Fibre Reinforced Facade Encyclopedia: Properties
Acoustic performance of a facade is the sound insulation of the complete external wall, not of the panel on its face. Airborne sound insulation is governed principally by mass, and a thin composite panel has very little of it. A 40 kg per square metre cladding panel acting alone gives a weighted sound reduction index in the low thirties, which is not enough for any occupied building on a noisy site. The insulation comes from the build-up behind the panel, from the backing wall, the cavity, the insulation and above all from the glazing, and the panel contributes a useful but secondary part of the total.
Doubling the mass of a single leaf buys about 6 dB. Going from a 40 kg per square metre panel to an 80 kg per square metre panel is a change most occupants would not notice, and it doubles the load on the structure.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Mass law, single leaf | about 6 dB gain per doubling of surface mass | Theoretical; real panels fall short of it |
| Mass law, per doubling of frequency | about 6 dB | Low frequency insulation is always the hard part |
| Cladding panel alone, 30 to 50 kg/m² | Rw of roughly 30 to 35 dB | Indicative for a single leaf at that mass; typical |
| Lightweight FRP panel, 5 to 32 kg/m² | Rw below about 30 dB at the light end | Indicative; mass is the limiting factor |
| Complete rainscreen wall build-up | Rw of roughly 50 to 60 dB achievable | Panel, cavity, insulation and a heavy backing wall; typical |
| Typical governing element | The glazing, then the junctions | Not usually the opaque panel |
| Spectrum adaptation term | Rw + Ctr is the relevant descriptor for traffic noise | Ctr is a negative correction, often 3 to 7 dB |
| Cavity absorption | Absorbent lining in the cavity improves the double leaf | Mass-spring-mass resonance must be pushed below the range of interest |
| Test methods | ISO 10140 and ISO 717-1 (laboratory), ISO 16283-3 (facade, field) | Rw, Rw + Ctr, and field measured Dnt,w |
Mass law, and why a thin panel cannot beat it
For a single homogeneous leaf, airborne sound insulation rises roughly six decibels for every doubling of surface mass and roughly six decibels for every doubling of frequency. That relationship is the mass law, and it is the reason acoustically demanding walls are heavy. It also sets a hard ceiling on what any thin facade panel can achieve on its own.
A glass fibre reinforced concrete (GRC, GFRC) panel at 30 to 50 kg per square metre is, acoustically, a light single leaf. It will give a weighted sound reduction index somewhere in the low thirties in a laboratory, and rather less in practice. A fibre reinforced polymer panel at 5 to 32 kg per square metre gives less again at the light end of that range, simply because it has less mass. Neither figure is a criticism of the material; it is arithmetic that applies equally to aluminium sheet, fibre cement and terracotta at the same surface density.
What the wall system does
A rainscreen or ventilated facade is a double leaf construction, and double leaf systems can substantially outperform mass law if they are designed for it. The outer panel and the backing wall are two masses separated by a cavity that acts as a spring. Above the mass-spring-mass resonance frequency of that system, insulation rises much faster than mass law predicts. Below it, the system performs worse than a single leaf of the same total mass.
Three things decide whether the double leaf helps or hurts. Cavity depth, because a deeper cavity lowers the resonance frequency and widens the useful range. Absorption in the cavity, because an absorbent lining damps the cavity resonances and is worth several decibels. And structural isolation, because every bracket that rigidly connects the panel to the backing wall is a sound bridge that short circuits the cavity.
The backing wall carries most of the load. A concrete or masonry backing wall, or a well built stud wall with two layers of board and insulation in the stud space, provides the mass and the damping. In practice a complete rainscreen build-up with a heavy backing wall reaches an Rw in the fifties, and the outer panel contributes perhaps a few decibels of that total.
The glazing governs, and so do the junctions
On almost every occupied facade, the window is the acoustic weak point and it sets the result. Glazing typically occupies a large fraction of the facade area and performs well below the opaque wall, so the composite performance of the facade is dominated by it. Improving a panel from Rw 32 to Rw 36 while leaving a large area of ordinary double glazing changes the overall facade performance very little.
After glazing, the failures are junctions rather than elements. Air paths transmit sound directly, and a facade with excellent elements and poor sealing performs badly.
- Panel to window head, jamb and sill junctions, where the membrane and the seal have to be continuous
- Slab edge and spandrel zones, where the backing wall is often interrupted and the fire and acoustic seal must be built and inspected
- Service penetrations: vents, outlets, mechanical louvres and conduits through the wall line
- Flanking transmission around the wall via the slab and the structure, which laboratory Rw figures do not capture at all
- Cavity barriers and closures, which have to be continuous for both fire and acoustic reasons
Specifying it honestly
Ask for the right descriptor. Rw alone is weighted toward speech frequencies. For road and rail noise the relevant descriptor is Rw + Ctr, and Ctr is a negative correction of commonly three to seven decibels that reflects the low frequency content of traffic noise. A facade specified in Rw and exposed to heavy traffic will disappoint.
Then apply the requirement that actually governs. In Australia the National Construction Code sound insulation provisions in Volume One deal principally with walls and floors between sole occupancy units rather than with external walls (check the edition adopted in your jurisdiction). External facade requirements usually arise from state planning policy for road and rail corridors, from AS 2021 for buildings near airports, or from a project specific acoustic report. Those documents set the target; the acoustic consultant then distributes it across glazing, opaque wall and ventilation, and the panel specification follows from that allocation rather than leading it.
Standards and references
- ISO 10140 series, laboratory measurement of sound insulation of building elements
- ISO 717-1, rating of airborne sound insulation, including Rw and the spectrum adaptation terms C and Ctr
- ISO 16283-3, field measurement of sound insulation of facades
- AS/NZS 1276.1, rating of sound insulation in buildings and of building elements, airborne sound insulation
- AS 2021, acoustics, aircraft noise intrusion, building siting and construction
- AS 3671, acoustics, road traffic noise intrusion, building siting and construction
- NCC 2022 Volume One Part F7 and Specification 28, sound transmission and insulation (check the edition adopted in your jurisdiction)
- CWCT Standard for Systemised Building Envelopes, acoustic performance requirements for facades
Related terms
Questions specifiers ask
What Rw does a GRC panel achieve?
On its own, roughly the low thirties in decibels at a surface mass of 30 to 50 kg per square metre. That is mass law, and no thin panel of any material at that weight does substantially better.
Can a lightweight facade panel meet a traffic noise requirement?
Not by itself. The requirement is met by the wall system: the backing wall, cavity, insulation and glazing. The panel contributes a few decibels of the total and is rarely the governing element.
Does a heavier panel solve a noise problem?
Barely. Doubling the surface mass of a single leaf buys about 6 dB, and doubling the weight of a facade has large consequences for the structure, the substructure and the craneage. Adding mass or performance to the backing wall and the glazing is far more effective.
Why does the cavity matter?
A rainscreen is a double leaf system. Above its mass-spring-mass resonance it outperforms mass law, and cavity depth, absorption in the cavity and isolation of the brackets determine by how much. Rigid brackets act as sound bridges and undo the benefit.
Should a specification use Rw or Rw + Ctr?
Rw + Ctr for road and rail noise, because Ctr corrects for the low frequency content of traffic. Rw alone is weighted toward speech frequencies and will overstate performance against traffic noise.
Acoustic targets are met by the wall system. Bring the acoustic consultant’s allocation to the facade package early and the panel specification follows from it. Talk to the ShapeShift team
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.