How to reach Rw 42 dB on a window next to a road

First, the terms: Rw and Rw + Ctr

Rw is the laboratory sound insulation rating to EN ISO 717-1: a single number that sums up the whole curve across the frequencies. It comes with two adaptation terms: C for sound similar to speech and music, and Ctr for traffic. Traffic noise is low-pitched, lorries and buses, and glass is at its weakest precisely at low frequencies. That is why Ctr for glass is negative, usually by a few decibels.

A window next to a road should be judged on Rw + Ctr, not on Rw alone. When someone offers you “42 dB”, the first question is: 42 of what, and for which make-up?

What actually works

Mass. Thicker glass stops sound better, but only slowly. You will not get to 42 on thickness alone, and the window gets heavy.

Asymmetry. Every pane has a frequency at which its performance dips, known as the coincidence frequency. For 4 mm glass it is around 3 kHz; for thicker glass it is lower. Two identical panes dip at the same point and the weakness adds up. Panes of different thickness, say 6 and 4 mm, dip at different points and each covers for the other. That is why 6+16+4 performs better than 4+16+4: Rw 34 against 30 dB.

Acoustic PVB interlayer. Laminated glass with a standard interlayer already helps, because the interlayer damps the vibration of the pane. An acoustic interlayer has a softer core and damps more strongly, so the coincidence dip is flattened out. The same 4.4.2 gives Rw 34 dB with a standard interlayer and 38 dB with an acoustic one. These are the cheapest decibels in the whole make-up.

Cavity. Going from 12 to 16 mm makes little difference. 20 mm adds a little more. The real jump only comes with a gap of several centimetres, and that is no longer an insulating glass unit but a window with two sashes. Argon does nothing for sound. It works for Ug.

What is just marketing

• Triple glazing is quieter. Not in itself. IG Low-E 4+16+4+16+Low-E 4 [44 mm] has three identical panes with three identical dips: Rw 32 dB, only 2 more than double glazing. Triple glazing with asymmetry and laminated glass can do it, but then you pay for both the mass and the thickness.
• Gas against noise. Argon and krypton do not change the sound insulation. The gas that once did (SF6) can no longer be used.
• A number without a make-up. “Acoustic glass 42 dB” without the panes, interlayer and cavity spelled out means nothing. The same goes for Rw without Ctr.
• The glass instead of the window. The Rw of the glass is not the Rw of the window. The frame, gaskets, roller shutter box, trickle vent and the perimeter joint sealing on installation can eat up five decibels or more. The target should be set for the installed window and confirmed by measurement on site.

The recipe for 42 by the road

IG Low-E 4+14+4+14+LamiLowe 4.4.2 SR [44.76 mm]: triple glazing with acoustic laminated glass on the inside. Rw 42 dB. If you need more, for example when the target is set as Rw + Ctr, move to a thicker acoustic laminate or laminated glass on both sides, up to 48 dB.

Besides the glass, the frame profile and the installation matter too: sealing the joint between frame and wall. The roller shutter box is a separate question and usually the weakest point of the façade.

In your enquiry, give the target as Rw + Ctr along with the façade orientation, so the make-up is worked out from that figure rather than from a catalogue.