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The Engineer: Sound control still making noise

Walking around the trade show floor at The Buildings Show in Toronto and the WinDoor show in Quebec City before Christmas gave me the opportunity to discuss trends in our industry with clients, contractors and manufacturers. Surprisingly, what’s old is now new again.

Even with a lot of talk focused on more efficient windows with lower U-values, I still had multiple conversations related to sound transmission.  Sound transmission issues are not new.  In the mid-1980s, the National Research Council published many articles on this topic and here we are 30 years later still discussing it.

A wall or window is typically rated in either sound transmission class (STC), apparent sound transmission class (ASTC), or by the outdoor-indoor transmission class (OITC).  The higher the number the better. Even though OITC is a better indicator of noise reduction at lower frequencies (such as highways, train tracks or airports), the STC rating is much more commonly used. Recommended STC ratings for meeting rooms and offices are 45 to 50, however a common STC rating for a standard double-glazed window is only between 28 and 32.   

This has given rise to the increased use of laminated glass for increased sound comfort. Bonding two pieces of glass together with a thin plastic interlayer has proven to increase the STC rating of glass by around four. Two laminates can increase the original STC rating by around eight. Even still, until the STC rating is above 40, building occupants will often complain about exterior noise.

According to the NRC’s published data, in order to exceed STC 40 you need one of the following configuration options:

• Double glazing with glass four millimeters thick and an interpane spacing of 100 millimeters
• Double glazing with glass six millimeters thick and an interpane spacing of 90 millimeters
• Triple glazing with glass three millimeters thick and interpane spaces of six and 100 millimeters  

They also clearly state that “The STC data listed in the table are for typical windows, but details of glass mounting, window seals, etc., may result in slightly different performance for some manufacturer’s products.” We have seen instances where missing heel beads, window seals and pinholes have resulted in noise complaints due to greater sound transmission through these voids. Since glass is only one part of the window or curtainwall system, consideration also needs to be given to the sound transmission occurring through the frame. By adding insulation into the framing components both the thermal and sound performance are improved.

So why were a couple people talking about this with me on the trade show floors? It was because both the contractor and supplier wanted to go above and beyond on specific projects they were working on. Our building codes are not pushing us to improve STC ratings and rarely will clients pay the premium cost to increase to triple-glazed or laminated windows when double-glazed will do. But as the push continues for lower U-value window systems, one of the side benefits will be improved STC ratings. And that sounds good to me.•

David Heska, P.Eng. is a director with WSP’s building sciences team in southwestern Ontario. He oversees the operation of the Hamilton, Kitchener and Windsor offices. David has been involved on window simulation projects as well as the design and replacement of windows in commercial and residential buildings. He can be reached at David.Heska@wsp.com