I recently noted with a great deal of interest that the National Fenestration Rating Council (NFRC) approved a new proposal for an optional ventilation rating procedure for operable fenestration products. As fenestration U-values have been pushed lower and lower in recent years, the NFRC is thinking about other important factors and test methods that may provide homeowners with additional comparative information to help them decide which door or window product is best for them.

Yes, we have spent a great deal of time talking about and comparing the energy efficiency of windows. But now that summer is approaching, the sound of lawnmowers in my neighborhood has me pondering another important aspect of window design — sound control. Yes, ventilation is nice in the spring. Open the windows and let that fresh air into the house. But now that summer is upon us, it is time to focus on another great reason to replace those old windows…more peace and quiet. Unwanted neighborhood sounds such as lawnmowers, screaming youngsters, barking dogs, motorcycles, trucks, planes, trains and automobiles…..can all be better dampened by new windows.

When it comes to rating windows for sound control, the typical homeowner looking at an NFRC label does not currently have access to a rating method to compare the relative degree of sound deadening that a fenestration product can provide. Test methods currently exist, but the testing is usually only done by window companies vying for commercial jobs with specific sound attenuation goals in mind, such as high-end residential or commercial buildings situated near airports, train stations or busy highways. In these situations, architects are faced with a project that entails installing windows with a minimum STC (Sound Transmission Classification) value. This might be dictated by a building code or simply specified by the building owner. The STC value is the standard way to rate a window for sound transmission. Introduced in 1970 as ASTM E413, STC is a value based upon the attenuation that is required to reduce each octave level of the noise spectrum to match what is defined as the NC-25 contour. This is a weighting method that is designed to compare the relative attenuation of fenestration systems of office-type noises (speech, radio, television and similar noises).

STC ratings can be influenced in the design of windows by paying attention to the glass thickness, air infiltration, air gap, type of glass (annealed vs. laminated), type of spacer (non-metal and foam spacers are quieter), the use of insulating glass films, argon filling, edge dampening and the use of dissimilar materials to break up harmonic waves.

Laminated glass also offers significant improvements in sound reduction while offering additional benefits such as improved security. There are specific interlayers that can be used to absorb sound waves. Also, by combining glass panes of dissimilar thickness within the IG unit, sound transmission can be reduced by preventing the buildup of harmonic waves. Harmonic waves are now canceling each other out when peaks in the waveforms are overlapping troughs as opposed to peaks overlapping peaks and troughs overlapping troughs. This results in reduced amplification, aka “sound deadening.” At one point in my career, I used this same technology as a tire design engineer when designing tread patterns. Look at your tread pattern around the circumference of your tires and you will see the tread elements are of different shapes. That’s intended to produce wave patterns that cancel each other out, resulting in less road noise.

Today’s forward-thinking architects are now digging deeper into sound data as opposed to just looking at the single-number rating systems such as STC or the optional OITC (Outdoor Indoor Transmission Classification) evaluation method. They appreciate the knowledge that certain materials or technologies might reduce sound transmission across a specific frequency range even though the STC value might not even change at all.

For example, in addition to lowering U-values, argon gas can also be used to reduce sound transmission across a specific bandwidth or sound frequency range. For example, if considering the bandwidth between 500 to 3000 Hertz, argon may provide sound deadening even though, because of the way STC is calculated, it may not show up as a lower STC value. If one compared only the STC values of two windows (with and without argon), you might conclude that the only benefit of argon is slightly better thermal performance (lower U-value). But by digging deeper into the data, a savvy architect may find that improving sound control in a specific frequency range is an added benefit the argon provides.

Technologies for designing windows for improved sound control are likely to become increasingly important as our nation grows and population density increases. This author envisions the day when the NFRC label also includes an optional rating for window systems that compares the relative sound deadening of the window to help window customers decide which window will add more peace and quiet in their type of neighborhood. Does this sound too sophisticated? Maybe so, but the technology exists — and it just may help us sell more windows!


  1. As I would expect, a valuable article Jim! I do get asked about sound control more and more. I’m glad you mentioned OITC, which to my limited knowledge makes more sense than STC for our industry. One issue, of course, is the cost to manufacturers of more testing, particularly when each glass will have a different level of performance. If OITC and STC could somehow be added to the calculated output of the LBL simulations, then adding it to labels and various web listing would be easy and very valuable to end users.

  2. Another Great Article, Jim!
    Windows that reduce “Noise Pollution” inside of living spaces are very much appreciated by homeowners. Ever since argon gas became prevalent, clients with new windows told us they enjoy a quieter home.
    But it seems homeowners only place real value on this benefit AFTER they experience the difference. Getting consumers to recognize the value of sound insulation before experiencing a quiet house is the tricky part…
    Maybe adding STC Value or OITC evaluations to the Energy Star label would help… But the Energy Star sticker is SO CONFUSING to the homeowner already.
    Maybe adding a number to reflect the sound deadening property of fenestration to the Energy Star Sticker could be part of an overall effort to make the sticker easier to read. The sticker tends to confuse consumers about what window or door is actually better for them…
    Before we add another number the consumer doesn’t understand to the Energy Star sticker, we should make the label as easy to read and understand as a mpg sticker on a car… We might start by using R-Value instead of the eternally baffling U-Factor!
    Sorry for digressing. Thanks for bringing up a great topic – a differentiating factor window dealers should present to the consumer during the sales process.

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