The general contractor of a 50-bed Midwest medical facility was facing a million dollar repair cost and a potential lawsuit. Winter frost and ice buildup was occurring in above-ceiling spaces, soffits and clerestories two years after completion. The frost/ice would eventually melt, wetting the ceiling surfaces, causing damage and leaking into the occupied rooms below. The owner, design team and a forensic consultant blamed the problem on a faulty air barrier installed by the contractor. Replacing this air barrier, as demanded by the owner, would have required removing exterior wall and ceiling components, including all finishes, in a functioning facility.
Frost forming on the underside of the metal roof deck in above-ceiling space.
Melting frost/ice dripping off the underside of the metal roof deck in above-ceiling space.
Warping and damage to wood beadboard ceiling in porch areas attributed to water from melting frost/ice in above-ceiling space.
Damage to ceiling tiles due to melting frost/ice in above-ceiling space.
The contractor called in their own forensic expert to investigate and provide guidance. What the contractor/expert team discovered ultimately solved the frost problem at very little cost. The expert also helped to convince the reluctant owner to implement the expert’s recommendations. The owner eventually bought into the fix when it was proven effective by the use of wireless data loggers which the owner could monitor in real time online.
The Problems
Indoor conditioned air was clearly entering unconditioned above-ceiling and cavity spaces. Moisture in that air was condensing on the colder surfaces, such as the underside of the roof, and then freezing. The owner and architect stated that the design called for an air barrier and that it was poorly installed allowing indoor air to travel to the unconditioned spaces. However, the architect had specified a material, polyethylene sheeting, that was not easily capable of providing a good air barrier.
Additionally, the architect had called this material a vapor retarder, not an air barrier. While some vapor retarders can be air barriers they must be properly selected and performance criteria provided. 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 specifications did not mention air barrier nor provide air barrier performance criteria. The only hint was a vague specification reference, on which the owner’s team was basing their case, to sealing joints at penetrations with vapor retarder tape to “create an airtight seal.”
The HVAC system contained humidifiers for patient comfort during the winter. What the contractor’s team found was these humidifiers were being operated improperly and were set too high, causing excess humidity in the interior that exfiltrated to the concealed cold spaces. That humid air was then condensing in the cold spaces. Additionally, the team found the building’s positive pressurization, as designed, was pushing the humidified interior air into the cold spaces. This pressurization was based on an incorrect interpretation of state regulations by the design team. Buildings in cold climates need to be slightly negative relative to the exterior atmosphere, not positive, to avoid exfiltration of indoor air, which in the winter will be more humid just from human activity regardless of the presence of humidifiers.
The presumptive “air barrier” as selected by the architect would never have been able to counteract the twin mechanisms of HVAC induced pressurization and vapor drive. Removing parts of the exterior wall to attempt to seal the polyethylene with tape, as demanded by the owner’s team, would have been useless in solving this problem.
The Solution
The solution by the contractor’s team was to use the “HVAC hammer” to solve the problem of exfiltration of humid air. Through quantitative analysis, it was determined that the proper operation of the humidification system would solve the frost/ice problem without changing pressurization and/or replacing/altering the presumptive “air barrier.” The fix including changing the location of humidity sensors in the ductwork to provide for more accurate readings and control as well as setting the wintertime humidity levels to around 25%.
To prove the efficacy of the recommended humidification fix, 69 wireless data loggers were installed by the contractor’s team in the concealed spaces, occupied rooms, HVAC system and outdoor air. These data loggers recorded dry-bulb temperatures and relative humidity (RH), which were then monitored in real time via the internet. The owner was given full real-time access to this data.
This solution worked. The frost problems stopped while patients and staff remained comfortable at the lower RH level. And the data from the data loggers helped to convince the owner that the fix was effective.
This chart generated from data-logger information was instrumental in proving to the owner the efficacy of the fix proposed and implemented by the contractor’s team. To avoid frost the difference between dry-bulb and dew point temperatures (blue line) in the soffit areas must be above the red dotted line. The left side of the graph (grey background) shows the as found conditions, with the humidifier set for an RH of 35%. The blue line is generally below the red dotted line meaning the potential for frost formation. With the humidifier either off (white background) or set to 10% (yellow background–right side of chart) the blue line stays above the red dotted line most of the time, meaning frost will not form. Note because of poor placement of RH sensors in the ductwork the actual room RH was higher than the setpoint; therefore a 35% setpoint resulted in 50% room RH and a 10% setpoint resulted in 20% room RH.
Conclusions
The contractor’s team, through analysis and real-time monitoring of HVAC operation changes, were able to prove the seven-figure building envelope fix was not needed. A much simpler and much less costly modification to the HVAC operation solved the problem.
When it comes to building failures, problem avoidance is always the best course of action to take. This medical facility combined several complex factors found in high-performance buildings, including added humidification and designed pressurization. A peer review focusing on the interaction between the HVAC system and building envelope would have revealed the problems during the design phase, allowing them to be corrected before construction.
