By Rick Scott, AIA

This is part 3 of 3.

Humidification Testing and Analysis

Through quantitative analysis, it was determined that proper operation of the humidification system would solve the frost/ice problem without changing pressurization and/or replacing/altering the presumptive “air barrier.” Replacing/altering the specified vapor retarder to act as an air barrier would have required tear-out of finish materials and disruption of this fully-operational facility.

To prove the efficacy of the recommended humidification fix, 69 wireless data loggers were installed in the concealed spaces, occupied rooms, HVAC system, and outdoor air (Figure 3). These data loggers recorded dry-bulb temperatures and relative humidity (RH), which were then monitored in real-time via the internet. Dew point temperatures were calculated from this information and subtracted from dry-bulb temperatures. To avoid condensation, this temperature difference needs to be five degrees Fahrenheit (ºF) or greater.

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

Figure 3: Omnisense wireless data-logger (right) and receiver/internet interface (left) used at the project.

During the datalogging test period, the humidifiers were first run at 35%, then turned off, and then put at the lowest setting. Results from the datalogging showed that operating the humidifier at 35% RH during winter resulted in increased risk of condensation and frost/ice formation (Figure 4). This setpoint of 35% RH (and sometimes higher) was what the owner had been using when condensation problems had occurred. This is a very high interior RH for northern climates in the winter and has been shown to result in moisture and mold problems in facilities that have humidifiers, such as museums.

When the humidifier was set to its lowest setting (reported by the owner to be 10% RH), the risk of condensation and ice decreased dramatically (Figure 4). No condensation, frost, ice, or water leaks were reported during this period. The design team was concerned that the lowest humidifier setting would result in occupant dryness complaints, but no such complaints occurred during the test period. Additionally, as had been predicted, the actual room RH averaged 25% at the lowest setting (not 10%), which was in the range that had been suggested and to which the design team agreed if occupants did not complain (Figure 5).

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

Figure 4: Chart of difference between dry-bulb and dew point temperatures in Soffit SF, from Dec. 16, 2008 through March 28, 2009. With humidifiers on and set to provide 35% RH (grey area in left side of chart), the difference (blue line) was below or close to five (red dotted line) which increased the risk for condensation and ice formation. With the humidifier off (white area in middle of chart) or on and set at 10% RH (yellow area in right side of chart), the differences were generally above 5, decreasing the risk for condensation and ice formation.

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

Figure 5: Occupied room RH measurements during the various humidifier settings. Note that with the humidifier set at its lowest setting (assumed by owner to be 10%), the RH averaged 25% was only below 20% three percent of the time.


Datalogging proved that correcting one (humidification) of the three issues causing condensation (gaps in the presumptive “air barrier”, pressurization, humidification) solved the problem. It was recommended that the conditions be monitored for another year. If the condensation re-occurred, then the next step would be to adjust the building pressurization to neutral or slightly negative.

Fortunately, condensation did not re-occur after the humidification system was altered. The alterations to the humidification system included:

  • Placement of RH sensors controlling the humidifiers in the occupied space to provide better control of building RH.
  • Setting the wintertime target RH at 20% to 25%.
  • Preventing the controls from allowing settings above 35%.
  • Training onsite maintenance personnel in proper operation of the humidification system.

The data loggers were left in place for use by the owner to monitor the efficacy of the humidification system alterations. During initial monitoring of the data loggers, access to information they were sending was granted to all involved parties, including contractors, designers, and the owner. Because of this, the owner was already set up to monitor the data after the consulting scope was complete. The batteries in these data loggers have a 15-year life, so the owner will not need to access them and will continue to receive data for that period of time.

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