What Window Acoustic Restrictions Impact Your Project?

In this installment of the Window Geeks: Master Class, we’re exploring the basics of a topic that comes up often with our partners and clients: window acoustics. Does the Building Code call for certain levels of performance? Are there any restrictions you need to worry about for your project? Let’s find out…

What does the Building Code say about window acoustics?

Window acoustic performance is not a requirement in the BC Building Code. There is mention of ‘Sound Transmission’ and ‘Protection from Airborne Noise,’ but these relate to situations where a dwelling unit has ‘adjoining constructions.’

These wall assemblies are interior only, and therefore reference STC (sound transmission class) as well as ASTC (apparent sound transmission class). STC ratings are not sufficient to determine outside noise, so OITC is the more commonly used rating for windows. OITC is not referenced in the BC Building Code at all.

When we see a requirement for a window to meet a certain acoustic performance, it’s likely because the local municipality mandated it as part of a zoning change. It is questionable whether even this is allowed, as the Building Act which governs the Building Code does not allow individual municipalities to create their own building bylaws – all building codes and bylaws are regulated by the province. There is, however, a path for municipalities to get special permission to create a bylaw in certain circumstances (see wildfire regulations).

When an acoustic performance requirement is specified as a result of a municipal zoning bylaw, it needs to be part of a building-specific acoustical study where the location-specific source noise and the frequency of that sound are measured. This will then dictate the barrier required to reduce the sound to a dB level that is deemed acceptable. This will trigger what is called an OITC rating, which is used for outdoor/indoor sound transmittance. STC uses only single frequencies and cannot accommodate the wider variety of outdoor sounds.

It is poor practice to specify an STC or OITC value for a window unless the sound source and dB are known, or you risk not meeting occupants’ expectations.

What are the different interpretations of acoustics in the Building Code?

OITC ratings for windows are applicable when a prospective project is looking to gain municipal approvals. Some municipalities will require a location-specific acoustical study as part of this approval process. This study will determine the varying frequencies and dB ratings that the proposed building will be exposed to, and as part of that study, a recommendation is made by the acoustical consultant to increase exterior wall and/or window and door design to mitigate sound transmission.

These acoustic recommendations for vinyl windows can be anything from glass thickness, air space between glass panes, or the addition of a laminated pane of glass. Sound transmission lessens as the OITC rating increases. For example, OITC 32 will block more noise than OITC 28. It is important to note that no assembly will mitigate 100% of noise.

As the window construction changes, the impact on other areas of window performance can be affected: energy performance, structural performance, ease of use, and general longevity of the product. As acoustics are not a part of the BC Building Code, it tends to take a backseat to energy and NAFS requirements, as these are areas of compliance that are non-negotiable.

How is this requirement actually enforced?

There is no specific enforcement of acoustics on a proposed project, as the acoustic consultant is not required to stamp or provide letters of assurance. However, the architect will typically defer to the acoustical consultant to approve assemblies, and this is where conflict can arise as not all recommendations are possible in a typical vinyl window frame, or the proposed acoustic upgrade conflicts with the energy specification for the project. Most of the concerns regarding acoustics stem from the fear of litigation if a homeowner moves in and deems their unit too noisy and the consultants on the project approved products that did not follow the acoustic report recommendations. The key word here is ‘recommendation,’ but most builders, developers, owners, and architects do not want to take the risk.

And there you have it – the basics of window acoustics and the Building Code. Don’t miss the next Master Class, when we’ll be providing another free white paper on the topic!