October 2012: Up On the Rooftop


Rooftop units (RTUs) can be appealing HVAC systems due to their ability to save space within the building and their comparatively low up-front cost. Sounds like a good idea, right? Here’s the catch: RTUs generate high levels of noise and vibration that can permeate a building’s interior, particularly in the spaces directly underneath them. Fortunately, there are ways to alleviate noise and vibration issues through multi-disciplinary design. Read on for an overview of what you should know about RTUs.

Location, Location, Location. Much like real estate, the location makes all the difference. Consider positioning RTUs above non-critical spaces such as storage rooms, utility rooms, or restrooms. When this option is not viable, it’s especially important that the design team develops a plan to address all three major sources of RTU noise:  1) case-radiated,  2) breakout, and 3) structure-borne noise. Think about the amount of sound that transmits through thin wall assemblies versus thick concrete walls. In general, partitions with greater mass block more sound. The same principle applies to controlling RTU case-radiated noise. Case-radiated noise can easily penetrate through light-weight roof deck and lay-in acoustic ceiling tile unless there is sufficient mass to block it.

RTU on steel dunnage with spring isolators

The design team should evaluate different options to add mass to the roof assembly, particularly under the RTU, to address this sound transmission path. Another approach is to install the RTU on raised dunnage allowing case-radiated noise to dissipate outdoors before it impinges on the roof deck.Low-frequency sound escapes through the walls of ductwork, especially within the first 25 feet of RTUs. This type of sound is referred to as duct breakout noise. The most effective way to control this noise is to route the ductwork across the roof, incorporating duct silencers as needed, before penetrating into the building. This routing allows much of the high-energy sound to breakout before the ducts funnel noise into the building. Round duct and proper fittings can also reduce breakout noise.

Improperly isolated RTUs can cause the roof or even the entire building to vibrate, which is known as structure-borne noise. Many RTUs come from the factory with internal spring isolators.  Although these isolators may cut down on the vibration from the fans and motors, they do not address the vibration caused by the turbulent airflow inside the unit. A more effective approach to isolate an RTU is to bolt-down the internal spring isolators and install the entire unit on a roof curb that incorporates spring isolators.

Rooftop Unit

RTU on vibration isolation curb

These vibration-isolating roof curbs are often available with enhanced noise control options such as layers of roofing board underneath the unit and duct silencers. Such a curb could address all three major sources of RTU noise in one fell swoop.

RTUs can be a part of a successful HVAC system. With a collaborative approach, the design team can tackle the major sources of RTU noise. The end result: noise and vibration control that meets the building occupants’ expectations.

Relevant Projects:
  • Bridesburg Elementary School – Philadelphia, PA
  • Cheltenham Elementary School – Cheltenham, PA
  • EEB Hub at Navy Yard – Philadelphia, PA
  • Esperanza Middle School – Philadelphia, PA
  • Fulton Elementary School – Lancaster, PA
  • Iroko Pharmaceuticals – Philadelphia, PA
  • Lawrence High School – Lawrenceville, NJ
  • Main Line Health – Berwyn, PA
  • Michener Art Museum – Doylestown, PA
  • Penn State University Chiller Plant – State College, PA
  • Seaford High School – Seaford, DE
  • University of Pennsylvania ARCH Renovation – Philadelphia, PA
  • West Point Visitor’s Center – West Point, NY
November 2012: Playing Smart
September 2012: Video Display Design