This post by Don Snell is part 2 of 2.

Rooftop Swimming Pool

Figure 2

Air is uncontrolled when it infiltrates. Air infiltration can be caused by mechanical fan effects or wind pressure. In this case, it was caused by strong negative fan pressure. Roof curb spaces should be maintained at a near-neutral pressure with respect to the outdoors. These, however, were not.

Liberty’s testing revealed that RTU-1 roof curb was operating at -674 Pa (-2.7 “WG) with respect to (wrt) outdoors. (See Figure 2.) This pressure is equivalent to being exposed to a wind speed of nearly 75 miles per hour (mph).

Similarly, RTU-3 roof curb was operating at -128 Pa (-0.5″WG) wrt outdoors. This pressure is equivalent to being exposed to a wind speed of greater than 30 mph.

Liberty’s analysis and calculations determined that the missing gaskets and sealant areas permitted an air infiltration rate of:

  • 45 CFM through RTU-1 roof curb
  • 20 CFM through RTU-3 roof curb

Liberty’s analysis and calculations determined that the airflows produced by these fan pressures were contributing to the accumulation of:

  • 2600 gallons of condensate per year – RTU-1
  • 1100 gallons of condensate per year – RTU-3
Figure 3

Figure 3

It was important to minimize this fan pressure and resultant air infiltration and condensation condition. Liberty determined the best approach would be to decouple the make-up air unit from the roof curb. This was accomplished by:

  • Lifting the make-up air units off the roof curb. (See Figure 3.)
  • Capping the top of the roof curbs.
  • Installing a rail support system, which decouples the make-up air units from the rail because the rail is open on two sides.
  • Installing a continuous gasket between the rail support and the make-up air unit.

While this hotel was a LEED-certified building, these defects which led to the failure could just have easily occurred in a conventionally constructed building.

Author Donald B. Snell, PE (Georgia, Virginia, and Pennsylvania), Certified Mechanical Contractor (Florida), and Senior Mechanical Consultant with Liberty Building Forensics Group, has provided moisture and IAQ-related forensic building investigations on more than 200 buildings. For more information, contact Donald B. Snell at d.snell@libertybuilding.com.