By Rick Scott, AIA

This is part 2 of 3.

As-Built Conditions In Accordance With Design

Humidification, Pressurization, and Air Leaks Cause Frost and Ice Damage to a Medical Facility

Damage to ceiling tiles due to melting frost/ice in above-ceiling space.

The exterior walls of the steel-framed building were finished with stucco (portland cement plaster) on gypsum sheathing attached to metal studs. A polyethylene vapor retarder was specified and installed. Although some gaps and loose sealing tape were observed in the vapor barrier, overall installation of the material was good when compared to the requirements of a functional vapor retarder, which does not require an airtight seal.

The polyethylene vapor retarder with taped joints was a poor design choice if the design team expected the vapor retarder to function as an air barrier. The use of taped joints on unbacked sheet membrane is considered a poor industry practice for air barriers. Additionally, the design did not provide for tying the wall barrier into the roof system to form airtight construction. The design specifications referred to the polyethylene sheeting as a vapor retarder and not an air barrier. The specifications did not mention air barrier nor provide air barrier performance criteria. The only hint was a vague specification reference to sealing joints at penetrations with vapor retarder tape to “create an airtight seal.”

Two humidifiers had been installed within the facility’s HVAC system. It was found that they were being operated in a manner that allowed wintertime interior RH to vary greatly from 30% to above 50%. Humidity sensors in the ductwork had also been located by design in a place that did not allow for accurate readings and control.

The building pressurization had been designed to be positive relative to outdoors (more outdoor air introduced into the building through the HVAC system than was exhausted) based on the design team’s interpretation of state regulations for this type of facility. This was contrary to industry knowledge (including ASHRAE) that in cold winter heating climates, neutral to slightly negative pressurization is desirable, especially for buildings that contain humidifiers. A review of state regulations revealed that the exterior patient rooms could be kept neutral relative to outdoors, as recommended by the industry, and still meet the regulation requirements.

After frost and ice problems began to show up, attempts were made to seal up the above-ceiling spaces and soffits by applying spray foam insulation in the exterior soffits at the exterior wall line. This approach did not appear to have any significant effect on reducing the problem.

To be continued…

With over 35 years of experience, Richard S. Scott is an expert in the areas of architecture, interior design, and building forensics, with a focus on moisture-related building problems. He is certified by both the American Institute of Architects/AIA Florida and the National Council of Architectural Registration Boards (NCARB). He has published over 30 articles, and has lectured or presented at nearly 40 seminars or events. Mr. Scott has developed various training courses, including a 16-hour IAQ training course for NASA and an 8-hour water intrusion prevention training course for the Naval Facilities Engineering Command (NAVFAC). He can be reached at