The Reality of Air Barriers
When it comes to building performance, too much of a good thing can become a bad thing.
When new building code requirements require high performance and innovation incentives, such as those found in green building rating systems, significant confusion and some building failure will ensue. This is the current situation that designers and contractors are facing in wall system air barrier design and performance.
Significant in 2012 was the issuance of the International Green Construction Code. This provided a vehicle for codifying many elements of the U.S. Green Building Council LEED rating systems and ASHRAE standards that have been issued over the last decade. (Most of them have been released in just the last two years.)
The development of codes, such as the IgCC, are often based on collaboration through cooperating industry professional society sponsors. Despite the benefits of collaboration, high performance and innovation initiatives are often driven by code empirical laboratory analysis, which sometimes does not translate well to field applications. This codification is then pushed out to contractors, who unfortunately must then face the task of interpreting sometimes puzzling requirements that don’t always make sense or work in the field.
Conscientious designers and contractors tend to be more familiar with qualitative testing (such as visual inspections) as a means to determine if an air barrier is continuous and visually leak-free. However, it is the air leak that is not visual that is important. Demonstrating and validating air barrier performance by using quantitative testing methods (such as field testing for compliance with air infiltration requirements) in order to comply with new codes and standards (Figure 1) is new and less familiar to designers and contractors. The difficulty for them is understanding what the numbers mean, interpreting different results and tightening the performance of what they can’t see. They know how to design and build a successful building using tried and true methods, but when mandated to use new products or methods to achieve high performance or accreditation, they have no track record or field studies to fall back on.
Because designers are often unfamiliar with air barrier and HVAC dynamics, our firm has been called upon on numerous occasions to resolve moisture and mold issues that arise as a result.
The following case study presents an example of what we encountered when excessive humidification, building positive pressurization, and a leaky building envelope combined to cause condensation, frost and ice damage in a Midwestern medical facility. Liberty was called in two years after construction to determine the most effective and least disruptive resolution to the now fully functioning facility’s problem. The designer had not properly specified an air barrier for the building, which led to the leaky envelope condition. These problems could have been avoided if the designer had specified proper materials, installation and testing for the air barrier system.
Humidification, Pressurization, and Air Leaks Cause Frost and Ice Damage to a Medical Facility
A 50-bed medical facility located in the Midwest experienced buildup of frost and ice in above-ceiling spaces, soffits, and in clerestories during winter months two years after construction was completed in 2006. The frost/ice (Figure 2a) would eventually melt (Figure 2b), wetting the ceiling surfaces, causing damage (Figures 3a and 3b), and leaking into the occupied rooms below. The facility owner and designer originally attributed the frost and ice buildup to an improperly installed air barrier system. While the presumptive “air barrier” was in fact not tight, an investigation also revealed the following:
- The designer had specified exterior wall materials that were not capable of providing a tight air barrier.
- The building’s HVAC system contained humidifiers for patient comfort during the winter. These humidifiers were improperly operating and were set too high, causing excess humidity that led to the buildup of frost and ice in concealed cold spaces.
- Building pressurization, as designed (based on interpretation of state regulations), played a role in pushing the humid interior air past the air barriers and into colder concealed spaces (exfiltration), where it condensed on cold surfaces and formed frost and ice.
Internet-connected wireless data loggers were used to show how properly operating the humidifiers without sacrificing occupant comfort solved the frost/ice problem.
As-Built Conditions In Accordance With Design
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 relative humidity (RH) to vary greatly from 30 to above 50 percent. 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.
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 4). These data loggers recorded dry-bulb temperatures and 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 5 degrees Fahrenheit or greater.
During the datalogging test period, the humidifiers were first run at 35 percent, then turned off and then put at the lowest setting. Results from the datalogging showed that operating the humidifier at 35 percent RH during winter resulted in increased risk of condensation and frost/ice formation (Figure 5). This setpoint of 35 percent 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 percent RH), the risk of condensation and ice decreased dramatically (Figure 5). 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 percent at the lowest setting (not 10 percent), which was in the range that had been suggested and to which the design team agreed if occupants did not complain (Figure 6).
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 percent.
- Preventing the controls from allowing settings above 35 percent.
- Training on-site 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, internet 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.
When it comes to building failures, problem avoidance is always the best course of action to take. This requires a pro-active peer review process that balances any attempts to achieve high performance through innovation with adherence to code regulation that is backed up by extensive quantitative analysis. This medical facility, while not necessarily “green,” 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 design phase, allowing them to be corrected before construction.