Horizontal assemblies — floor-ceiling constructions separating one storey from the next — carry two distinct types of noise. Airborne sound travels as pressure waves through the air and through the materials themselves. Impact sound is generated by a physical force applied directly to the structure: footsteps, dropped objects, furniture movement. These two categories require different treatment strategies, and many assemblies need to address both simultaneously.
In Canadian multi-storey construction, floor-ceiling assemblies between dwelling units are regulated under the National Building Code. The code specifies minimum ratings for both Sound Transmission Class (STC, for airborne noise) and Impact Insulation Class (IIC, for impact noise). Both minimums are set at 47 in the current NBC, though many acoustic consultants and building owners aim higher, particularly in urban settings.
Understanding the Two Noise Types
Airborne Noise Through Floors and Ceilings
Airborne noise from an upper unit — conversation, television, music — causes the floor assembly to vibrate, which then radiates sound into the lower unit. The primary countermeasures are the same as for walls: mass, absorption, and decoupling. A well-insulated joist cavity with mineral wool and a resilient ceiling system below can achieve STC ratings in the mid-50s.
Impact Noise
Impact noise is more difficult to control than airborne noise because it is introduced directly into the structure. A person walking in hard-soled shoes transmits energy into the floor, which propagates through joists and into the ceiling below, often with very little attenuation. IIC performance is measured using a standardized tapping machine; the resulting number describes how well the assembly reduces this standardized impact.
Rugs and carpets on the upper floor are the most straightforward impact noise reducer available. A carpet with a thick pad can raise IIC values by 20 or more points without any structural modification. However, in many Canadian condominiums, hardwood or tile floors are the norm and cannot be replaced with carpet. Structural solutions are then required from below or within the assembly.
Ceiling-Side Treatment Approaches
Resilient Channels on Ceilings
Resilient channels are the most common retrofit approach for existing ceiling assemblies. Metal channels are screwed to the underside of the joists, and new drywall is attached to the channels rather than directly to the joists. This creates a mechanical decoupling that prevents vibration from passing directly from the structural floor to the ceiling surface.
Correct installation is critical. The channel must be attached to joists with a single screw per joist crossing, placed slightly off-centre on the channel's mounting leg. Drywall screws must penetrate the channel only, never the joist behind it. This error — known as "short-circuiting" the channel — connects the ceiling directly to the structure and negates almost all of the acoustic benefit.
Sound Isolation Clips (RSIC / Resilient Sound Isolation Clips)
Isolation clips provide a more robust form of decoupling than resilient channels. A rubber or neoprene element within each clip absorbs vibration before it reaches the hat channel and drywall. RSIC-type systems typically achieve 5 to 10 STC points better than standard resilient channels in laboratory conditions, with similar gains in IIC performance.
The higher material cost of isolation clips is often offset by their more consistent field performance. Resilient channels are more sensitive to installation errors; clips with integrated rubber isolators are somewhat more tolerant of minor deviations in installation technique.
Joist Cavity Insulation
Filling the joist cavity between floors with mineral wool absorbs airborne sound energy within the cavity before it can vibrate the ceiling surface. An open joist cavity allows sound to resonate and amplify within the space; insulation eliminates this resonance chamber effect.
For impact noise, cavity insulation alone does little. The structural path carries impact energy through the joists themselves, bypassing the insulated cavity. Decoupling the ceiling surface is necessary for IIC improvement.
Adding a second layer of 5/8-inch drywall to a resilient channel ceiling, with acoustic sealant at all perimeter joints, contributes additional mass and reduces the likelihood of air leakage. The combination of resilient channels, mineral wool cavity fill, and double drywall is a common specification for upgraded floor-ceiling assemblies in Canadian mid-rise renovation projects.
Floor-Side Treatment Approaches
Floating Floors
A floating floor assembly rests on a resilient underlayment layer rather than being fastened directly to the subfloor. The underlayment — typically rubber, cork, foam, or a composite material — acts as a spring-mass system that absorbs and dissipates impact energy before it enters the structure.
The effectiveness of a floating floor depends on the density and compression characteristics of the underlayment material. Products marketed as "acoustic underlayment" vary considerably in performance. Materials with independent lab testing data and published IIC values provide more reliable information than marketing claims alone.
Concrete Floating Slabs
In higher-performance applications, a concrete or gypsum-based floating slab is poured over a resilient mat, separated from the structural slab by the mat's thickness. This approach is used in purpose-built music practice rooms, recording facilities, and some high-end residential construction. The mass of the concrete combined with the resilient support achieves IIC values above 70 in many configurations.
For most Canadian residential applications, this level of treatment is not practical in existing buildings due to weight constraints, ceiling height loss, and cost. Lighter floating floor systems using dense rubber or cork underlayment with engineered wood flooring are more commonly specified in renovation contexts.
Area Rugs and Underlays
Where structural treatment is not feasible, area rugs with rubber or felt underlays on the upper floor provide meaningful reduction in impact noise. A rug covering the majority of the floor area above a bedroom, for example, can reduce perceived impact noise levels noticeably. This is not a substitute for structural treatment but is a practical interim or supplementary measure.
Performance Reference Table
| Assembly Configuration | Approx. STC | Approx. IIC |
|---|---|---|
| Wood joist, no insulation, single drywall ceiling, hardwood floor | 30–35 | 25–30 |
| Wood joist, mineral wool cavity, single drywall ceiling | 40–44 | 30–35 |
| Resilient channel ceiling, mineral wool, double drywall | 50–55 | 40–47 |
| RSIC clips, mineral wool, double drywall ceiling | 55–60 | 48–55 |
| RSIC clips + floating rubber underlayment floor above | 58–63 | 55–65 |
These values are approximations based on common assembly configurations. Actual field performance (FIIC, FSTC) is typically 3 to 7 points lower than laboratory ratings due to flanking paths through the building structure.
Flanking in Floor-Ceiling Assemblies
Flanking is the transmission of sound along indirect paths that bypass the primary assembly. In floor-ceiling construction, the most common flanking routes are:
- Through continuous joist framing that connects the two units structurally
- Through shared walls that are rigidly connected to both floor and ceiling
- Through plumbing stacks and drains running between floors
- Through HVAC ducts and plenums
In some older Canadian apartment buildings, addressing flanking through joist continuity requires either cutting and isolating the joists (a structural intervention) or accepting that the floor-ceiling treatment will not achieve its theoretical performance. Acoustic consultants can assess the relative contribution of flanking versus direct transmission through field testing using standardized procedures.