Dwellings with Cross Laminated Timber (CLT) have become very popular lately. When designing a multi-family dwelling in CLT there are some critical factors to consider with regards to acoustics and sound insulation. A characteristic property of CLT is that it is very sensitive to flanking transmission i.e. sound transmission between two rooms another path than through the directly separating partition. What this means in practice, is that whatever you do, do NOT mount CLT elements that are continuous between two dwellings. That is a recipe for disaster. It might work in premises or between rooms with lower requirements, but in dwellings it is a big NO-NO. So, what solutions are there?
A well-known issue with timber constructions (in general) is that they have a reputation of having problems with impact sound insulation, especially in the low frequencies. However, with CLT buildings it is not impact sound that is the issue. We have that under control and there are plenty of solutions that will safely fulfill the requirements. The primary challenge with CLT buildings is airborne sound insulation, due to the flanking transmission in both horizontal and vertical direction.
You know the saying that the chain is never stronger than its weakest link and that is also the case here. Even if you have a floor structure capable of something extreme, like Rw = ~65 dB, that will be all for nothing if the flanking path only can reach let’s say Rw = ~47 dB. An analogy with electronic circuit theory would be to connect two (or more) resistors in parallel and let each resistor represent one sound transmission path in the building. First connect a 1 MOhm resistor and then connect a 100 Ohm resistor in parallel. What happens? ALL the current will go through the 100 Ohm – the path of least resistance. Sound transmission is no different. To achieve good sound insulation, each of the resistors in your acoustic circuit needs to be of the same magnitude. Sometimes you can get by with a weaker path, if it is only a small part of the total partition. One example would be a window in a large outer wall. The wall with have very high sound insulation and surface area compared to the window and compensate somewhat. If you have a very large window in that same wall, it won’t matter at all that the wall has much better sound insulation. The total will be determined primarily by the window.
Let’s consider vertical flanking transmission. The primary flanking transmission path will be through the load bearing structure. A common solution for unit separating partitions, is to use one load bearing CLT element, and then a separate light framed wall on the other side mounted with air gap and mineral wool as a double construction. That solution seems to offer the most “bang for the buck” with regards to wall thickness. A double CLT frame is also possible, but they tend to become somewhat thicker. On the other hand, with a double CLT frame, you have the option of visible wood on both sides! That is not possible with the first solution. To take care of the vertical sound transmission a common solution is to mount elastic interlayers on the top side of the floor slab, or even better, two thinner elastic interlayers both above and below each floor slab. Structural engineers seem to prefer the solution with two thinner strips compared to one larger strip. This is because two 6 mm strips will not deform nearly as much as one 12 mm strip. Consequently, we have now avoided continuous CLT elements in the vertical direction.
We have indications that there are huge savings to be made when optimizing the usage of elastic interlayers. They are very expensive and thus need to be carefully designed. Another option to get rid of them altogether, is to “hide” the whole CLT load bearing frame on both sides. Consider the above mentioned construction, but now add a second layer on the CLT side with 10 lath, 25 acoustics/resilient steel profile and one layer of gypsum. Now the whole structure is hidden and can be mounted without elastic layers. This can be useful when designing high-rise buildings or premium buildings where very high sound quality is desired. On the downside, wall thickness increases and valuable square meters are lost.
In this blog we have only scratched the surface on the topic of flanking transmission in CLT buildings. There are many other transmission paths to explore and options to consider. And a lot of money to save. Stay tuned!