The project is a school located in a quiet suburban area in California where the environmental noise level is below 60 dB DNL. The proposed window assemblies have STC ratings up to 34. Interior environmental noise levels have been calculated to be below 30 dBA with the proposed assemblies. Please confirm that at "quieter" sites like the subject projects, window assemblies that have STC ratings less than 35 are acceptable. The window assemblies have been selected to achieve energy and thermal efficiency goals. A one-STC point reduction in performance is not considered to be acoustically significant. In addition, the roof-ceiling assembly at the project is calculated to have an STC that is modestly below 40, due to skylights and other daylighting components. In the CIR dated 19 October 2007, roof-ceiling assemblies that are less than STC 50 (i.e., STC 40) are acceptable, provided that the site DNL is below 60 dB and that rooftop mechanical noise is controlled to meet the prerequisite\'s requirements. We have calculated rooftop equipment noise through our roof assembly and have designed noise reduction measures for the equipment (through quiet equipment selections and acoustical barriers surrounding the rooftop equipment), and also added mass to the roof assembly in a targeted way to meet the background noise requirement in classrooms of 45 dBA. Thirdly, certain core learning spaces will be science lab/classrooms with fume hoods. We assume that the requirement for a maximum background noise level in classrooms of 45 dBA does not apply to fume hoods. The hoods will be on during experiments, but will be either turned off or covered during instructional times. While fume hood noise will be mitigated to the ASHRAE Chapter 47 NC 35 - 45 standard, it will exceed 45 dBA. Supply and return ventilation system noise will be designed to meet the required 45 dBA.
There are three questions posed in this CIR - one relating to the STC value for window assemblies, the second pertaining to the roof-ceiling assembly STC value, and the third pertaining to fume hoods. In regard to the window assemblies, an STC of 34 is acceptable under the described circumstances as the interior environmental noise levels have been calculated to be below 30 dBA, and the environmental noise level is below 60 dB DNL. Similarly, the roof-ceiling assembly is acceptable under the specific conditions as the design achieves a background noise level of 45 dBA. Lastly, fume hoods can be excluded from the requirement for background noise levels, and as noted, should be turned off during instructional periods. Applicable Internationally.