Question #1 The current prerequisite identifies the minimum STC rating that is needed, but not the minimum F-STC rating that is needed. In relying on ANSI Standard S12.60-2002, the credit accepts F-STC testing, but does not give the requirement for it. Without the requirement for F-STC, are we to assume that the requirement for STC and F-STC are the same even though the test methods and industry standard practice always presents different requirements for these two different test methods - ASTM E413 and E336, respectively? For example, the 2009 International Building Code Section 1207, "areas shall have a sound transmission class (STC) of not less than 50 (45 if field tested)." The "F" on F-STC represents the F in Field for the field sound transmission class. What is LEED\'s requirement for the F-STC? Question #2 - (dependant on answer to #1) With the current acoustical language in this credit, our project is left with the following dilemma. There is an official field test on an assembly similar to our project. It contains a 1/2" less wood mass and a 1/2" less Maxxon Gypsum Based Underlayment mass than what we will be doing. The rating achieved in that test is an F-STC 47 (Twin City Testing #02 31573-6). While this meets the industry standard practice of F-STC rating of 45 if a 50 STC was requested, at this point it may not meet the LEED credit. There are thousands of ways buildings are constructed. Only the most commonly constructed assemblies are tested for STC ratings. When a test does not exist for an exact assembly, most code bodies accept estimations or calculations based on solid data, experience, or historical performance. In our case there is a practical ways to accurately estimate the STC of an assembly. One of the laws of physics is expressed as follows: TL = 20 log (f * m) -34 where TL = transmission loss (dB) f = frequency m = mass The known mass of each component in an assembly can be added up and the total inserted into this equation. From the data obtained from the equation, an STC can be calculated. By adding more mass to the current assembly of this project, an STC of over 50 can be calculated. This calculation is not an actual lab or field test. Will this calculation be acceptable for LEED?
It should be noted that as an alternative to addressing the submitted questions, the EQp3 STC requirements can be met by submitting a narrative as specified in the LEED for Schools Performance/Intent Equivalent Alternative Compliance Path (PIEACP), "Indoor Environmental Quality, Prerequisite 3: Minimum Acoustical Performance", dated April 23, 2008. The PIEACP requires a narrative describing measures taken to limit sound transmission between core learning spaces, but does not require specific STC ratings for demising walls or other sound isolation elements. This enables design teams to meet the STC prerequisites by employing and describing sound control measures, without meeting or documenting specific STC performance requirements. Submitting a narrative per the PIEACP is a straightforward approach for compliance, and will eliminate the need to consider Questions 1 and 2. The rulings below apply if the primary compliance path for STC performance per LEED for Schools 2007 is pursued. Question #1: The LEED requirement for compliance with STC ratings is based on laboratory ratings. Post-construction measurement to confirm compliance is not required, but STC performance must be clearly documented in the design phase with reliable test data confirming that STC ratings will be met. The supporting test data must be conducted in accordance with appropriate ASTM standards, and provided by a NVLAP- accredited test lab or other reliable source such as an acoustical consulting firm that is a member of the National Council of Acoustical Consultants (NCAC), or the Institute of Noise Control Engineering (INCE). Question #2: It is widely recognized that field measurements are typically lower than laboratory ratings, with a 5 point difference allowed per building codes and many municipalities. The FSTC 47 assembly can be considered compliant with the STC 50 requirement, particularly considering that the design assembly has higher mass than the tested assembly. Applicable Internationally.