We are in the process of designing a new school registered under the LEED for Schools Rating System. Our team is planning to use 6" CMU classroom wall construction - which is the preferred construction for schools of this type and location - but it appears that this material will not meet the required sound transmission class (STC) ratings for classroom partitions. Add-ons of paint, grout-filling, gypsum board, and sound-attenuation blankets are either insufficient to meet the minimum STC\'s of 50-60, or are not possible for the nature of this construction. Previous communications with USGBC Customer Service suggested that we consider acoustic block for classroom walls. However, our research indicates that this strategy will help with reverberation time requirements (which we are meeting) more than it will help with STC requirements (which we are struggling with). In choosing this wall design for durability, stability, cost-effectiveness, and contributions to energy-efficiency, we believe we were making the best decision for the balance required to attain LEED credits from all categories within our available budget. However, it appears that the only available option to meet the LEED-S STC requirements is increasing the CMU wall thickness to 8" or 10", which would impact materials costs, structure cost and available square footage such that we would no longer be able to earn enough credits to certify the building. Because this prerequisite does not have a history of case experience to draw from, we would like to confirm that we are interpreting the requirements properly. If so, we would ask whether it is possible to pursue an Alternate Compliance Path to meet the Intent of this prerequisite, wherein we would keep the planned 6" CMU walls, increase their STC ratings as much as possible, and meet the rest of the prerequisite\'s requirements (window and door STC\'s, classroom reverberation time and background noise levels) as normal. If using an alternate compliance as described above is not sufficient to meet the intent of the prerequisite, we would appreciate any guidance you could provide on how to cost-effectively meet the technical requirements as written.
The CIR is requesting an alternate compliance path to this credit, where the minimum STC rating is not achieved but the structural wall system that has been chosen to meet a wide variety of criteria is improved to achieve the maximum STC rating possible for that type of construction. Consistent with the CIR ruling published 12/19/07 for this credit, the prescriptive requirements can be exempted when other criteria are being met. If 6-inch thick CMU classroom wall construction is the preferred for schools of this type and location, there should be existing walls that are virtually identical to the proposed constructions. Therefore, several of these walls should be field tested to quantify their actual noise reduction. If the measured noise is NIC (Noise Isolation Class) 45 or more, then it complies with the STC 50 minimum rating requirement for classroom walls according to ANSI S12.60-2002 [4.7 (3)]. Applicable Internationally.