When engineers talk about the acoustic performance of an insulating glass unit (IGU), the conversation typically centers on the glass: its thickness, the number of lites and the interlayer. But research suggests there may be another piece of the equation worth considering: What happens around the perimeter?
The edge of an IGU can influence how sound travels through the assembly, though the effect may depend on more than the spacer alone. Researchers have found that the broader edge system, including the spacer, sealants and their mechanical interaction, can affect acoustic performance, although the size of that effect varies.
In a 2024 study presented at the 30th International Congress on Sound and Vibration, Mark Koehler, Yohko Aoki and Sven Öhler of the Fraunhofer Institute for Building Physics IBP examined whether the edge system, particularly a spacer’s stiffness, affects the acoustic performance of modern triple-glazed IGUs. The researchers note that up until then, there was limited information about spacer dynamic properties.
Testing the Edge
Koehler et al. sought to address a practical problem: modern IGU acoustic performance is often predicted from glass thickness and cavity width, while the edge bond, spacer properties, geometry and mounting conditions can cause the performance of an actual window to differ from laboratory ratings.
“The acoustic performance of glass lites is usually estimated by the glass and air layer thickness, disregarding the edge bond and is proven by a test certificate, which refers to the ISO standard measurement of the sound reduction index in a laboratory,” note Koehler et al. “Since windows in real construction generally have different dimensions, edge connections and mounting conditions, the acoustic performance of the window in construction often differs from the value specified in the test certificate. This project aims to develop a method to predict the sound transmission loss of glass panes by measuring only certain parameters of the edge bond.”
Koehler and his team tested 20 glass lites with five different formats and four lite configurations according to ISO 10140 (methods and requirements for testing the sound insulation of building elements) in third octave and narrow band frequencies. They also tested the elasticity and loss factors of window putty, IG sealant and spacers made of different materials (stainless steel, polymer and hybrid). The results of the ISO standard measurements and simulations were then compared with each other.
The researchers concluded that softer spacers provided somewhat better acoustic performance, but the effect was not significant within the range of spacer stiffnesses they measured. They reasoned that spacer stiffness matters more to accurately model the system than it does to change the acoustic performance of the IGU.
Technoform’s Helen Sanders agrees that spacers change the acoustic performance of an IGU minimally, with the difference between a hybrid spacer and a flexible spacer being around one point in Sound Transmission Class (STC)/Outdoor-Indoor Transmission Class (OITC) of the glass itself (not the fenestration assembly). She says, however, that Technoform’s data indicates the inverse of Koehler et al.’s conclusion, finding that a more rigid hybrid spacer performs better acoustically (and closer to an aluminum spacer) than a more flexible compressible spacer.
Of course, the finding that spacer stiffness has a relatively small effect does not necessarily mean the IGU perimeter can be ignored acoustically.
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