The construction industry is responsible for approximately 42% of European energy consumption and 35% of CO2 emissions. With the UK and the European Union moving the construction market towards more sustainable practices, many professionals are currently searching for new solutions.
Wood stores carbon and is generally able to re-grow rapidly providing an excellent opportunity for the construction sector in general, as long as sourced from well-managed forests.
Cross-laminated timber (CLT) is now becoming a reference model in Europe, being used not only for residential development but also for multi-storey and multi-use buildings.
The many aspects of CLT design and construction have been consolidated, however, in depth understanding of the acoustics performance of such solutions has started only recently.
This blog post aims to explain the main limitations in the acoustics performance predictions for CLT buildings and provides recommendations to overcome some of the issues.
The acoustic properties of CLT systems and the lack of laboratory data
CLT panels have a low density, combined with relatively high stiffness. Moreover, CLT elements are considered to be orthotropic, i.e. its material properties differ along three mutually orthogonal twofold axes. Such properties add complexity to the understanding and prediction of the acoustics performance of CLT systems.
Like other modular and pre-fabricated systems, CLT panels need to be completed with additional layers (for thermal insulation, sound insulation etc.). However, there is an evident lack of laboratory data for wooden constructions when compared with other solutions.
Also, more often than not, CLT manufacturers only supply the panels as a semi-finished product, and as such, it is relatively difficult to find reliable data for CLT systems with additional layers.
Tests carried out to date, indicate that lined CLT panels can provide sound insulation levels up to 55 dB Rw, for single-side lining and up to 65 dB Rw, for double-sided linings. However, sound-absorbing layers, resilient studs and at least 2 layers of dense plasterboard are required which means typical thicknesses of approximately 250-350 mm.
Unfortunately, current standardised procedures to determine the improvement of airborne and impact sound insulation by linings refer to base structures that differ considerably from CLT panels.
The current issues with flanking transmission predictions
BS EN ISO 13254-1 sets out a detailed prediction method for airborne and impact sound insulation between different rooms, which has been proven to be reliable. The calculation method combines direct and flanking transmission as a function of the features of all the elements involved and the interaction between their connections.
According to ISO 12354-1, flanking noise can be defined as “the transmission of sound energy from an excited element in the source room to a receiving room via structural (vibration) paths in the building construction, e.g. walls, floors, ceilings”.
If a building is not properly analysed and flanking paths are left “untreated”, even when the main separation wall/floor offers high acoustics resistance performance, the overall sound insulation between two spaces may be substantially lower than initially predicted.
When calculating flanking transmission, the most critical parameter is the vibration reduction index Kij which in line with ISO 12354-1 needs to be calculated for each independent transmission path. This quantity represents the vibration energy dissipating into each element junction. In simples terms, the higher the values of Kij, the better is the junction performance and the less relevant a specific transmission path is.
The last version of ISO 12354-1 has introduced parametric relations to help determine values of Kij for CLT panels - see Figure 1. However, these are limited to X and T shaped junctions between mass elements per unit area 0.5<m1/m2<2.
Moreover, following studies which focused on the effects of the various types of metal connectors used to fix the CLT panels together, it has been determined that vibration transmission which occurs through vertical CLT junctions is mainly due to the metallic connections rather than to the actual transmission of vibration at the panel-panel interface. As such, significant differences have been noticed between the kij values determined and the ones included in BS EN ISO 12354-1, for similar types of junctions.
It is important to keep in mind that certain types of connections may be needed to provide dimensional stability and stiffness, which make the panels suitable for earthquake resistant constructions. Thus, a balance between structural and acoustics requirements is necessary.
Given the above, it becomes evident that to improve the accuracy of the prediction methods used, it is crucial to correctly define installation methods beforehand.
Figure 2 shows some examples of typical connections for CLT shear walls.
Recommendations
The limitations with predicting sound insulation for CLT systems are evident. Despite the numerous researches already carried out, due to the complexity associated with the different types of connections and the lack of laboratory data, this knowledge is not yet consolidated.
An effective approach used by others would consist of running simulations and carrying out site testing simultaneously to calibrate and improve performance simulations. To add even more complexity, symmetrical configurations of the panels may not correspond to a symmetrical connection of the screws which in turn may create differences in lateral transmission.
Reference (pre-tested) Kij values can also be used to determine the performance of various junction configurations. The most relevant studies we could find are the following:
Morandi, F.; De Cesaris, S.; Garai, M.; Barbaresi, L. Measurement of flanking transmission for the characterisation and classification of cross laminated timber junctions. Appl. Acoust. 2018, 141, 213–222.
Speranza,A.;Barbaresi,L.;Morandi,F.Experimental analysis of flanking transmission of different connection systems for CLT panels. In Proceedings of the WCTE 2016—World Conference on Timber Engineering, Vienna, Austria, 22–25 August 2016.
Barbaresi,L.;Morandi,F.;Garai,M.;Speranza,A.Experimental measurements of flanking transmissionin CLT structures. Proc. Mtgs. Acoust. 2016, 28, 015015. [CrossRef]
Reference Rw laboratory values for various constructions with different lining layers can be found here:
Canadian CLT Handbook - https://web.fpinnovations.ca/clt/
Rothoblaas systems - https://www.rothoblaas.com/catalogues-rothoblaas
Stora Enso systems - https://www.storaenso.com/en
A less economically but effective approach is to consider multilayer linings with sound-absorbing materials where certain transmission paths performance are associated with high levels of uncertainty.
References
Di Bella, Antonino & Mitrovic, Milica. (2020). Acoustic Characteristics of Cross-Laminated Timber Systems. Sustainability. 12. 5612. 10.3390/su12145612.
Jayalath, Amitha & Navaratnam, Satheeskumar & Gunawardena, Tharaka & Mendis, Priyan & Aye, Lu. (2021). Airborne and impact sound performance of modern lightweight timber buildings in the Australian construction industry. Case Studies in Construction Materials. 15. e00632. 10.1016/j.cscm.2021.e00632.
Bettarello, Federica & Gasparella, Andrea & Caniato, Marco. (2021). The Influence of Floor Layering on Airborne Sound Insulation and Impact Noise Reduction: A Study on Cross Laminated Timber (CLT) Structures. Applied Sciences. 11. 5938. 10.3390/app11135938.