Changing acoustical standards for residential buildings: what designers should know
The most recent National Building Code of Canada (NBCC), published in 2015, lays out new acoustic requirements for multi-family residential buildings. Some provincial building codes already include the updates, and with other jurisdictions expected to follow, designers and builders are preparing to adapt.
What should designers know?
The 2015 code changes how acoustic requirements are expressed. The previous version focused on the lab-rated acoustical performance of the separating assembly only. The new focuses on occupants’ acoustical experience. The updated requirements take a “whole system” view, considering how well all construction assemblies work together to reduce the transmission of noise from one space into adjacent dwelling units.
STC vs ASTC: from testing components to evaluating occupant experience
In the past, code requirements were given in sound transmission class (STC) ratings: numerical values indicating the performance of individual elements – typically walls or floors – in limiting the transmission of sound. To be certified as compliant, only the party wall or floor (a partition separating units and blocking the most direct path of sound transmission) had to achieve an STC rating of at least 50 in laboratory tests. (Higher ratings indicate stronger performance.)
A significant shortcoming of STC ratings is that even a wall or floor/ceiling that performs exceptionally well in the lab can, once installed in a building, be undermined by flanking sound transmission. Flanking transmission occurs when sound that would not penetrate the partition itself travels along surrounding elements. In the case of a wall, sound can also travel along the adjacent floor and ceiling, for instance. Other common flanking paths such as window mullions, pipes, ducts or penetrations like electrical sockets can also allow sound to intrude into an adjacent residence.
The new NBCC addresses this shortcoming by offering an additional performance measurement approach: apparent sound transmission class (ASTC). ASTC ratings focus on an occupant’s experience in the space itself. That experience can be shaped by the entire partition assembly (walls plus adjoining elements) that separates an occupant from their neighbour.
Under Canada’s new building code, designers may choose to achieve compliance in one of two ways. One option is to select from a set of approved materials and assembly details, essentially building a design from a menu of options that are known to meet the minimum code requirements. A second option is to develop a custom design and then demonstrate that it meets the standard of ASTC 47. (As with STC ratings, higher ASTC numbers indicate stronger sound insulation performance.) Whereas STC ratings are determined under the controlled conditions of a laboratory, ASTC ratings are site-specific and have to be established either through field testing or calculations stipulated in the building code. More below on how exactly a residential unit’s ASTC rating can be determined.
Impact insulation class
Managing noise from footfalls in units overhead is a key factor in the acoustic comfort of a residential unit. Indeed, footfall complaints are the most common concern we hear.
The new NBCC has not changed with respect to impact insulation class (IIC); an IIC rating is a single number assigned to a floor/ceiling based on the extent to which impact sound travels into the space below the source. The NBCC continues to recommend (but does not require) a minimum rating of IIC 55 when testing the bare floor (i.e., no carpet). In the unit below a floor/ceiling assembly which achieves an IIC rating of 45, footfalls overhead would be clearly audible, as would other impacts such as the movement of a chair or the sound of a small object being dropped on the floor. By contrast, a floor/ceiling assembly that transmitted virtually no sound from footfalls or other normal domestic activity would earn an IIC rating of 80 or greater.
As with ASTC, it’s the final IIC performance – as opposed to a lab-tested rating – that matters most. We strongly recommend meeting or exceeding the NBCC recommendation for IIC-55 of bare floors to limit complaints and ensure comfort for residents.
Demonstrating compliance: measurement versus calculation
Designers have a few options when it comes to both ensuring that they meet the new code requirements and demonstrating to authorities that they’ve done so.
a. Carrying out field measurements.
One option is to design your separating assemblies and adjoining construction to meet a minimum ASTC rating of 47, and then engage acoustical experts to carry out field measurements in order to demonstrate that occupant experience in the space indeed meets that standard. The risk in relying on field measurements is that you can’t fully establish performance until the units are built, at which point achieving meaningful performance improvements may be costly or even impossible.
b. Estimating performance with existing measured data, or using measurements from similar builds.
If your design relies on certain common assemblies, there may be enough existing data associated with your design to fulfill the code requirements without carrying out any additional measurements yourself. If your design has some particularities that make it difficult to estimate its acoustic performance with confidence, then testing a mock-up or an existing building with the same construction to demonstrate compliance may be the way to go.
Another way of demonstrating compliance is through calculation. The calculation approach considers the direct sound path through the party wall, and also considers three flanking paths from each of the four adjoining partitions (see below).
The code permits either a simple or a detailed method of calculation. The simple approach only considers overall sound insulation performance, expressing that performance in a single number quantity: the ASTC rating. The detailed approach evaluates performance in each one-third octave band frequency, and results in detailed transmission loss values, as well as the resultant ASTC-rating. The detailed approach may be most appropriate in cases where acoustic performance is especially critical and designers want to study particular frequencies in greater depth to build confidence in their designs. Both the simple and detailed calculation methods are based on an adapted ISO-12354 standard.
For design teams navigating the calculation approach, the National Research Council offers a fair amount of guidance, as well as an online tool called soundPATHs that will execute the calculations. The greatest challenge in the calculation method is that data are only available for basic assemblies. Unique or unusual assemblies are difficult to model and may benefit from testing mock-ups or previous projects.
Regardless of the methodology you use, it’s important to demonstrate mitigation efforts with respect to sound leaks (e.g., electrical outlets). None of the calculation approaches include provisions for sound leaks, but addressing penetrations and other possible leaks is vital to achieving your ASTC targets.
Early evaluation can go a long way
RWDI’s Acoustics, Noise and Vibration team works with clients across many Canadian jurisdictions to help them achieve and demonstrate code compliance. We also work with clients whose projects (such as health care facilities and laboratories) require exceptional levels of noise insulation and vibration control – far beyond standard residential code requirements.
Wherever possible, we advise clients to seek early design evaluations focused specifically on noise and acoustics; identifying potential noise issues early in the design process tends to make solutions considerably easier to implement and less costly. As projects proceed, we typically work closely with design teams to evaluate performance along the way to ensure that constructed units ultimately achieve the levels of acoustic comfort their designers intended.
If your next project could benefit from enhanced acoustical performance, or if you have any questions about what changes to the acoustic elements of the code requirements in your province might mean for you and your team, please contact Steve Meszaros.