment of the advantages and disadvantages of each blower door test approach, and recommend the best approaches to test units for energy code compliance.