Scope
Air seal and compartmentalize multifamily buildings to reduce uncontrolled infiltration, exfiltration, and airflow between dwelling units.
- Air seal to provide a complete air barrier for each individual unit and for the whole building in multifamily buildings, whether low-rise, midrise, or high-rise multistory buildings. Approaches may differ for interior-to-interior air control layers and interior-to-exterior air control layers.
- Address the fire rating requirements for shared partition walls (“party walls”).
- Address the effect of mechanical service cavities on compartmentalization in multistory buildings.
- Conduct air pressurization testing to verify the building and individual units meet program and code requirements for air tightness.
See the Compliance Tab for related codes and standards requirements, and criteria to meet national programs.
Description
This guide provides design and construction details and approaches to meet the required air sealing requirements for air leakage of no more than 0.30 cfm50/ft2 of dwelling unit enclosure area for ENERGY STAR Multifamily new construction and no more than 0.25 cfm50/ft2 of enclosure area for the U.S. Department of Energy’s Zero Energy Ready Home Program.
Air sealing strategies using either interior air control layers or a hybrid approach of interior and exterior control layers are presented. When an interior control layer, notably gypsum board, is used, approaches and details are similar whether the building assembly consists of wood frame, steel frame, or mass building enclosures. Complications associated with air sealing of vertical mechanical service cavities in multistory buildings are discussed. Guidance is also provided on enclosure leakage testing.
Compartmentalization
Compartmentalization is the term used to describe approaches to control air flow in multifamily buildings by essentially turning the building into a series of single building units that share the same space. Each unit is isolated from the others so that they do not communicate with each other in terms of air flow. In other words, compartmentalization is like turning a ten-story building into ten one-story buildings that are stacked on top of one another. Or, for rowhouses or townhouses, it is like turning a ten-unit building into a row of ten individual houses that abut each other. Figure 1 illustrates this concept.
Compartmentalization is necessary to reduce fire and smoke risk, provide safe egress during fire events, enhance indoor air quality, and help control odors. When the individual units of a multistory, multifamily building are not compartmentalized, or separated from each other by installing air sealing and air barriers, then outside air can be pulled into the lower part of the building and conditioned air can be pulled out of the upper floors due to the stack effect. Additionally, air from one unit can be drawn into adjacent units as air is pulled through the building. Figure 2 illustrates the impact of the stack effect on non-compartmentalized units in a multistory multifamily building. The term “stack effect” takes its name from a chimney stack where buoyant heated air flows out of the top of a chimney while air is drawn into the base of the chimney from the room below. This stack effect can be observed when trying to open an outside door on the ground floor of a tall building. The stack effect can create such an inward and upward pull that it can be difficult to open the door.
Compartmentalization, or thorough air sealing of each unit and between floors, stops this vertical air flow. Figure 3 shows the effect of compartmentalization - instead of a single large building stack effect, a “one-story” stack effect is seen at each floor. This is like turning a very tall chimney with a strong draw into much shorter chimneys with weak draws. This reduction in pressure differentials between the bottom floors and top floors results in reduced air leakage throughout the building.
Figure 4 illustrates the concept of compartmentalization in multistory multifamily buildings, where each dwelling unit is considered to be an individual unit consisting of a “six-sided” cube that needs to be “air sealed” on all six surfaces to isolate it from the other units in the building so that they do not communicate with each other in terms of air flow. Compartmentalization reduces pressure differences across each unit by preventing horizontal air flow between units.
Figure 5 shows the “plan view” of one floor of a multistory multifamily building. The gray lines indicate the continuous air barriers that need to be established around each dwelling unit. The red lines indicate the air barriers separating each common space. The elevator shafts are isolated from the corridors by using “vestibules” like the one shown in Figure 6. The vestibules need to be air sealed on all six sides and have tightly fitting weather-stripped doors. The vertical service shaft and trash chute are isolated from the corridors by locating them within the air barrier of the vestibules (Figure 7). The stairwells are isolated from the corridors with weather-stripped fire doors. The individual dwelling units need to be air sealed and isolated from these non-unit spaces as well as from adjacent dwelling units. The principles of air sealing and compartmentalization are the same for wood frame, steel frame, and mass building enclosures.
Doors from individual units to the corridors are weather-stripped (Figure 8). When unit air leakage pressure testing is conducted, the fan testing equipment is often installed in the unit-to-corridor door and as such will not identify any leakage associated with the unit-to-corridor door. The unit-to-corridor door should be visually inspected to verify the presence of weather-stripping.
Mechanical Systems and Compartmentalization
For compartmentalization to be effective, mechanical systems should also be “compartmentalized” or designed to address compartmentalization. Air sealing must be performed around ductwork where it passes through floors, and the walls of shafts and chase ways and the ducts themselves must be air sealed.
Hot water, chilled water, and refrigerant systems do not move air and are contained in sealed pipes. It is common in multistory multifamily buildings to locate individual-unit air conditioning or heat pump systems on roofs or at ground level. These “remote” individual unit systems are coupled to air handlers or coils located in each individual dwelling unit. This approach keeps the air-moving system within the boundaries of the compartmentalized dwelling unit. Similarly, each corridor should have its own system and each elevator vestibule should have its own system.
Construction Details - Corridors
Figure 9 shows the air sealing approach for isolating corridor service chases from individual dwelling units. Gypsum board is installed at the ceiling and sides of the service chase and is sealed with sealant, tape, and drywall mud prior to installing HVAC ducts, pipes, and wiring. This air sealing is critical to fire safety and smoke control. Most corridors have “dropped ceilings” that allow space for services to run to individual units. Note this gypsum will be installed before installation of gypsum for the remaining ceilings and walls and must be planned for in advance.
Figure 10 shows a method for providing corridor compartmentalization when a truss floor structural system is used. The gypsum board on the corridor walls extends upward past the corridor trusses and is sealed to the underside of the floor sheathing of the floor above. These approaches work whether wood framing or steel framing is used.
Construction Details – Dwelling Unit Separation
As noted previously, an effective approach to isolate individual units is to use gypsum board to provide the air sealing and airtightness of the individual dwelling units. Note that the units need to be isolated from the exterior as well as from neighboring adjacent units. Think of a gypsum board “cube” where 4 of the 6 sides of the cube are gypsum board and the bottom of the cube is a floor assembly (sheathing or concrete) and the top of the cube is the floor sheathing or floor assembly above (or attic ceiling gypsum board in the case of the top floor). It is necessary to seal the tops and bottoms of the exterior walls, window openings, and intersecting interior walls (Figures 11, 12, and 13).
The demising walls that separate the dwelling units and the interior walls that intersect corridor walls should be air sealed with fire-rated sealants and fire-rated assemblies (Figures 14 and 15). Joint compound by itself is not rated but, when used in a fire-rated assembly, joint compound is acceptable – the components acting together establish the rating. Further note in Figure 15 that OSB was installed as a solid air barrier between the floor trusses. These approaches work whether wood or steel stud faming is used.
It is necessary to air seal where an interior wall intersects a corridor wall. Figure 16 illustrates how joint compound is used to air seal along the first stud adjoining the intersecting wall. Joint compound is also used for air sealing at all gypsum board seams, nail holes, and points where the floor trusses cross the gypsum board to provide air-sealing in the corridor wall.
Air Sealing Bathtub and Shower Enclosures
Bathtub and shower enclosures can be a large source of air leakage to the outside in single- and multifamily buildings, especially when the tub or shower is installed along an exterior wall or a demising wall separating one dwelling unit from another. The bathtub or shower enclosure should be air sealed with a rigid material such as gypsum board or fiber cement sheet products that have been tested to provide a fire-rated assembly (Figures 17-20). The bathtub and bathtub enclosure air sealing has to be in place prior to installation of the bathtub and bathtub enclosures. This is often referred to as “prerocking.” Air sealing includes sealing joints in the gypsum board and sealing the gypsum board or other rigid material to the bottom plates and floor assembly as well as air sealing around any pipes that go through the walls or subfloor.
Air Sealing Electrical Outlets and Plumbing Pipes
Electrical outlets on exterior walls or unit-separation demising walls should be sealed with a fire-rated sealant (Figures 21 and 22). Plumbing penetrations need to be sealed at the top plates (Figure 23).
Construction Details – Shared Partition Walls (“Party Walls”) – Multifamily Row Houses
Shared partition walls (“party walls”) need to meet fire rating and separation requirements. Numerous assemblies are possible. The three most common are:
- 2-hour multifamily row-house separation wall (Figure 24)
- Double-1-hour multifamily row-house separation wall (Figure 25)
- 1-hour multifamily row-house separation wall (Figure 26).
Each approach will be described. The use of one approach versus another is typically based on whether plumbing or mechanical ducts are included in the assemblies. Although each approach described in Figures 24-26 may meet the “listed” fire rating, they do not provide sufficient airtightness to meet the required airtightness limits of 0.30 cfm50/ft2 of enclosure specified in the ENERGY STAR Multifamily New Construction (MFNC) program or 0.25 cfm50/ft2 of enclosure specified in the DOE Zero Energy Ready Home program. To meet the required airtightness, air sealing modifications are needed. These modifications, which are described below, do not affect the listed fire ratings.
Note that, although mechanical ducts can be included in some fire-rated demising wall assemblies, this should be avoided. It is akin to installing ductwork in exterior walls. Ductwork adjacent to separation walls should always be located in interior walls or shafts located to the interior of the rated assemblies.
Figures 27-30 show details for 2-hour separation walls using two layers of fire-rated gypsum between two frame walls. Figure 29 shows the air sealing necessary at party wall-to-attic interfaces. Figures 30-32 show the air sealing necessary at party wall-to-exterior wall interfaces to meet the required performance requirements of 0.30 cfm50/ft2 of enclosure specified in the ENERGY STAR Multifamily New Construction (MFNC) program or the 0.25 cfm50/ft2 of enclosure specified in the DOE Zero Energy Ready Home program. The flexible membrane connections allow for differential movement. The air sealing of the gypsum board that provides compartmentalization needs to be continuous from the ceiling gypsum board to the party wall gypsum. This is accomplished by sealing the ceiling to the top plate framing and the top plate framing to the party wall gypsum.
The air sealing in Figure 30 requires that the gypsum fire wall is airtight to the exterior sheathing. Installing a single strip of a membrane air sealing tape across the vertical gap between the two framed demising walls is not an effective air seal. Two strips are necessary. The first strip seals the gypsum fire wall to the framing of one demising wall. The second strip seals the second frame demising wall to the gypsum fire wall. Two vertical beads of sealant are then added to provide an air seal of the two frame demising walls and gypsum fire wall to the exterior sheathing.
The membrane air seals shown in Figures 31 and 32 are typically an exterior-rated fully adhered membrane sheathing flashing that is used to provide air management and water management on exterior sheathings.
The two other typical fire-rated frame walls (double-1-hour separation walls and 1-hour separation walls) are described in Figures 33-36. Note the flexible membrane air seal connections between wall assemblies to allow for differential movement. Figure 34 shows the air seal connection of the intermediate floor framing to the exterior wall. Note the two steps shown in Figure 34. Figure 36 shows the air seal connection of the upper ceiling assembly to the exterior wall prior to the installation of the roof framing assembly (trusses, rafters). The flexible membrane air seal is typically a fully adhered roofing underlayment that is cut into widths appropriate to the geometry of the assembly being sealed. Note that Figures 34-36 apply to corridor walls as well as to exterior walls.
Success
The building enclosure surfaces, corridor assemblies, and party walls (shared partition walls) between units should be visually checked to ensure that the gaps and connections between sheathing and gypsum board layers and framing are sealed. Air sealing and compartmentalization effectiveness is measured in individual units using air leakage testing conducted with a blower door tester as shown in Figure 1. The ENERGY STAR Multifamily New Construction (MFNC) program requires that air leakage be no more than 0.30 cfm50/ft2 of enclosure. The DOE Zero Energy Ready Home program specifies air leakage can be no more than 0.25 cfm50/ft2 of enclosure.
Climate
The approaches are climate independent – they work in all climates.
See the Compliance tab for more information on the minimum levels of roof assembly thermal resistance.
Training
Compliance
Compliance
The Compliance tab contains both program and code information. Code language is excerpted and summarized below. For exact code language, refer to the applicable code, which may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.
ENERGY STAR Multifamily New Homes, Version 1 (Rev. 03)
National Rater Field Checklist
Thermal Enclosure System.
4. Air Sealing (Unless otherwise noted below, “sealed” indicates the use of caulk, foam, or equivalent material).
4.8 In multifamily buildings, the gap between the common wall (e.g. the drywall shaft wall) and the structural framing between units sealed at all exterior boundaries.
Please see the ENERGY STAR Single-Family New Homes Implementation Timeline for the program version and revision currently applicable in your state.
DOE Zero Energy Ready Home Multifamily
Exhibit 1 Mandatory Requirements.
Exhibit 1, Item 2) Certified under the ENERGY STAR Multifamily New Construction Program, V 1.2.
4.10 Rater-measured compartmentalization is no greater than 0.30 CFM50 per square feet of dwelling unit enclosure area, following procedures in ANSI / RESNET / ICC 380.38
2009 International Energy Conservation Code (IECC)
Table 402.4.2 Air Barrier and Insulation Inspection Component Criteria, Common wall: Air barrier and sealing exists on common walls between dwelling units.
2012, 2015, 2018, and 2021 IECC
The IECC does not specifically address sealing multifamily party walls. Table R402.4.1.1 Air Barrier and Insulation Installation, Walls: Junction of foundation and wall sill plates, wall top plate and top of wall, sill plate and rim-band, and rim band and subfloor are sealed. Corners, headers, and rim joists making up the thermal envelope are insulated.
Retrofit: 2009, 2012, 2015, 2018, and 2021 IECC
Section R101.4.3 (in 2009 and 2012). Additions, alterations, renovations, or repairs shall conform to the provisions of this code, without requiring the unaltered portions of the existing building to comply with this code. (See code for additional requirements and exceptions.)
Chapter 5 (in 2015, 2018, 2021). The provisions of this chapter shall control the alteration, repair, addition, and change of occupancy of existing buildings and structures.
2009 International Residential Code (IRC)
Table N1102.4.2 Air Barrier and Insulation Inspection, Common wall: Air barrier and sealing exists on common walls between dwelling units.
2012, 2015, 2018, and 2021 IRC
Table N1102.4.1.1 Air Barrier and Insulation Installation, Walls: Junction of foundation and wall sill plates, wall top plate and top of wall, sill plate and rim-band, and rim band and subfloor are sealed. Corners, headers, and rim joists making up the thermal envelope are insulated.
Retrofit: 2009, 2012, 2015, 2018, and 2021 IRC
Section R102.7.1 Additions, alterations, or repairs. Additions, alterations, renovations, or repairs shall conform to the provisions of this code, without requiring the unaltered portions of the existing building to comply with the requirements of this code, unless otherwise stated. (See code for additional requirements and exceptions.)
Appendix J regulates the repair, renovation, alteration, and reconstruction of existing buildings and is intended to encourage their continued safe use.
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Access to some references may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.
The following authors and organizations contributed to the content in this Guide.
Building Science Corporation