STC 43, ASTC 40, or the Recipe: How BC Code Actually Rates a Suite Wall

The general rule, and why a suite gets an easier one

Start with the baseline that applies between any two dwelling units. Sentence 9.11.1.1.(1) requires that a dwelling unit be separated from every other space in the building where noise may be generated, by one of two things: a separating assembly and its adjoining constructions together providing an apparent sound transmission class (ASTC) rating of not less than 47, or a separating assembly providing a sound transmission class (STC) rating of not less than 50 with adjoining constructions conforming to Article 9.11.1.4.

Those two numbers describe the same real-world result by different methods. STC measures the assembly on its own, in a laboratory, where sound can only pass through the wall. ASTC measures the finished building, where sound also travels around the wall through the floor and the side walls. That extra route is called flanking. Because ASTC counts the flanking noise and STC does not, the ASTC threshold sits lower for the same outcome: 47 rather than 50.

Then comes the part that matters here. Sentence 9.11.1.1.(2) sets a different, lower standard specifically for secondary suites, and the code explains its own reasoning in Note A-9.11.1.1.(2). A reduced level of performance is acceptable in a secondary suite because the occupants of the house are only affected by the sound of one other unit, and in many cases it is the owner of the house who decides on the desired level of protection.

That is worth reading carefully, because it hands the decision back to you. The code sets the floor. Whether you build to the floor or well above it is a choice you make once, at framing, and cannot revisit later without opening the ceiling again.

The three compliance paths for a secondary suite

Sentence 9.11.1.1.(2) gives three options, and you only need to satisfy one. They are not equivalent in effort, in cost, or in what they demand from the design team.

Path (a) is prescriptive. Build the assembly this way and it complies, with no rating attached: joist spaces filled with sound-absorbing material of not less than 150 mm nominal thickness, stud spaces filled with sound-absorbing material, a resilient channel on one side of the separation spaced at 400 or 600 mm on centre, and not less than 12.7 mm thick gypsum board on ceilings and on both sides of walls. No test report, no consultant, no number to prove.

Path (b) is a tested performance number: construction providing an STC rating of not less than 43. This is where the assembly tables come in, since the code publishes typical STC values for standard assemblies and you can select one that meets or beats 43.

Path (c) is the whole-system number: a separating assembly and adjoining constructions together providing an ASTC rating of not less than 40. This one accounts for flanking, which makes it the most honest description of what an occupant will actually hear, and also the most work to demonstrate.

For a house with a suite, path (a) is usually the practical answer. A building official can verify it by looking at the framing and reading a product label. Paths (b) and (c) both require someone to establish a rating, and path (c) requires modelling the whole junction assembly, which on a typical basement suite is a consultant's fee attached to a result the recipe would have delivered anyway.

The assemblies, and the number that looks backwards

Table 9.10.3.1.-A carries a column titled Typical Sound Transmission Class alongside the fire-resistance data, so the same table that tells you an assembly's fire rating also tells you roughly what it does for sound. A handful of the wall assemblies are worth knowing by name.

Look at W4a and W4b. Same studs, same absorptive material, same layers of gypsum board, same resilient channel. The only difference is stud spacing: 400 mm on centre for W4a and 600 mm for W4b. And the wider spacing rates higher, 54 against 51.

That surprises people, so here is the reason. Every stud is a rigid piece of wood touching the gypsum board on both sides of the wall. Sound travels through it directly, bypassing the insulation in the cavity. Fewer studs means fewer of those rigid paths, so the wall performs better. The same logic explains why the resilient channel matters so much: it exists to break that contact on one side entirely.

Note also where W3c sits. Studs at 400 or 600 on centre with 12.7 mm Type X gypsum board rates STC 43, which lands exactly on the secondary suite minimum under path (b). It clears the bar with nothing to spare, which means any site error, any unsealed penetration, any back-to-back outlet, and the real performance drops below what you designed for.

Where the ratings come from, and what they do not include

Article 9.11.1.2 sets out how the ratings are determined. STC is measured to ASTM E413 using sound transmission loss values from ASTM E90, a laboratory method. ASTC uses the same ASTM E413 classification but with field measurements to ASTM E336. Article 9.11.1.3 covers compliance with those required ratings, and Article 9.11.1.4 covers adjoining constructions, including Table 9.11.1.4 for floor treatments.

That last article is the one people skip, and the code itself flags the risk. Note A-9.11.1.3.(2)(b) says that selecting an appropriate separating assembly is only one part of the solution, and that to fully address the sound performance of the whole system, flanking assemblies must be connected to the separating assembly in accordance with Article 9.11.1.4.

In plain terms: a wall rated STC 54 does not deliver STC 54 if sound walks around it through the subfloor and the side walls. The rating describes a panel tested in isolation. Your basement is not isolated.

One more limitation worth naming. STC and ASTC describe airborne sound: voices, music, television. Footsteps are impact sound, which travels as vibration through the structure, and neither number describes it. On the jobs we run, footsteps overhead are the complaint that arrives most often after move-in, and it arrives from suites that met the code requirement exactly as written. Floor coverings, underlay, and how the ceiling is hung are where that gets addressed.

What actually ruins the separation on site

Everything above is design. Here is what we see fail during construction, and none of it shows up on a rating table.

The single most common error on a suite job is resilient channel installed wrong. The channel works by holding the gypsum board away from the framing so vibration cannot pass straight through. Drive a screw through the board, through the channel, and into the stud behind it, and you have bolted the two together again. The channel is now a decorative strip of metal. It does nothing at all, and nothing about the finished wall looks different, so it is never caught until the tenant moves in.

The second is back-to-back electrical boxes in the same stud cavity. Two boxes cut opposite each other leave a thin gypsum membrane between two rooms, and both faces of the assembly are now open into the same air space. Offset them by at least one stud bay.

After that it is penetrations, in every form. Plumbing stacks that pass through the separation, ducts crossing it, pot lights cut into the ceiling, unsealed gaps where the top plate meets the floor above. Air paths are sound paths. A gap you can slide a business card through will carry conversation.

And then rigid contact at the edges: where the new ceiling assembly ties into the existing side walls, where the wall meets the floor slab. Those junctions are the flanking paths that Article 9.11.1.4 exists to control, and on a retrofit in an existing house they are usually the hardest part of the job.

• Resilient channel screwed through into the stud behind it, which cancels it completely
• Back-to-back electrical boxes sharing one stud cavity
• Plumbing stacks and vent pipes passing through the separation unsealed
• Ducts crossing between the two units
• Pot lights cut through a rated ceiling assembly
• Unsealed gaps at the top plate, the floor edge, and around penetrations
• New assembly tied rigidly into existing side walls, creating a flanking path

The ductwork problem, which is two problems at once

A house with a suite very often ends up with one furnace serving both units, because separating the systems is expensive and the existing equipment is sitting right there. The code has something to say about that.

Sentence 9.32.3.2.(4) states that in a house with a secondary suite, where a heating or ventilation system serves more than a single dwelling unit, the system must be designed and installed to prevent the circulation of smoke upon a signal from a duct-type smoke detector. That is a life safety requirement, and it is checked.

The sound consequence rides along with it. A duct that runs from the upstairs return, through the basement, and into the suite is a continuous air path between two units, and air paths carry sound. You can build a ceiling assembly to STC 55 and still hear a conversation clearly through a shared return, because the sound never touched the ceiling. It went around through the sheet metal.

Where the budget allows, fully separate systems solve both the smoke requirement and the sound problem in one move, and they simplify the utility question if the suite is ever rented separately. Where it does not, the ducts need lined sections, offsets, and proper sealing at every point they cross the separation, and that work has to be planned before the ceiling closes.

Adding a suite to an existing house is a different rulebook

New construction and retrofit are treated differently, and this catches people who read the code straight through without noticing.

Table 1.1.1.1.(6) provides alternate compliance requirements for adding a secondary suite to an existing house. Several standards are relaxed, including ceiling heights, which drop from 2.1 m to 1.95 m, along with doorway sizes, fire ratings, and sound requirements. That relaxation is the difference between a suite being possible and impossible in a lot of older North Shore basements, where the joists sit lower than anything built today would allow.

Which set applies to your project is a determination the building department makes, based on the age of the house, the scope of what you are doing, and how the municipality reads it. The City of North Vancouver and the District of North Vancouver are separate jurisdictions with their own building departments, and this is a question to ask at the counter early, before the design is fixed. On the projects we run, that single conversation reshapes the ceiling detail more often than any other.

The one thing we would not do is design down to the relaxed minimum just because it is available. Ceiling height you cannot recover. Sound separation you can improve later only by demolishing the ceiling you just paid for.

What we would build, and why

Given the choice on a basement suite, follow the prescriptive recipe in path (a) and then spend a little more on the things the recipe does not cover.

The recipe is the practical path because it is verifiable. A building official can see 150 mm of sound-absorbing material in the joist spaces, see the resilient channel spacing, and read the thickness off the gypsum board. Nobody has to trust a report. And it is genuinely good construction: absorptive material in the cavity, a broken contact path, and mass on both faces are the three things that make a separation work.

The extras worth adding on top are cheap at framing stage and expensive afterwards. Seal every penetration properly with acoustic sealant rather than expanding foam. Offset the electrical boxes. Keep the ducts on separate systems or line and seal them where they cross. Brief the drywall crew specifically about the resilient channel screw length before they start, and check the first wall yourself.

For an owner living directly above the suite, going past the minimum is the decision the code note anticipated. Two layers of gypsum board instead of one costs very little relative to the whole project, and W6a at STC 55 exists in the same table as the assemblies that scrape by at 43. The gap between them is a few hundred dollars in material and the willingness to decide before the ceiling goes up.