TLDR: Most Toronto multi-unit builders hit the code minimum for airborne sound and call it done. That's not enough. Impact sound — footsteps, dropped objects, heavy heels on hardwood — gets overlooked constantly, and tenants feel it immediately after move-in. Here's what actually works, what it costs, and why the code floor is a terrible ceiling.
The Complaint Every Multi-Unit Owner Gets After Year One
Three weeks after your tenants move in, the calls start. Not about a leaky faucet. About the upstairs neighbour walking to the bathroom at 2am. About every footstep on hardwood landing like a drumbeat in the unit below.
We've seen it on duplexes in East York, triplexes near the Junction, stacked townhomes across Scarborough — time and again. The builder hit the code. The walls tested fine. But nobody properly addressed the floor-ceiling assembly for impact sound. And now it's the building owner's problem.
Soundproofing in multi-unit residential builds isn't just about meeting the Ontario Building Code. It's about whether tenants actually want to stay.
Airborne Sound vs. Impact Sound: Two Different Problems
Most people — including plenty of contractors — conflate the two types of sound transmission. They're separate problems, and they need separate solutions.
Airborne sound is what you'd typically imagine — voices, music, TV, conversations through a shared wall. This is what the Ontario Building Code directly addresses with STC (Sound Transmission Class) ratings. It travels through the air and then vibrates through walls and ceilings.
Impact sound is physical energy transferred through structure. Footsteps. A dropped pot. A dog running across hardwood. Heavy heels on flooring. This type of sound travels directly through the building's structure — it doesn't need to travel through air first. That's what makes it so hard to stop after the fact.
Most Toronto builders do a decent job managing airborne sound. The code forces them to. Impact sound? That's where things fall apart, especially in wood-framed buildings — which is the majority of smaller multi-unit residential construction in the city.
Wood-framed buildings are lightweight. They flex and transfer energy easily. A concrete slab between units naturally damps impact sound far better than a wood joist floor system. When you're building stacked units on wood framing, you need to be deliberate about impact isolation from the very start of your floor assembly design.
What the Ontario Building Code Actually Requires
The OBC sets a minimum STC rating of 50 for assemblies separating dwelling units from other noise-generating spaces. Where a unit borders an elevator shaft or garbage chute, that jumps to STC 55.
Here's the important nuance: as of January 1, 2020, Ontario amended the Building Code to require assemblies meet an ASTC (Apparent Sound Transmission Class) rating of 47 — or demonstrate STC 50 compliance under very specific construction conditions. The difference matters.
STC is a lab rating. It measures how a wall or floor assembly performs in a controlled setting. ASTC is field-measured — it accounts for flanking paths, the indirect routes sound takes around an assembly through adjacent floors, ceilings, and walls. Flanking is almost always present in real-world wood-frame construction, and it eats into your lab-tested STC number considerably.
The code is now more realistic about this. But it's still a minimum. We go beyond it on our projects because we've seen — and heard — the difference.
What Minimum Code Compliance Actually Sounds Like
We'll be direct: a building that barely meets the OBC is a building where tenants will hear each other. Maybe not every word, but enough. And in a wood-framed Toronto triplex where units are stacked vertically, impact sound compounds the problem.
Hitting ASTC 47 is passing. That's it. There's no award for passing.
Anyone who's ever walked into a properly soundproofed apartment — one where the walls absorb, the floors don't transmit, and upstairs is genuinely another world — knows the difference isn't subtle. It's night and day. A properly soundproofed unit is worth more. It rents faster. It retains tenants longer. The additional investment pays for itself.
What Most Contractors Get Wrong
The blind spot is almost always the floor-ceiling assembly.
Contractors will specify a double-stud wall, pack it with mineral wool, and feel good about their airborne numbers. Then they'll frame the floor system the same way they always have — without any dedicated impact isolation layer — and assume the insulation in the joist cavity will handle it.
It won't.
The consequence is tenants who can track each other's movement through the building. In smaller wood-frame construction — Toronto's duplexes and triplexes especially — this becomes a genuine quality-of-life problem that damages the building's reputation and the owner's relationship with their tenants.
What We Recommend for Floor-Ceiling Assemblies
For horizontal separations (floors and ceilings between stacked units), you need to address both airborne and impact sound independently.
For airborne sound, a properly designed floor assembly with acoustic insulation in the joist cavity and a resilient channel or sound clip system to decouple the ceiling drywall will get you where you need to be.
For impact sound, we regularly specify a Sonopan floor panel system. Sonopan manufactures panels that install directly under your finished flooring — hardwood, LVP, carpet — and absorb impact energy before it has a chance to travel through the structure. It's one of the cleanest, most practical solutions available for existing floor assemblies or new construction, and it's compatible with nearly every flooring finish you'd specify in a Toronto multi-unit project.
The combination of proper ceiling decoupling for airborne isolation and an impact-absorbing underlayment like Sonopan is how you build a stacked unit someone is genuinely happy to live in.
For Wall Assemblies
Walls between side-by-side units are primarily an airborne sound problem. A double-stud wall system with acoustic mineral wool (like Rockwool Safe'n'Sound or equivalent) is the standard approach. Attention to detail on penetrations — electrical boxes, plumbing runs, HVAC — is critical. Sound finds every gap.
What Does Proper Soundproofing Cost in Toronto?
Budget figures from real Toronto multi-unit projects:
Floor/ceiling assemblies (combined airborne + impact treatment): Budget $10–20 per square foot depending on the level of performance you're targeting and the complexity of the floor system.
Wall assemblies: Budget $5–15 per square foot depending on the system specified.
The factors that push costs higher: retrofitting an existing building (more disruptive), targeting higher-than-code performance levels, complex floor systems, or dealing with existing plumbing or mechanical that runs through demising walls or floor assemblies.
The factor that keeps costs manageable: designing it in from the start. Retrofit soundproofing is always more expensive than doing it right during construction. Always.
On a Toronto multi-unit project, proper soundproofing isn't a luxury line item. It's the line item that protects your investment for the next 30 years.
What Most Homeowners Get Wrong
First-time multi-unit developers in Toronto tend to make the same few mistakes:
Treating soundproofing as a finishing touch. It's not. Soundproofing decisions — wall assemblies, floor systems, the location and detailing of penetrations — are made during design and framing. By the time you're finishing floors, some of your options are gone.
Assuming code compliance equals quality. Minimum code is a legal threshold, not a design goal. For a unit you're renting at Toronto market rates, "barely meets code" is not the positioning you want.
Only specifying STC, not IIC. IIC (Impact Isolation Class) is the companion rating to STC — it measures how well a floor-ceiling assembly resists impact sound transmission. You need both numbers, and you need both to be strong. Most specs only reference STC.
Not detailing penetrations. Every hole through a demising wall or floor assembly is a potential flanking path. Electrical, plumbing, HVAC, and data runs all need to be sealed acoustically. This gets missed constantly on smaller projects.
The BVM Approach
Our projects start with the end in mind. Soundproofing isn't an afterthought — it's specified during the design phase, confirmed during framing, and checked during finishing.
We look at every stacked unit project through the lens of both sound types. We design wall assemblies for airborne isolation. We design floor-ceiling assemblies for both airborne and impact. We flag penetrations before they're drilled and ensure they're sealed properly.
Beyond the assembly specs, we vet products. Sonopan for impact control. Acoustic mineral wool for wall and ceiling cavities. Resilient channels or sound isolation clips for ceiling decoupling. These aren't exotic materials — they're widely available and practically priced. The issue is never the product; it's whether the contractor knows to specify them and installs them correctly.
We also push clients beyond code minimums on projects where long-term rental performance matters. The extra cost is real. The payoff — in tenancy retention, fewer complaints, and building value — is more real.
Key Takeaways
The Ontario Building Code requires ASTC 47 or STC 50 (under specific conditions) for assemblies between dwelling units — that's the floor, not the target
Airborne sound and impact sound require different solutions — most contractors address one but not the other
Floor-ceiling assemblies in wood-frame construction are the biggest soundproofing risk in Toronto multi-unit builds
Dedicated impact isolation (like Sonopan floor panels) installed under finished flooring is one of the most effective and practical solutions available
Budget $10–20/sf for floor/ceiling treatment and $5–15/sf for wall systems
Design it in from the start — retrofit is always more expensive
Frequently Asked Questions
Q: Does the Ontario Building Code require impact sound ratings as well as airborne?
A: The OBC primarily specifies STC/ASTC ratings, which address airborne sound transmission. Impact isolation requirements (IIC ratings) are less prescriptive in the OBC, which is exactly why they get missed. We recommend targeting IIC 50+ for floor-ceiling assemblies between stacked units regardless of what's technically required.
Q: Can soundproofing be added after a multi-unit build is completed?
A: Yes, but it's harder and more expensive. Retrofit ceiling assemblies require opening up ceilings and adding decoupling systems. Floor assemblies can often be improved by adding an impact isolation underlayment like Sonopan under new flooring — but this works best when you're already replacing floors. Design it right from the start.
Q: What's the difference between STC and ASTC ratings?
A: STC is measured in a lab — it only captures direct sound transmission through the assembly. ASTC is field-measured and includes flanking paths (sound traveling around the assembly through adjacent structure). In real-world wood-frame construction, ASTC is almost always lower than STC for the same assembly. The current OBC requires ASTC 47 to account for this.
Ready to Talk About Your Multi-Unit Project?
If you're planning a duplex, triplex, fourplex, or any stacked multi-unit development in Toronto, let's talk about soundproofing before you finalize your design. The decisions you make in the design phase will define the quality of the building for decades. We've worked on these projects firsthand and know where the common failure points are.
Book a conversation with the BVM team directly at bvmcontracting.com.
Related Reading:
Here's Why Accessory Dwelling Units Cost So Much — understanding total build costs for multi-unit projects
A Summary of Toronto's New Divisive Sixplex Plans — what the new density rules mean for multi-unit development
Unexpected Home Building Costs in Toronto — the full picture on what surprises developers