By Rich Porayko
Fenestration West delivers strong education.
By Rich Porayko
Eastern Canada’s flagship fenestration event, Fenestration West, returned to the Delta Burnaby this past October.
Eastern Canada’s flagship fenestration event, Fenestration West, returned to the Delta Burnaby this past October. The event was created by the Fenestration Association of British Columbia (FEN-BC) following a merger of the non-profit Glazing Contractors Association of B.C. (GCABC) and the Window and Door Manufacturers Association of B.C. (WDMA-BC). Eligible for AIBC Core Learning Units, Fenestration West 2013 was sponsored by Apex Aluminum Extrusions.
|Leonard Pianalto of RJC brought attendees up to speed on how to specify the energy performance of glazing components in B.C.’s new, more complicated regime under the Energy Efficiency Act. Photo credit: Rich Porayko|
Topics from the sessions included dynamic eletrochromic programmable glass, specifying the energy performance of glazing products in B.C., thermal bridging for building assemblies and building a water-tight door.
One of the standout presentations had to be “Acoustic Performance of Windows” presented by Briét (Brizi) Coetzer, from North Vancouver’s BKL Consultants. According to Coetzer, urban planners in most Metro Vancouver municipalities have requirements in place to ensure that new residential projects proposed for high noise environments will be designed to achieve acceptable interior noise levels. Although exterior walls, roofs, doors and ventilation will sometimes require upgrading, it isn’t a surprise, as Coetzer explained to the attendees, that windows are the controlling factor with respect to the interior noise levels.
Even where there are no specific requirements imposed by municipalities, designers are paying careful attention to the acoustic performance of windows where noise-sensitive buildings such as residences, hotels, churches, schools and hospitals are involved. Speaking on a very timely, technical topic, Coetzer provided an excellent introduction and clearly explained the basics of sound, magnitude, frequencies, decibels, noise types and how humans hear all of the above.
“Single number ratings such as Sound Transmission Class (STC) and Outside to Inside Transmission Class (OITC) are both a weighted average of the performance of a product which can assist designers in evaluating the acoustic performance of two similar products to determine which one is better,” said Coetzer. “A single number rating is a single number that represents a whole range of frequencies. Can you really rely on one number to give you an accurate performance of the window? I would suggest not. It is helpful to make initial decisions; however, there is a lot of detail which gets lost in coming up with this single number. These numbers perform adequately over a range of generalized cases but perform extremely well in almost none.”
“OITC is more suited for applications where you are dealing with outdoor to indoor noise transmission whereas STC is more suited for indoor room to room noise transmission.” The STC classification system is based on the amount of attenuation required to reduce a standard household noise spectrum to be subjectively quiet. Noise sources used in generating this standard household spectrum consisted of loud speech, radio, television, vacuum cleaner noise and air conditioner noise. “So if that is the kind of noise that you want to mitigate, then STC is the appropriate choice,” said Coetzer.
Building envelope companies deal with outdoor to indoor sound transmission of traffic noise, for example. “That is a different frequency spectrum completely which has a much bigger low-frequency component that isn’t easy to mitigate with windows,” stated Coetzer. “So the STC rating is not going to give you an accurate indication of relative acoustic performance if you are trying to mitigate low frequency noise because the STC classification system is, by nature, principally controlled by mid- to high-frequency noise.
“The OITC test method came about during the late 1980s in response to a perceived need for a more robust rating system that performed adequately for low-frequency incident sounds,” said Coetzer. “The OITC classification system performs well in situations where the incident sound is broadband in nature and is dominated by low-frequency sound such as typical vehicle, aircraft and railway traffic noise. This is more representative of the real situation in which designers would be trying to choose appropriate exterior fenestration products,” said Coetzer.
According to Coetzer, the rule of thumb for glass thickness is the mass law, which says “for fixed incident frequency, the transmission loss across a barrier can be increased by approximately six decibels by doubling the mass per unit area.” So if you replace three-millimetre lites with six-millimetre lites, for example, you could increase transmission loss by six decibels, which is the equivalent of allowing only one quarter of the noise energy incident on the glass to pass through to the indoor space.
However, this is complicated by the “co-incidence effect” and “mass-air-mass resonance.” It is always a good idea to call in the acoustical experts to evaluate the performance of an assembly in the context of the proposed construction.
Airspace is also important. Coetzer explained to attendees that if you double the airspace, you could get an additional three-decibel increase in sound transmission loss. This rule of thumb seems to work well for STC ratings on assemblies with air spaces over 19 millimetres but is less correlated with OITC ratings.
Another key takeaway from Coetzer’s presentation was that laminated glass has constrained layer damping that significantly improves the transmission loss. “If you think of a sound wave moving through the glass, once it hits that constrained layer, there is a lot of energy that gets absorbed (transferred to heat). Laminated glass definitely helps. The use of laminated glass with appropriate interlayer increases transmission loss by approximately five decibels.”
Interestingly enough, if you add a gas such as argon to an insulated glass unit, it does change the shape of the transmission loss curve but usually does not result in any significant change to the OITC or STC rating.
Coetzer also explained that edge damping improves transmission loss; however, it’s marginal and is really only in the lower and upper frequencies. Most of the transmission loss that is of benefit to human hearing is in the middle frequency range, which is not improved by edge damping. Triple glazing also performs no better than double glazing with the same total glass weight and the same overall section depth.
In order to improve acoustical performance, you want to focus foremost on glass thickness and airspace size, with consideration given to panel size, use of laminated glass and airtight installation.
“If you have a large panel of glass, it is more flexible and is able to vibrate more when exposed to a noise source. A more rigid, smaller panel of glass will improve the transmission loss because the glass stiffness reduces the ability of the glass to vibrate. So it’s better to have a smaller panel size if acoustics are a concern but it’s very important to look at the TL data from the lab and know what panel size was used in the test which produced the STC/OITC rating,” explained Coetzer.
Air leakage is another acoustic degrading factor that is most apparent at high frequencies. “Gaskets must seal positively when the window is closed,” said Coetzer. “If you have air leakage around the edge of a panel, the high frequency performance is significantly degraded. If the window isn’t sealed airtight, all of the efforts of extra glass thickness, laminated glass and increased airspace can be wasted. Double gaskets should be employed to provide sealing redundancy.”