Soundproofing a Toronto Multiplex: STC Ratings, Resilient Channel and Floor Assemblies
A Toronto multiplex can pass inspection and still become a daily noise problem if the wall or floor assembly is treated as a list of products instead of a complete acoustic system. Voices may travel through a party wall, footsteps may transmit through the joists, and sound can bypass the separating assembly through ceilings, side walls, ducts and framing junctions. The right question is not whether a project uses insulation or resilient channel; it is whether the complete tested or Code-compliant assembly controls direct and flanking transmission.
Quick Answer: What Sound Rating Does a Toronto Multiplex Need?
Ontario’s current sound-separation framework generally requires a dwelling unit to be separated from adjoining spaces where noise may be generated by either an assembly plus adjoining construction providing an apparent sound transmission class (ASTC) of at least 47, or a separating assembly providing STC 50 with adjoining construction that meets the prescribed flanking-path requirements. STC measures direct airborne sound through the assembly; ASTC includes flanking paths around it.
Key Takeaways
STC is a laboratory-style rating for direct airborne sound transmission through a wall or floor assembly; ASTC includes flanking paths through adjoining construction.
For dwelling-unit separations, Ontario’s framework generally uses ASTC 47 or STC 50 with compliant adjoining construction.
A product does not have the same rating as the complete tested assembly in which it was tested.
Resilient channel can improve performance by decoupling gypsum from framing, but poor screw placement can short-circuit the channel and reduce the benefit.
Floor/ceiling assemblies must address both airborne noise and impact noise such as footsteps.
The Ontario Building Code’s minimum sound provisions should not be confused with a guarantee of luxury acoustic privacy.
Penetrations, ducts, continuous framing and wall/ceiling junctions can become flanking paths even when the separating wall itself is high-performing.
The sound-control assembly should be selected before framing, mechanical rough-ins and drywall are finalized.
STC, ASTC and IIC: What Do the Ratings Mean?
STC: Sound Transmission Class
STC is a single-number rating used to describe airborne sound insulation through a separating assembly. Voices, television and music are typical airborne sources. STC is based on the tested assembly—not simply on one layer of insulation, drywall or acoustic board.
Ontario’s sound provisions define STC as a rating of the airborne sound attenuation of a separating assembly using the direct transmission path. The Code permits compliance through testing or through qualifying assemblies in Supplementary Standard SB-3. See Ontario Regulation 88/19 sound-transmission provisions.
ASTC: Apparent Sound Transmission Class
ASTC looks beyond the direct wall or floor and accounts for sound that travels through adjacent walls, floors, ceilings and junctions. This matters in multiplexes because sound can bypass a very good demising wall through a continuous floor system or ceiling cavity.
The National Research Council’s soundPATHS overview explains that direct and flanking transmission together produce apparent sound transmission and that occupants’ perceived sound insulation correlates well with ASTC.
IIC: Impact Insulation Class
IIC is used to describe how a floor/ceiling assembly controls impact sound, such as footsteps or objects striking the floor. It is a different problem from airborne sound. A floor can perform well for airborne STC and still transmit objectionable footfall noise.
NRC guidance on reducing noise through joist or truss floors distinguishes airborne sound from impact sound and notes that resilient ceiling support and floor toppings can play different roles in controlling the two.
What Is the Ontario Building Code Minimum for Dwelling-Unit Sound Separation?
Ontario’s current framework generally requires a dwelling unit to be separated from every other space in the building where noise may be generated by either ASTC 47 for the separating assembly plus adjoining construction, or STC 50 for the separating assembly with adjoining construction that meets the prescribed flanking provisions.
That minimum is a compliance threshold, not a promise that tenants will never hear each other. Low-frequency bass, impact noise, plumbing, doors, ducts and imperfect construction can still be noticeable.
Where sound privacy is a major project goal, the design team can select a higher-performing tested assembly than the minimum, but any claimed rating should be tied to a documented assembly and its installation details.
Why STC 50 Alone Does Not Tell the Whole Story
A common mistake is specifying a wall as STC 50 and assuming the entire room-to-room performance is therefore STC 50. In the field, sound can travel around that wall through the floor, ceiling, side walls, structural framing or service penetrations.
That is precisely why ASTC is useful: it represents the combined direct and flanking paths.
NRC’s flanking research shows that junction conditions and ceiling attachment can materially affect apparent sound transmission between rooms. See The Effect of Resilient Channels on Ceiling Flanking Transmission Paths.
How Resilient Channel Works
Resilient channel is a light-gauge metal channel used to reduce the rigid connection between gypsum board and the wall studs or floor joists behind it. The goal is decoupling: vibration travelling through the framing has a less direct path into the finished gypsum surface.
Resilient channel is not magic, and the performance depends on the complete system: channel type and spacing, framing, cavity insulation, gypsum layers, fasteners, junction details and penetrations.
NRC research on sound insulation of gypsum board on resilient supports confirms that resilient support can improve sound transmission loss but that the practical benefit depends on the complete construction.
The Biggest Resilient-Channel Installation Mistake: Short-Circuiting
Resilient channel works only when the finish layer remains resiliently supported. If drywall screws pass through the channel and bite directly into the stud or joist, the screw creates a rigid bridge that bypasses the intended decoupling.
NRC testing documented a floor/ceiling example where only ten misplaced screws that connected the gypsum directly to the framing reduced the apparent STC from 60 to 57. See Flanking Transmission in Multi-Family Dwellings: Phase II.
The lesson for homeowners is not that every bad screw costs a fixed number of STC points. The result depends on the assembly. The practical lesson is that acoustic performance can be lost through small installation errors that are invisible once drywall is finished.
What Should a Multiplex Party Wall Include?
A high-performing party wall commonly combines four principles: mass, absorption, decoupling and airtightness. The exact materials and sequence should come from a tested or Code-recognized assembly rather than a generic shopping list.
Mass: one or more appropriate layers of gypsum or other rated board.
Absorption: fibrous cavity insulation that reduces cavity resonance.
Decoupling: resilient channel, clips or separated framing where part of the tested design.
Airtightness: careful perimeter and penetration sealing so sound does not leak through gaps.
Flanking control: junction details that prevent sound from bypassing the wall through floors, ceilings or adjacent framing.
Fire performance: the sound assembly must also satisfy the project’s required fire-resistance and continuity details.
Can You Just Add More Insulation?
No. Cavity insulation can be an important part of an acoustic assembly, but insulation alone does not create a high STC wall. Sound-control performance depends on the mass of the faces, spacing between them, structural connections, cavity absorption, sealing and flanking paths.
A homeowner comparing products should ask for the tested wall or floor assembly—not only the acoustic board’s marketing claim.
Floor Assemblies: Airborne Noise and Footsteps Need Different Strategies
A floor between two dwelling units has to control airborne sound such as voices and televisions as well as structure-borne impact from footsteps, dropped objects and moving furniture.
NRC explains that carpet often improves IIC significantly while having only a small effect on STC. It also notes that lightweight floors can continue to transmit low-frequency footstep noise, which is why heavier toppings and resilient ceiling details are commonly considered in acoustic floor design. See Reducing Noise Transmission Through Joist or Truss Floors.
Typical floor/ceiling sound-control layers
Depending on the tested assembly and project conditions, a wood-framed multiplex floor/ceiling system may use some combination of:
finished flooring;
an acoustical underlayment or resilient layer;
a heavier topping or additional subfloor layer;
wood joists or engineered joists;
cavity insulation;
resilient channels or resilient clips below the joists;
one or more layers of gypsum board;
sealed perimeters and carefully detailed penetrations.
Changing one layer can change the measured performance. An IIC or STC number from a tested system should not be transferred to a different floor buildup simply because it uses the same underlayment.
Does the Ontario Building Code Require an IIC Rating?
Ontario’s dwelling-unit sound-separation provisions focus on airborne sound through STC and ASTC. They do not establish the same general minimum IIC requirement for residential floors. That does not make impact noise unimportant; footfall complaints are one of the main reasons an otherwise compliant multiplex can feel poorly sound-isolated.
A recent NRC/CCMC evaluation notes an IIC 55 recommendation for bare floor/ceiling assemblies as a performance target in the evaluated context, while distinguishing that recommendation from the Code’s STC/ASTC requirements. See CCMC 12419-R.
For a Toronto rental conversion, an impact-performance target above the bare minimum can be a practical owner decision, especially when hard flooring is planned above bedrooms.
Why Carpet, Vinyl and Hardwood Behave Differently
Floor finish changes impact performance. Soft finishes can cushion high-frequency footsteps, while hard finishes transmit more energy unless the floor buildup includes a resilient layer or another tested impact-control strategy.
Luxury vinyl or engineered hardwood installed over a thin foam pad should not automatically be assumed to provide the same IIC as a laboratory-tested floor system. The substrate, topping, underlayment, ceiling and room dimensions matter.
Flanking Paths: Where Sound Sneaks Around the Assembly
Flanking transmission is sound that reaches the neighbouring unit by a route other than straight through the separating wall or floor.
continuous joists crossing a separating wall;
a gypsum ceiling that is continuous across both units;
side walls rigidly connected to the party wall;
ductwork serving multiple units;
open stair cavities;
electrical boxes placed back-to-back;
recessed lights and access panels;
plumbing chases;
gaps at wall-to-floor and wall-to-ceiling junctions.
This is why treating only the party wall can disappoint. A complete multiplex design has to coordinate the separating assembly with the surrounding construction.
Mechanical and Plumbing Noise Need Their Own Controls
STC and ASTC deal primarily with sound transmission through building assemblies. A multiplex also has equipment noise and vibration from furnaces, heat pumps, pumps, exhaust fans, plumbing stacks and duct systems.
Mechanical sound control can require equipment isolation, flexible connections, appropriate duct routing, lined transitions or silencers, and avoiding rigid service bridges through separating assemblies. These details must be coordinated with fire stopping and ventilation requirements.
Do not block vents or reduce required airflow in an attempt to stop sound.
What About Doors?
A high-performing wall can be undermined by a lightweight, poorly sealed door. Where a door occurs in or near a sound-separating assembly, its mass, perimeter seals, undercut, frame installation and fire rating may all matter.
Open common stairs and poorly sealed suite doors can also transmit conversation and hallway noise even when the main wall assembly is strong.
Toronto Multiplex Permit Planning and Sound Assemblies
Toronto’s Multiplex Conversion permit guide confirms that a building permit is required to convert an existing house into a duplex, triplex, fourplex, fiveplex or sixplex.
Where sound-separation assemblies are part of the permit design, the drawings and specifications should clearly identify the assembly rather than relying on a note that says soundproof wall. The fire rating, acoustic layers, framing, resilient components and continuity details should be coordinated so the contractor knows what must actually be built.
The permit designer may use qualifying assemblies from the applicable Code resources or another accepted compliance path. Any field changes that alter layers, framing or attachment should be reviewed before construction continues.
Soundproofing vs. Fire Separation
Sound control and fire separation often occupy the same wall or floor assembly, but they are not the same requirement. Adding an acoustic product does not prove the required fire resistance, and adding Type X drywall does not automatically prove the acoustic rating.
The wall or floor should be selected as one coordinated assembly that satisfies both objectives where both apply.
Common Soundproofing Mistakes in Toronto Multiplexes
Specifying STC 50 without checking flanking transmission.
Buying a single acoustic product based on its advertised rating instead of using a tested complete assembly.
Installing resilient channel and then screwing the gypsum into the framing.
Using direct-attached ceilings where the selected tested assembly requires resilient support.
Assuming batt insulation alone will soundproof a party wall.
Ignoring footfall noise because the floor meets an airborne STC requirement.
Selecting hardwood or vinyl without checking the complete floor/ceiling impact performance.
Installing back-to-back electrical boxes in separating walls without an approved detail.
Leaving gaps around pipes, ducts, perimeter joints and access panels.
Changing a tested assembly on site because another material looks equivalent.
416 Construction Practical Guidance
For a Toronto duplex, triplex or fourplex, choose the acoustic strategy before the framing and mechanical rough-ins are finalized. Identify every dwelling-unit separation on the plans, then decide how each wall and floor will control direct airborne sound, impact noise and flanking paths.
When the budget allows, prioritize the locations where occupants are most likely to notice sound: bedroom-over-living-room floors, units separated by lightweight wood framing, mechanical rooms beside bedrooms, common stairs and basement ceilings below hard-surface flooring.
Do not chase the highest advertised STC product. A properly installed, documented assembly with controlled flanking paths is more valuable than a premium acoustic layer installed inside an otherwise weak system.
Step-by-Step Soundproofing Planning for a Toronto Multiplex
Map every wall and floor that separates dwelling units or a dwelling unit from a noisy common/mechanical space.
Identify the required fire-resistance and acoustic performance for each separation.
Choose a tested or Code-accepted complete wall and floor assembly.
Coordinate resilient channel or clips, cavity insulation, board layers and fastener lengths.
Review floor finishes and determine whether additional impact-noise control is appropriate.
Coordinate ducts, pipes, electrical boxes, lights and access panels before rough-ins.
Detail perimeter sealing and flanking junctions.
Include the assembly references and key installation notes on the permit/construction drawings.
Inspect the resilient components and penetrations before drywall is fully closed.
Avoid unauthorized substitutions that change the tested assembly.
When to Contact Each Professional
Architect or qualified BCIN designer: to select and document compliant wall/floor assemblies and coordinate them with the permit drawings within scope.
Acoustical consultant: when higher-than-Code privacy, music rooms, mechanical noise, difficult flanking conditions or field testing is important.
Structural engineer: when floor toppings, framing changes or added assembly weight may affect the structure.
Mechanical designer: where ducts, equipment or ventilation cross sound/fire separations.
Electrician: to coordinate outlets, panels, penetrations and devices without undermining required assemblies.
Contractor or design-build team: to sequence framing, resilient channels, services, sealing and drywall so the specified assembly is actually constructed.
Conclusion: Soundproof the Complete Multiplex Assembly
Soundproofing a Toronto multiplex is not about installing one product. Ontario’s framework generally requires ASTC 47 or STC 50 with compliant adjoining construction for dwelling-unit sound separation, but good real-world performance also depends on flanking paths, impact noise, doors, ducts, penetrations and installation quality.
Use tested assemblies, coordinate the floor and ceiling as one system, and protect the resilient details during construction. The strongest specification is the one that can actually be built and inspected correctly.
416 Construction can coordinate rental-conversion design, permit drawings, fire and sound separation details, engineering and construction through one design-build process. Learn more about Toronto rental conversions or call 647-624-8110.
Informational Disclaimer
This article provides general information for Toronto homeowners as of October 1, 2026. It is not a project-specific Building Code, acoustical, engineering or fire-resistance design. STC, ASTC and IIC ratings apply to tested or calculated assemblies and conditions; the actual project must be reviewed using the applicable Code, approved drawings and product/assembly documentation.
Frequently Asked Questions:
What STC rating is required between apartments in Ontario?
Ontario’s dwelling-unit sound provisions generally allow compliance with an assembly plus adjoining construction providing ASTC 47, or a separating assembly providing STC 50 with adjoining construction meeting the prescribed flanking requirements. The exact compliance path should be documented for the project.
Is STC 50 good enough for a Toronto multiplex?
STC 50 can satisfy one Code compliance path when the adjoining construction also meets the required flanking provisions, but it is a minimum compliance threshold rather than a guarantee of complete privacy. Impact noise, ducts, doors and flanking paths can still be noticeable.
What is the difference between STC and ASTC?
STC describes direct airborne sound transmission through the separating assembly. ASTC includes the direct path plus flanking transmission through adjoining walls, floors, ceilings and junctions. ASTC is therefore closer to the actual room-to-room performance occupants experience.
Does resilient channel increase STC?
It can improve the sound performance of an assembly by reducing the rigid connection between gypsum and the framing, but the benefit depends on the complete tested system and correct installation. It should not be assigned a stand-alone STC value.
Can resilient channel be installed wrong?
Yes. One common error is driving drywall screws through the channel into the studs or joists, which creates a rigid bridge. NRC testing has shown that a small number of these short circuits can measurably reduce apparent sound performance.
Is insulation enough to soundproof a duplex wall?
No. Cavity insulation helps absorb energy inside the wall cavity, but strong sound isolation also depends on mass, decoupling, airtightness, framing configuration and control of flanking paths. Use a tested complete assembly.
Do floors need a different soundproofing strategy than walls?
Yes. Floors must control airborne sound and impact noise such as footsteps. A ceiling with resilient support, cavity insulation and gypsum can help airborne performance, while floor toppings, underlayments and resilient layers can be important for impact sound.
Does the Ontario Building Code require an IIC rating?
Ontario’s general dwelling-unit sound-separation provisions focus on STC and ASTC rather than establishing the same universal minimum IIC requirement. Owners may still choose an impact-performance target because footsteps and hard-floor noise can be a major comfort issue.
Does carpet improve STC?
Carpet generally has a much larger effect on impact performance than on airborne STC. NRC guidance notes that carpet can significantly improve IIC while having only a small effect on STC.
Can hardwood be installed over a rental unit?
Yes, but hard flooring can make impact noise more noticeable. The complete floor/ceiling buildup should be reviewed, including any underlayment, topping, joist system, cavity insulation and ceiling below.
What is flanking transmission?
Flanking transmission is sound that bypasses the main separating wall or floor by travelling through connected construction such as ceilings, floors, side walls, joists, ducts and junctions. It is one reason a laboratory STC number may not match field performance.
Should acoustic sealant be used around the perimeter?
Airtight perimeter and penetration detailing is commonly part of high-performing sound-control assemblies because small gaps can transmit airborne sound. The sealant and location should follow the selected tested assembly and must remain compatible with fire-stopping requirements where applicable.
Can I copy a neighbour’s STC wall assembly?
Not safely. Their framing, joist direction, fire rating, ceiling, wall junctions and permit conditions may differ. The sound rating belongs to a specific tested or calculated assembly and set of adjoining conditions.
How much does multiplex soundproofing cost?
There is no defensible flat cost per square foot because the scope can range from upgrading one party wall to rebuilding several floor/ceiling systems with resilient support, additional gypsum, underlayments and mechanical changes. A project-specific quotation should separate acoustic work from fire, structural and finish work, with HST identified.
When should I hire an acoustical consultant?
Consider one when the project requires higher-than-minimum privacy, music or entertainment spaces, difficult low-frequency noise, mechanical equipment near bedrooms, unusual flanking paths or field testing. Many standard residential conversions can use documented Code-compliant assemblies without a separate acoustical consultant.
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