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Specialty Buildings Column Series, Part 3 or 6
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| Photo 1. Condensation on exterior glazing system in a humidified hospital | |||||||||||||||||||||||||||||||||||||||||||
When discussing moisture-related problems in high-humidity buildings, natatoriums and museums typically come to mind as the most challenging building types. However, specific design requirements for temperature, relative humidity (RH), and air pressure differentials in hospitals and healthcare facilities can create moisture conditions that are equally problematic. The impact of the interior environment on the building enclosure is often overlooked in favor of other design criteria in what are often considered “mission critical” facilities. This can result in both visible and concealed moisture problems that are difficult and expensive to repair.
However, some special-use spaces have more extreme temperature and RH requirements—including radiology (78-80°F and 40% winter RH) and post-operative suites (75°F and 45-55% winter RH). The exterior enclosure for these two spaces must be designed to accommodate interior dew points of up to 54°F and 58°F, respectively. Other areas, such as specialized equipment rooms for medical imaging, may similarly require humidification to control electrostatic discharge (ESD), which could cause damage to the sensitive equipment. Although 30% RH is relatively low when compared to the 50-60% levels encountered in museums or indoor swimming pools, it is sufficient to cause condensation, especially during the winter months. In ventilated, non-humidified buildings, interior moisture levels are largely dependent on the exterior conditions. During the winter, when low exterior temperatures create the greatest risk of interior condensation, the ambient moisture levels are also low.
Another feature of hospital environments that can lead to airflow and moisture problems is building pressurization. Many areas in hospitals, such as isolation rooms for immune-compromised patients, are maintained at a positive air pressure with respect to their surrounding spaces. The positive air pressure helps prevent contaminants (e.g., bacteria) from entering the space. Other areas, such laboratory or medical gas storage, are maintained at a negative pressure to prevent the migration of harmful substances to other areas of the building. Other areas have no dedicated pressure control. Such pressure differentials between adjacent spaces can create air flow patterns within the exterior walls, roofs, and ceilings that expose the airborne water vapor to cold surfaces, thereby increasing the condensation risk. Avoiding surface condensation on exterior walls The most common (and visible) problem associated with high interior humidity levels is surface condensation on exterior walls, typically on glazed components such as windows and
Hospital buildings are often constructed with precast concrete panels as the exterior cladding material. When using precast concrete cladding, the position of the window within openings is critical. Thermally-broken windows with insulating glass units can provide relatively good thermal resistance when aligned with the insulating component of exterior walls. For precast concrete, the easiest method of attachment for windows is to anchor them directly to the precast, placing them out of alignment with the wall insulation, which is typically adhered or mechanically fastened to the back of the precast. This misalignment significantly decreases the interior surface temperature of the window frame.
Due to the financial constraints and operational difficulties associated with shutting down a hospital to replace windows, in-situ repairs, such as the addition of electric heat trace cables (Photos 3 and 4), are often necessary to address condensation issues. Although effective at preventing condensation, heat trace systems represent an additional operational burden and maintenance cost to owners. In addition to artificial humidification, high interior humidity levels can also result from the high ventilation rates required in hospitals, particularly during the summer and “swing” seasons. If mechanical systems are not designed with sufficient dehumidification capacity, the interior RH can reach damaging levels due to the introduction of humid outside air. High ventilation rates also necessitate the use of energy recovery ventilation to minimize the amount of energy used to condition the ventilation air. Concealed condensation: A hidden risk in hospitals A more insidious problem than surface condensation is concealed condensation within walls and roofs. Positive air pressure within hospitals will tend to “push” humid air to the exterior through cracks or gaps in the building enclosure, causing significant condensation on cold surfaces along
A continuous air barrier in the building enclosure can minimize this risk by eliminating airflow paths through and within the enclosure assembly. Additional complications arise when multiple pressure-controlled spaces are located in close proximity. The uneven distribution of interior air pressure can create airflows between spaces, often leading to additional airflow across the exterior walls even when those walls are airtight (Figure 2, below). To address these airflows, interior partitions between pressure-controlled spaces must also be made airtight—not a common practice in typical construction. |
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| Figure 1. Temperature plot showing difference between aligned and misaligned window system | |||||||||||||||||||||||||||||||||||||||||||
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| Figure 2. Airflow between interior spaces resulting in condensation on exterior walls | |||||||||||||||||||||||||||||||||||||||||||
| Sean O’Brien is a Senior Project Manager in the New York City office of Boston-based Simpson Gumpertz & Heger Inc. O’Brien specializes in building science and building envelope performance, including computer simulation of heat, air, and moisture migration issues. He has investigated and designed repairs for a variety of buildings types, from condominiums to art museums, and has published papers on topics including moisture migration in masonry wall systems and condensation resistance of windows and curtain walls. He can be reached at smobrien@sgh.com . |
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