The Three Insulation Requirements in Multifamily
Unlike single-family residential, multifamily construction has three distinct insulation requirements that need to be addressed separately:
- Thermal performance — Meeting energy code R-values for exterior walls, attic/roof, and floor assemblies
- Fire resistance — Meeting IBC 1-hour or 2-hour fire-rated assembly requirements at party walls, corridor walls, and floor/ceiling separations
- Sound attenuation — Meeting STC requirements at unit separation walls and floor/ceiling assemblies
Each requirement drives different product choices. Most multifamily projects use fiberglass on the exterior and mineral wool at party walls and corridors.
Exterior Walls: Fiberglass Meets the Spec
For the building envelope — exterior walls facing outside — fiberglass batts meet energy code requirements in all climate zones. Typical specs:
- Zone 1–3: R-13 or R-15 (2×4 wall), R-21 (2×6 wall)
- Zone 4: R-15 HD (2×4 wall), R-21 HD (2×6 wall)
- Zone 5+: R-21 HD (2×6 wall), often with continuous exterior insulation
Party Walls and Corridor Walls: Mineral Wool Required
IBC requires 1-hour or 2-hour fire-rated construction at walls between dwelling units, walls between units and corridors, and walls between units and other occupancies. These assemblies must use non-combustible insulation — specifically, mineral wool that meets ASTM E136.
Fiberglass does not meet ASTM E136 and cannot be used in these fire-rated assemblies. The standard product is mineral wool AFB (Acoustical Fire Batts) or SAFB (Sound Attenuation Fire Batts).
Standard multifamily party wall assembly: 2-hour rated — double stud wall with SAFB mineral wool fills the cavity. This achieves both fire-resistance and STC 55–65 depending on the specific assembly.
STC Requirements
Most state building codes require STC 50 minimum for walls separating dwelling units. Some jurisdictions require higher. Mineral wool SAFB in a properly constructed assembly can reach STC 65+.
STC performance by assembly type (approximate):
- Single-stud wall with fiberglass: STC 40–45
- Single-stud wall with mineral wool AFB: STC 48–52
- Double-stud wall with mineral wool SAFB: STC 58–65
- Staggered-stud wall with mineral wool SAFB: STC 55–62
Floor/Ceiling Assemblies
For multistory multifamily, floor/ceiling assemblies typically require:
- 1-hour fire resistance (IBC requirement between dwelling units)
- IIC (Impact Insulation Class) 50+ — covering footfall and impact noise
- STC 50+ — covering airborne sound transmission
Mineral wool in the floor joist cavity contributes to both STC and fire-resistance ratings. Combination assemblies with resilient channels and acoustic mat underlayment can achieve IIC 55+ and STC 60+.
Typical Insulation Order for a 100-Unit Apartment Building
A 100-unit mid-rise in zone 4 (e.g., Washington DC, Philadelphia) might require:
- R-15 HD fiberglass — exterior walls (~8,000 sq ft)
- R-21 HD fiberglass — 2×6 punched openings and corner conditions (~1,200 sq ft)
- Mineral Wool AFB — party walls and corridor walls (~12,000 sq ft)
- Mineral Wool SAFB — high-STC separation walls per architect spec (~4,000 sq ft)
- R-49 fiberglass — roof/attic if wood-frame top floor (~10,000 sq ft)
We can quote all products in a single order with line items for each assembly. Call (929) 466-1426 with your specs and sq footage and we'll put together a complete quote.
Managing Insulation Supply Across Multiple Units
For phased construction — where floors or wings are completed on different timelines — we can stage deliveries. Give us your completion schedule and we'll coordinate delivery for each phase so you're not storing product on-site longer than necessary.