Moisture control design tips for hospitals and healthcare facilities

Specialty Buildings Column Series, Part 3 or 6
August 11, 2010

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.
This special report is the third installment of a six-part series from Sean O'Brien on moisture-related design for specialty buildings. 

The series includes:
Indoor swimming pools/natatoriums

Museums and archives
Hospitals
Ice rinks
Cold storage facilities
Indoor ski parks 






The American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) provides guidelines and recommendations for healthcare and outpatient facilities in its 2007 HVAC Applications handbook. Conditions for most spaces are generally in the range of 68-75°F and 30-60% RH annually with relatively high ventilation rates (up to 20 air changes per hour).
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.
Design tips

The following must be considered when designing hospitals:
• Carefully review requirements for interior temperature, relative humidity, and pressure control at the beginning of the project.
• Design window and curtain wall systems for condensation resistance, both in terms of system selection and system location within the exterior walls.
• Design a continuous air barrier system for the exterior enclosure to prevent airflow and condensation on exterior wall components.
• Design interior partitions between pressure-controlled spaces to be airtight to reduce inter-zone airflow.





These tend to reduce the humidity of the interior air and decrease the risk of condensation. Whereas a low dew point in a non-humidified building (less than 30°F) may be sufficient to prevent condensation on windows or curtain walls, raising that to 37°F, the dew point corresponding to 70°F and 30% RH, may create condensation problems during periods of low exterior temperatures.
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
     
Photo 2. Frost/ice accumulation on exterior glazing
      
curtain walls. Although off-the-shelf systems capable of resisting condensation at 30-40% RH are available, lesser-performing systems are often installed because hospitals are not considered high humidity buildings. Depending on the particular fenestration system used and interior RH levels, problems can vary from slight surface condensation (Photo 1, at top) to frost and ice accumulation on interior frames (Photo 2). Wetting of interior finishes, especially paper-faced gypsum wallboard, can lead to both material degradation and microbial growth—an intolerable condition for a hospital environment. 
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. 
   
Photo 3. Control window for mock-up heat trace installation
        
Photo 4. Heat trace system installed on mock-up window (condensation eliminated)
       
Figure 1 (at bottom) shows thermal analysis results for a window, including a thermally-broken sill receptor, installed both in and out of alignment with wall insulation in a precast concrete wall system. The model shows that with an interior temperature of 70°F and an exterior temperature of 10°F, misalignment with the insulation lowers surface temperatures by 10°F and creates a condensation (and icing) problem when the interior RH exceeds approximately 23%.
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
          
Photo 5. Ice accumulation on interior surface of exterior precast concrete
             
the airflow path. This is particularly problematic where fiberglass insulation—an air-permeable material—is installed inboard of precast concrete. The fiberglass keeps the precast cold, but does not prevent airflow (and thus interior moisture) from reaching the precast and condensing/freezing (Photo 5).
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.


























Figure 1. Temperature plot showing difference between aligned and misaligned window system
        
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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