A New Way to Reduce Multifamily Air Leakage

August 06, 2017
Fall 2017
A version of this article appears in the Fall 2017 issue of Home Energy Magazine.
Click here to read more articles about Multifamily

While tight exterior envelopes have become standard for single-family homes, they have been slow to reach the multifamily sector. Multifamily buildings have many of the same leakage paths as houses, as well as additional paths hidden in walls or other cavities that are difficult to seal with conventional methods. Researchers at the Western Cooling Efficiency Center (WCEC) at the University of California at Davis recently developed an aerosol sealant to seal leaks in building walls, floors, and ceilings. The process can bemore effective and convenient than conventional methods for sealing envelopes, because it requires less time and effort, and it seals the leakage area more quickly.

In 2014-15, researchers at the Center for Energy and Environment (CEE) and WCEC conducted a project to determine whether this sealant could be used in multifamily buildings. At the start of this project, the aerosol envelope-sealing technology was in precommercial development. The project team performed aerosol envelope-sealing demonstrations on six multifamily buildings. Three of these buildings were new construction and three were existing construction. While the work was performed on Minnesota multifamily buildings, the air leakage reduction results are generally applicable to all multifamily buildings.

Air Sealing Demonstration

Air Sealing Demonstration
Figure 1. As air escapes the building through leaks in the envelope, the sealant particles are carried to the leaks, stick to the edges of the leaks, and eventually seal them.

How the Technology Works

The aerosol envelope-sealing technology that was used in this project was developed by WCEC. It uses an automated approach to produce extremely tight envelopes. Air is blown into a unit while an aerosol sealant fog is released in the interior. As air escapes the building through leaks in the envelope, the sealant particles are carried to the leaks, stick to the edges of the leaks, and eventually seal them (see Figure 1). standard house or duct air leakage test fan is used to pressurize the building and also provide real-time feedback and a permanent record of the sealing. The technology is thus capable of simultaneously measuring, locating, and sealing leaks in a building.

Energy Program and Code Envelope Airtightness Requirements

There is a growing recognition of the need for tight multifamily building envelopes. The 2012 and 2015 versions of the

Sealed air leaks

International Energy Conservation Code require that one- to three-story multifamily buildings meet the residential energy code envelope tightness requirement of 3 ACH50. In addition, the EPA Energy Star Multifamily High Rise Requirements include a requirement for a maximum air leakage rate of 0.3 CFM50/ft2 of enclosure (EPA 2013). LEED v4 also has a prerequisite of 3 ACH50 for one- to three-story multifamily buildings with air infiltration credits for tighter envelopes. There is a prerequisite of 0.3 CFM50/ft2 or air-sealing checklist for midrise (four- to eight-story) buildings and annual energy use points for tighter envelopes. Effective air sealing technologies are necessary to meet these requirements.

Study Objectives

The aeroseal study on new and existing multifamily buildings had several objectives.

  • Refine the aerosol sealing technique.
  • Measure leakage and noise transmission reduction.
  • Determine how to incorporate aerosol technique into sealing strategy (for example, pre-seal “large” leaks and protect horizontal surfaces).
  • Estimate the labor and time needed for sealing.
  • Model energy savings and ventilation/inter-unit airflow.
Air Sealing

Aerosol envelope air sealing was performed on 9 units in three existing-construction multifamily buildings and 18 units in three new-construction multifamily buildings. The air-sealing protocol was adapted based on experience with past laboratory and field projects. The type of sealant deposition protection measures, temporary seals, manual presealing, and time required for all tasks were broken out for a subset of the sealed units. Multipoint, compartmentalization tests to measure total-unit air leakage were conducted on all units before and after the sealing. In addition, guarded air leakage tests were performed to break out exterior and interior envelope leakage. These tests used an additional fan to pressurize an adjoining unit or hallway during an air leakage test of the target unit. The reduced blower door flow rate of the target unit is approximately equal to the leakage to the adjoining space being pressurized. Pre- and postacoustic tests and documentation of sealant locations using a fluorescent dye in the sealant and black-light photography were conducted for some of the units.

Airflow and Energy Use Modeling

The airflow and energy use modeling was performed with EnergyPlus simulations to determine building airflows from wind, stack, and mechanical effects as well as the air leakage characteristics of each unit. Whole-building simulations often assume a constant air infiltration rate to represent the effects of uncontrolled infiltration driven by the natural forces of wind and stack effect and unbalanced mechanical ventilation. However, comparing the performance of different multifamily envelope tightness and ventilation strategies requires simulations that compute infiltration. The building airflows are computed from detailed information on the location and size of envelope air leaks along with inside air temperature and relative humidity (RH), outside air temperature and RH, wind speed and direction, and mechanical ventilation flow rates. The models were developed for four ventilation strategies, and the energy consumption was compared for each strategy before and after sealing.

Aerosol Sealing Process

The air sealing process consisted of several steps, as follows.

  1. Preseal large gaps. Gaps wider than 3/8 to 5/8 inch and leaks located where the aerosol will not stay in suspension should be manually sealed.
  2. Cover finished horizontal surfaces. Some of the sealant will settle on horizontal surfaces during the aerosol sealing process. These surfaces should be protected with plastic, duct mask, or masking tape.
  3. Set up equipment and perform sealing. One sealant spray nozzle is typically placed in every bedroom and living area. The unit is then pressurized to about 100 Pa while an aerosol sealant fog is released in the interior. The sealing typically takes 45 minutes to a couple of hours.
  4. Remove coverings and clean surfaces. Windows must be opened and fans set to high to purge remaining sealant; surface protection should be removed, and any remaining residue cleaned up.
  5. Conduct a postsealing air leakage test. This test should be conducted after all penetrations in the envelope have been made.


Aerosol envelope sealing was performed on a convenience sample consisting of 18 units in three new- construction buildings and 9 units in three existing-construction buildings, as described above. Key characteristics and presealing leakage results are shown in Table 1.

New Construction

Presealing Leakage
Figure 2. Variation in unit leakage for units in newly constructed buildings.


Existing Construction

Existing Construction

Existing Construction
Figure 3. Variation in unit leakage for units in existing buildings.

The research team conducted the sealing using an equipment design modified from previous field tests and the protocol described above under “Methodology.” Figures 2 and 3 show reduction in unit leakage for four new-construction units and six existing-construction units, respectively. In general, the sealing rate was greatest for the first 30 minutes and decreased steadily thereafter.

The aerosol envelope sealing of new-construction and existing-construction units successfully demonstrated high levels of air leakage reduction with no damage to the finished surfaces. For the new-construction units, the reduction varied from 67% to 94% with an average of 81% (see Figure 4). For all of the units the total leakage was more than 50% tighter than the 3 ACH50 code requirement for low-rise residential buildings, and half of the units met the Passive House tightness requirement of 0.6 ACH50. In addition, all of the units were at least 80% tighter than the EPA Energy Star Multifamily High Rise requirement of 0.3 CFM50/ft2.

Postsealing Leakage: New Construction

Postsealing Leakage: New Construction
Figure 4. Pre- and postsealing unit leakage and percent reduction for new-construction units.

Results were equally impressive for existing buildings, sealing an average of 68% of the unit leakage (see Figure 5). The tightness achieved was less consistent, with two of the tests sealing only 39% of the available leakage—which in one case was due to large hidden leaks behind a kitchen cabinet. The preseal results show initial total leakage levels of 12–17 ACH50 and postseal results of 1.4–10.5 ACH50. This indicates that with manual presealing of larger leaks, the aerosol sealing process can realistically reduce air leakage in existing apartments to meet or exceed the new-construction low-rise residential code requirement of 3 ACH50.

Postsealing Leakage: Existing Construction

Postsealing Leakage: New Construction
Figure 5. Pre- and postsealing unit leakage and percent reduction for existing-construction units.

Labor Requirements

The total time required to complete the ten tasks for the air sealing process was tracked for three of the six buildings. The average task labor times for all sealed units for the three buildings are shown in Figure 6. The total time per unit for the sealing process varied from 14 to 22 person hours. However, this was a research project with staff who were being trained on the process. With trained personnel labor time might well be reduced by a factor of 2 or greater. Labor time would be reduced by

  • presealing large leaks;
  • performing sealing at a time when there are a minimum of finished surfaces to cover; and
  • using new, more-portable and automated equipment.
Time Required to Complete Six Tasks

Time Required to Complete Six Tasks
Figure 6. The total time per unit required to complete the ten tasks that comprise the air sealing process was tracked for three of the six buildings. Average task labor times for all sealed units of each building are shown in hours per unit.

Energy Savings Modeling: New Construction

The new-construction modeling compared the energy performance of two buildings. The units in the first building had a total (exterior and interior) envelope leakage of 3 ACH50. The units in the second building, which were sealed using the aerosol process, had a total envelope leakage of 0.6 ACH50, for a reduction of 80%. This 80% reduction in envelope leakage approximates the 81% average reduction for the aerosol sealing of the 18 new- construction units completed for this project. These savings would be applicable to a building that needs to meet a code-required tightness level of 3 ACH50 and that uses aerosol sealing to exceed the code requirement. This might be done either for a high-performance utility program or for a green-building rating program.

The 80% reduction in envelope leakage also saved heating energy (see Figure 7). This can be translated into annual cost savings. For example, a system using balanced ventilation would save 27 therms at $0.58/therm for an annual cost savings of $15. Thus the cost of sealing by the aerosol method would have to be $150–225 per unit for a 10–15-year payback. This assumes that the aerosol process is an add-on that reduces the leakage of a unit in a low-rise multifamily building from the code-required level to a very tight level. However, aerosol sealing might eliminate the need to achieve a tighter envelope using conventional methods and higher levels of quality control. In that case it might actually cost less than conventional alternatives.

When the modeling for this project was performed, researchers expected that the 3 ACH50 code requirement would apply to the total unit leakage. However, Minnesota code officials have indicated that the 3 ACH50 requirement applies to the exterior leakage only. This allows units to be leakier than they would be if the requirement applied to the total leakage. Increasing the leakage of the baseline model results in higher absolute savings for the new-construction sealing, and is closer to the savings reported for the sealing of existing construction, as described below.

Modeled Energy Savings for New Construction

Modeled Energy Savings for New Construction Figure 7. Modeled annual space-heating energy use and savings for new-construction units.

Energy Savings Modeling: Existing Construction

The modeling for existing construction focused on comparing the energy performance of an existing building that was sealed to the low-rise multifamily code requirement for new construction. The two total envelope leakage levels modeled for the existing buildings were 9.5 ACH50 and 3.0 ACH50.

The results show an 11% to 25% reduction in heating energy use due to sealing the envelope with annual gas savings of 41 to 68 therms and cost savings from $24 to $39, which may not be sufficient for many building owners (see Figure 8). However, the modeling results were based on a 68% reduction from a starting leakage of 9.5 ACH50, and the average pre-sealing leakage of the nine existing units was over 14 ACH50. A pre-sealing leakage of 15 ACH50 and a reduction of 75% would increase annual savings by about a factor of two. The simulations assumed that 43% to 47% of the total leakage was to the exterior, but if the percent exterior leakage for the models was 68%, the savings would have been about 50% greater. Under certain factors, leakier units could see higher savings of three times or more (e.g. $70 to $120 per year).

Modeled Energy Savings for Existing Construction

Figure 8. Modeled annual space-heating energy use and savings for existing-construction units.

Utility Program Recommendations

Existing Construction. Three Minnesota utilities provide incentives for envelope air sealing. The CenterPoint Energy/Xcel Energy Multifamily Building Efficiency program includes envelope air sealing as a custom measure. To qualify for an incentive, the air sealing work must have a payback of less than 20 years, representing the life of the measure. The Minnesota Energy Resources Multifamily Direct Install Plus program lists envelope air sealing as one of the measures targeted for investigation, and air sealing work may qualify for a custom rebate. We recommend that all Minnesota utility programs for existing multifamily buildings include incentives for envelope air sealing.

Energy Savings Calculation. The State of Minnesota Technical Reference Manual for Energy Conservation Improvement Programs (2016) includes an algorithm for residential and small commercial buildings, but it is not directly applicable to multifamily units. There is currently no generally accepted methodology for computing multifamily envelope air sealing savings. The current calculation includes a value for n_heat, which is the conversion factor from leakage at 50 Pa to leakage at natural conditions, building height, and exposure level. The modeling results from this project indicate that a value of 25 should be used for n_heat of existing multifamily buildings with less than 50 CFM of continuous unbalanced mechanical ventilation that are well shielded from wind. The value should be reduced to 21 for normal wind shielding and 19 for exposed shielding. The relationship between exterior envelope leakage reduction and space heating savings is not only impacted by the type of ventilation system and wind shielding, the building height and climate also affect energy savings from air sealing. Similar building air flow modelling should be performed to properly relate sealing reductions to energy savings for other parts of the country.

An evaluation of the building ventilation system should be conducted and recommended upgrades completed when any significant exterior envelope air sealing is performed. Exterior air sealing is not recommended when units do not have a mechanical ventilation system.

New Construction. Xcel Energy and CenterPoint Energy offer design assistance programs for commercial and industrial new construction and major renovation, including construction and renovation for multifamily buildings. The program[s] provides consulting services and energy modeling, as well as electricity and natural gas efficiency implementation rebates. Although a tighter building envelope and associated air infiltration reduction is not a standard measure for the program, these factors can be modeled if requested by the design team. We recommend that the modeled air infiltration results from this demonstration project be used for baseline and reduced envelope tightness infiltration values for design assistance programs.

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The airflow modeling conducted for this project suggests that for Minnesota multifamily buildings design assistance program building energy models should use a baseline air infiltration rate of 0.16 ACH for buildings with normal wind shielding. The baseline is reduced to 0.13 ACH for well-shielded buildings and increased to 0.18 ACH for exposed buildings. The percent reduction in modeled air infiltration should be the difference between the measured exterior envelope leakage and the low-rise residential code requirement of 3 ACH50. Given the high level of energy savings achieved in this demonstration project, aerosol envelope sealing may be one of the most cost-effective sealing methods for multifamily units that are required to meet more-stringent compartmentalization requirements.

Dave Bohac is the director of research at the Center for Energy and the Environment (CEE).  Curtis Harrington is a senior engineer at the UC Davis Western Cooling Efficiency Center where he manages research projects on a variety of topics including evaporative cooling technologies, aerosol-based sealing methods, and building energy modeling.

This project was supported in part by a grant from the Minnesota Department of Commerce, Division of Energy Resources, through the Conservation Applied Research and Development program.

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