A new multiplex gives each family their own home and their own front door. For that to feel true day to day, each home also needs to feel quiet inside. Sound that travels through walls and floors between suites is one of the first things owners and tenants notice, and it is much easier to plan for before construction starts than to fix after.
This post covers what the BC Building Code requires for sound control between dwelling units, what the two numbers (STC and FIIC) mean, the wall and floor systems builders use to meet them, and what to plan for plumbing and other penetrations. Every code reference below is from the BC Building Code 2024. The City of Vancouver applies its own Building By-law, which may have additional requirements, so ask the City what your specific project must meet.
The BC Building Code sets minimum standards for how much sound can pass between dwelling units. Two assemblies matter here: the walls between suites side by side, and the floor-ceiling assembly between a suite below and a suite above.
For both, the Code requires a minimum rating of 50. That 50 applies to two different types of sound, which is why there are two separate numbers.
STC stands for Sound Transmission Class. It measures how much an assembly blocks airborne sounds: voices, television, music. A higher number means better blocking.
At STC 50, loud speech heard through the assembly is hard to understand. Normal speech is hard to hear. This is the minimum the Code sets.
STC is measured in a laboratory on a test specimen. When the same assembly is built in a real building, the result is usually lower, because real construction has gaps, penetrations and paths that a lab specimen does not. For that reason, many builders aim for assemblies with a laboratory STC of 55 or higher, to arrive at a field result that comfortably meets 50.
FIIC stands for Field Impact Isolation Class. It measures how well a floor-ceiling assembly reduces impact sounds: footsteps, a chair scraping, something dropped on the floor above. Impact sound travels through the structure itself, so it is a different problem from airborne sound.
A higher FIIC is better. The Code also sets the minimum at 50 for FIIC between dwelling units.
Impact sound is the harder problem in wood-frame construction. Even if the floor-ceiling assembly blocks airborne sound well, footsteps can still be clearly audible below if the impact path through the structure is not interrupted.
A standard wood stud wall with one layer of drywall on each side will not reach STC 50 between suites. A standard wood joist floor with drywall below and hardwood on top will not reach FIIC 50. Both need extra measures.
The measures depend on the assembly type, but they all work by adding mass, adding an air gap, adding flexible connections, or some combination of the three.
There are three common approaches to walls that separate suites side by side.
A staggered-stud wall uses a wider bottom and top plate, usually 2x6, with 2x4 studs placed alternately on each edge of the plate. The studs on each side of the wall do not touch each other. Drywall on one side of the wall is attached only to the studs on that side. Because the two sides of the wall are not connected by any shared stud, sound vibration has less of a direct path from one face to the other.
Filling the cavity with acoustic insulation, such as mineral wool or dense-pack glass fibre, adds absorption for the sound that does travel through the air in the cavity.
Staggered-stud walls are common and can reach STC 50 to 54 in laboratory tests when built correctly with two layers of drywall on one or both sides.
A double-stud wall builds two entirely separate stud walls with a gap between them. The two walls share no structural connection at all. This is the best approach for sound isolation between suites, and it can reach STC 60 or higher in laboratory tests with proper insulation and drywall.
The trade-off is wall thickness. A double-stud wall with a small gap can be 12 to 16 centimetres thick, compared to 14 centimetres for a staggered-stud wall on a 2x6 plate. On a tight lot, that thickness comes out of the floor plan.
A single-stud wall can be upgraded with resilient channel. A resilient channel is a thin bent metal strip that holds the drywall away from the studs. The drywall is screwed to the channel, not to the studs. Sound vibrates the studs, but the flexible metal slows down how much of that vibration passes to the drywall on the living side.
Resilient channel works, but it is easy to install wrong. If a drywall screw accidentally touches the stud behind the channel (called short-circuiting the channel), the decoupling is lost and the assembly behaves like a solid wall. The installation needs careful site supervision.
For floors between a suite below and a suite above, the assembly must address both airborne sound (STC 50) and impact sound (FIIC 50).
The basic approach for a wood joist floor has three layers:
- A resilient mat or underlayment under the finished floor or under the subfloor. This cushions impact before it enters the structure.
- Acoustic insulation between the joists. This absorbs airborne sound travelling through the air space.
- Resilient channel holding the drywall ceiling below, with two layers of drywall.
The resilient mat is the most important part for impact sound. It must go under the subfloor or under the finish floor, depending on the product. Installing it after the subfloor is nailed down is a later change that costs more and gives a less reliable result, because the subfloor is already attached to the joists.
Engineered wood flooring and laminate both transmit impact sound well. Hardwood, luxury vinyl plank and carpet all respond differently to the mat below them. Confirm with your supplier which mat products are tested with the flooring you plan to use.
Drain pipes are a source of sound that is easy to miss. Plastic drain pipes (ABS or PVC) allow water noise to travel through the building structure when they are attached directly to framing.
Two measures help. First, wrapping drain pipes in mass-loaded vinyl (a heavy, flexible material) before the wall is drywalled absorbs the sound before it reaches the structure. Second, using firestopping and acoustic sealant at every point where a pipe penetrates a fire-separation assembly seals both fire and sound paths.
The BC Building Code requires that plumbing penetrations through fire separations be sealed to maintain fire resistance. Using products that also provide acoustic performance at those penetrations adds sound control at a stage when it is straightforward to do.
Electrical outlets, pot lights and other wall or ceiling penetrations also carry sound between suites if they are placed back to back or if the boxes are open to the wall cavity. Placing outlets on opposite studs rather than back to back, and using putty pads or acoustic boxes, helps maintain the sound rating of the assembly.
Sound control decisions must be in the permit drawings. The wall and floor-ceiling assemblies shown on the drawings set the standard the inspector will check. Changing the assembly after framing is done costs more and may require opening walls that are already closed.
The right time to choose the wall type, the resilient mat, the channel, and the insulation is at the design stage, when the architect or designer is drawing the assemblies. At that stage, adding a resilient mat or specifying a staggered-stud wall instead of a standard wall is a drawing change. After framing, it is a construction change.
A builder who understands sound control works with your designer at that stage to confirm that the assemblies in the drawings will meet the Code and will work as a system. The gap at the top of a party wall, the connection between the wall and the floor above, and the chase for plumbing all need to be detailed on the drawings, not solved on site.
When we build a multiplex, the families who will live in it are our reference point. A grandparent in the ground-floor suite should not hear every step from the suite above. A tenant in the top unit should feel at home without hearing the family below.
We review the sound assemblies with the project designer before permit submission and confirm the installation with our team on site. For the resilient channel on a ceiling, we follow up during the drywall stage to check that no screws have made contact with the joists behind the channel. These checks happen during construction, while the wall is still open.
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Related on this site: Multiplex construction in Vancouver · Secondary suite requirements in BC · Planning a secondary suite in a new house
