Write an Effective Specification for Rooftop Unit In-Curb Acoustical Treatments
The Top 5 Considerations of an Effective In-curb Acoustical Specification
1) Require a reviewable material submittal for the In curb treatment
Whether or not there is a reviewable submittal for the in-curb acoustical treatment on an RTU project depends mostly on WHERE the specification is embedded in the project documents. The most logical and frequently used spec section is in the Vibration and Sound Section but the best location is to embed the acoustical treatment spec in with the RTU equipment specification. Contractors routinely take "liberties" on the in-curb treatment when it falls inside the boilerplate V&S spec.

2) Determine the product minimum performance rating to an industry appropriate test methodology
First, let's talk about the rating methodology. For this application the appropriate rating methodology is Standard ASTM E90-09(2016) which was last revised in 2009 and last reapproved in 2016, "Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements". Be sure to include in your specification BOTH the STC overall single digit rating AND the Transmission Loss (TL) values across the frequency bands because all materials with the same STC rating can have drastically different TL. The below pictures show the HUSHCORE in-curb testing initiative at independent testing laboratory Intertek in York, PA.
3) Clearly state performance critical Installation Guidelines for the construction team
The rating of an acoustical treatment to achieve a given performance is a start. Specifying HOW it is to be installed assures that the installation contractor does everything necessary to realize the maximum attenuation possible. Examples of how contractors' installation practices can compromise performance is the sealing of the roof deck openings inside the curb for the SA and RA duct drops. Performance loss of at least 5 dBA and sometimes upwards of 10 dBA are the result of overcutting the duct drop openings. See Below in RED where most RTU installations are leaking radiated noise around the roof deck openings for downflow supply and return direct ducted units.
4) Assure compatibility of the In-curb treatment with the RTU/Ductwork Systems
This aspect of the specification pertains mainly to Plenum Curbs where the acoustical treatment of the radiated sound is installed inside the curb bottom and is exposed to the airstream. On these applications the acoustical in-curb treatment must have a Class "A" flammability rating per ASTM E84 and the treatment must be suitable to deter migration of media fibers into the airstream.

5) Performance Accountability
Let's understand the difference between the acoustical treatment MATERIAL performance and the SPACE acoustical performance in the Built Environment. Best Practices "Cut and Paste" specification strategies make a lot of sense when specifying an RTU in-curb acoustical treatment because we can model performance once and standardize on a treatment as long as we continue to design buildings using consistent equipment types with similar baseline sound, as long as the building shell is comparable and as long as the building use has similar acceptance criteria (NC). Having a GOOD/BETTER/BEST library of in-curb acoustical treatment specifications can help the specifier customize the Best Practice approach. PLEASE NOTE: Whenever the acceptance criteria of the building space is lower than Noise Criteria NC-35 a project specific noise modeling such as HUSHCORE "Just Right Acoustics" is strongly recommended.
Here is What an Effective RTU In-curb Specification Looks Like for a HUSHCORE DS-53 Deck System
The BRD Team is ready to support your ongoing applications with curb mounted Rooftop Units. We are happy to review your Best Practices and Master Specifications or make project specific recommendations. Have a HUSHCORE Day!
BRD Noise and Vibration Control, Inc.
PO Box 127
Wind Gap, PA 18091-0127
610 863-6300
www.hushcore.net