Multifamily Party Walls
No climate specific information applies.
Common walls (also called party walls) between units in multifamily housing (e.g., townhouses, duplexes, and apartments) should be constructed as airtight assemblies for sound, smoke, fire, and air quality control. However, experience has shown that these common walls can often be significant sources of air and heat loss if gaps or cracks exist in the connections between each unit’s walls.
To reduce air leakage, common wall assemblies should be air sealed at all boundaries. Wood-framed walls are sealed with fireproof spray foam. Masonry block party walls, which form “chimneys” because of their open cores, can be air sealed with two-component urethane foam, which also reduces sound and odor transfer, and dust, insect, and moisture entry. These walls are fire-rated assemblies for each unit. Acceptable materials for air sealing common walls can vary significantly around the country. Confirm with local code officials which material is preferred for fire safety reasons. Any plumbing and wiring penetrations through the drywall surfaces of the common walls should be sealed with fire-rated sealant materials (BSC 2009). The 2009 IECC requires that air barriers be installed in common walls; for more information, see the Compliance tab in this resource guide (Otis and Maxwell 2012).
Air sealing of common walls might be done by the framer. This task should be included in the contract for the appropriate trade depending on the workflow at the specific job site.
How to Air Seal a Common Wall
Check local code requirements to determine appropriate fire-rated materials for air sealing common walls between dwelling units. Such materials include intumescent caulks, fire-rated (high-temperature) caulks, and fire-rated one-component foam (Otis and Maxwell 2012).
Use caulk, foam or equivalent material to seal the drywall to the top plates and bottom plates of the common walls. Also seal at the seam between the top and bottom plates and the exterior sheathing.
Caulk the first stud of each partition wall to the exterior wall studs.
Figure 3 - The partitions walls are caulked to the exterior wall. A continuous piece of sheathing is run across the property line or if there is a seam in the sheathing at the dividing line it is sealed with elastomeric caulk or mastic paste
Common walls (also called party walls or shared partition walls) between units in multi-family buildings should be visually checked to ensure that the gap between the drywall and the structural framing is sealed with caulk, foam or other sealing material. Air barrier effectiveness is measured at the whole-house level. Blower door testing, which is conducted as part of the whole-house energy performance test, may help indicate whether common walls have been successfully sealed.
In multifamily buildings, the gap between the drywall shaft wall (i.e., common wall) and the structural framing between units fully sealed at all exterior boundaries.
- The gap between walls must be declared an approved assembly before being air sealed.
- Seal the bottom plate to sub-floor.
- Seal the bottom plate to sheathing connection.
- Seal gap between units from exterior at all common wall locations with caulk, foam, or equivalent material. (Typically fire rated foam is required by code).
Thermal Enclosure Checklist, Air Sealing. Cracks in the building envelope fully sealed. In multifamily buildings, the gap between the drywall shaft wall (i.e. common wall) and the structural framing between units fully sealed at all exterior boundaries
DOE Challenge Home
Exhibit 1: Mandatory Requirements. Certified under ENERGY STAR Qualified Homes Version 3
Table 402.4.2 Air Barrier and Insulation Inspection Component Criteria, Common wall: Air barrier and sealing exists on common walls between dwelling units.*
Table N1102.4.2 Air Barrier and Insulation Inspection, Common wall: Air barrier and sealing exists on common walls between dwelling units.*
The 2012 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.*
Table N11402.4.1.1 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.*
*Due to copyright restrictions, exact code text is not provided. For specific code text, refer to the applicable code.