STC ratings of exterior walls and roof assemblies

We have our first new non-res shell building project subject to CALGreen.

It is within 1000' of a freeway right of way, is a proposed bank so it is not an occupancy that falls under the exception, so CALGreen 5.507.4.1 requires that the envelope wall and roof-ceiling assembies have an STC rating of at least 50. Proximity to property line, type of construction and use do not mandate any fire-resistant construction, so it only the STC we are dealing with.

Both the GA manual and UL do not list STC ratings for exterior walls or roof assemblies. only fire-resistance ratings. The CBSC Non-res guide has a link that provides some wall and floor-ceiling assemblies, but does not include roof assemblies,

The architect has proposed an interior wall assembly with an STC rating of 51-54 for the exterior wall, and adding a 7/8" stucco to the exterior membrane. Their Acoustical consultant is proposing adding insulation to the ceiling assembly, but I can't find any quantifiable way to determine whether it meets code or not.

Any thoughts?
Original Post
Adding mass (weight) to most any light frame type construction assembly will increase the STC rating. So if you are starting with an assembly that already meets or exceeds the minimum, you should be OK with additional layers or mass elements added to the assembly.

Since the sound ratings are now more pervasive in their application, hopefully there will be better information available in the near future (doesn't help much today). Ask the Acoustical Consultant for reference material sources so you can use them on plan reviews.
Your acoustic engineer can use the aggregate calculation method to determine within close tolerance what the resultant performance will be. The inherent weakness will be the joints/interfaces of components, recessed items, utility penetrations, etc. which are the acoustic 'leakers' - so regardless of what is 'calculated' unless the assemblies are all installed exactly as specified, performance will always fall short. That is why in-place field test result requirements are set lower than what you have called for when using an 'rated, tested, approved assembly'.

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