Standing water within roof curb space

Figure 1: Inside of roof curb following removal of RTU-3. Note: Standing water within roof curb space. (Photo provided by building owner.)

This post by Don Snell is part 1 of 2.

It’s not uncommon for mid-rise buildings in warm, humid climates to have rooftop make-up air units to serve the ventilation and pressurization requirements in these buildings. What is uncommon is to unknowingly have the roof curbs of these make-up air units become a swimming pool-size reservoir for condensate (water). That’s exactly happened in the following case study. (See Figure 1.)

When these units were turned off, the suction pressure from the make-up air unit fan, which held the unintended condensate in the roof curb, now released unexpectedly. This condensate then flowed under the roof curbs and across the roof deck before dumping hundreds of gallons of water into the building through shafts intended for air ducts and pipe risers. The result? Six-figure damage to this four-star, LEED-certified hotel in Florida.

Based on our investigation, Liberty believes that construction defects consisting of unsealed openings permitted warm, humid air to infiltrate initially into the roof curb and finally to the make-up air unit. These unsealed openings consisted of missing gaskets and sealant, as well as a missing conduit plate in the floor of RTU-1. All of these issues failed to comply with the manufacturer’s installation instructions.

As this warm, humid air infiltrated through the roof curb, it came in contact with the cold (approximately 55°F) bottom-curb side of the make-up air unit and condensed. This condensing condition was nearly continuous because:

1. the make-up air units operated continuously (except during maintenance) to deliver cooled and dehumidified ventilation and pressurization air to the building.
2. this warm, humid Florida air stays above 55°F most hours of the year.

A household example of this condensing condition occurs when you have a glass of ice water on your kitchen table. The glass, which is approximately 32°F, is colder than the temperature of the air around it. As a result, condensation collects on the surface of the glass. This is what was occurring within the roof curbs.

To be continued…

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