Diagnosing and Solving Moisture Problems in Natatoriums

Specialty Buildings Column Series, Part 1 of 6

August 11, 2010

Photo 1. Condensation on interior surfaces, such as window wall frames and surrounding materials, is one of the most common moisture-related problems in indoor swimming pools.
        

Natatoriums, particularly when located in cold climates, are among the most challenging building types for architects and engineers. Interior moisture levels are extremely high, with dew points ranging from 60°F to 70°F. At these levels, even natatoriums in mild climates are susceptible to moisture-related problems. Issues typically encountered in natatoriums are co ndensation on interior surfaces, condensation within walls and roofs, and
         
  This special report is the first 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
       
corrosion of interior components. 

This column diagnoses these three issues and provides design guidance on how to avoid them.
1. Condensation on interior surfaces
Interior surfaces in natatoriums must be kept quite warm, often as high as 70°F, to prevent condensation. Opaque walls and roofs can typically be designed with continuous insulation and high R-values sufficient to meet this criterion. Windows, doors, and curtain walls need to be high-performance, thermally broken systems designed specifically for high-humidity applications. 

The actual installed performance of fenestration, however, is typically different from the tested performance that designers reference when specifying systems.1 Consequently, even the best building envelope systems are likely to experience surface condensation during the coldest times of the year. Marginal systems can experience condensation problems ranging from mild condensation on frames (Photo 1) to fogging on glass surfaces
       
 
  Photo 2. Condensation on window frame and large portions of glass
       
(Photo 2). In addition to problems with system durability and degradation of adjacent materials, condensation on interior surfaces can affect the public perception of a building—occupants and visitors will tend to view condensation as an indication of a design deficiency.
Several design strategies can help reduce condensation, or increase the condensation resistance of a lower-performance system. Providing air curtains, or directed flows of warm air, over window components is one effective way of increasing the surface temperature of enclosure components. Similar results can be obtained by using electric heat trace cables or radiant heaters to deliver heat directly to glazing and framing systems.

Natatoriums often include skylights to provide natural light. The condensation risk at skylights is high, due, in part, to their orientation. Mounted in low-slope roofs or near-horizontal applications, skylights tend to
   
  Design tips

The following should be considered when designing natatoriums:
• Design enclosure systems to include air barrier, vapor retarder, and thermal insulation systems that are sufficient and continuous
• Use the mechanical system to maintain negative air pressure within the building
• Use high-performance fenestration, with supplemental heat if necessary
• Use only moisture-tolerant, corrosion resistant materials both within the building and in the enclosure assemblies
   
lose more heat through radiation (to the sky) than similarly sized windows and doors. Supplemental heating systems can be difficult to install and undesirable due to the high visibility of skylight systems. A typical compromise in this situation is selecting a high-performance skylight product combined with designing a water-management system, consisting of a system of rafter and perimeter gutters, to prevent condensate from dripping and to collect and drain surface condensate to the exterior of the building.
2. Condensation within walls and roof systems
Condensation may also form within wall or roof assemblies, leading to concealed damage to materials and possibly microbial growth. Moisture can be carried through the envelope in two ways: vapor diffusion and airflow.

Water vapor naturally flows from areas of high moisture content to areas of low moisture content and can diffuse through porous materials such as gypsum board and concrete. This water vapor will condense if it accumulates in a region of the assembly that is cold enough, which can lead to, among other things, steel corrosion and efflorescence on masonry
               
 
  Photo 3. Efflorescence on exterior masonry due to moisture migration from interior
              
(Photo 3). In natatoriums, this “vapor drive” is almost always from the interior to the exterior, making a well-detailed vapor retarder a critical component.
In addition to vapor diffusion, moisture can be carried into walls and roofs via moving air. In this case, moisture in an air stream will condense as soon as the air stream contacts a surface that is below the dew point of the air. For exterior walls, this surface is typically the exterior sheathing or the back side of the cladding (Figure 1). Visible signs of air leakage include icicle formation on the exterior and staining on the interior of the building, often caused by degradation of roof and wall materials (Photos 4 and 5). The amount of moisture transported by airstreams is significantly greater than that transported by vapor diffusion alone (50 to 100 times as severe), making a continuous, well-detailed air barrier a critical element in walls and roofs. Typical problem areas include exterior soffits, building wall intersections, and wall-to-roof interfaces (Figures 2 and 3).
                
 
  Figure 1. Illustration of air leakage path through exterior wall
         
 
  Photo 4. Icicles on building exterior due to air leakage from natatorium space
               
 
  Photo 5. Interior staining caused by moisture-related degradation of roof materials
           
 
  Figure 2. Air leakage at roof-to-wall intersection visualized using infrared thermography.  Red/white coloring indicates higher temperatures caused by warm interior air leaking through the envelope.
             
 
  Figure 3. Air leakage at building wall intersection visualized using infrared thermography. Yellow coloring indicates air leakage sites.
              
If the natatorium is located within a larger building, the air barrier should separate the natatorium from all adjacent buildings and spaces. Self-adhered rubberized-asphalt membranes are a good choice for air barriers, since they also retard vapor diffusion, but they must be placed at a suitable location within the enclosure (typically the winter-warm side of the insulation).
Finally, the natatorium mechanical systems should be designed to maintain the space at a negative air pressure with respect to the exterior (including adjacent buildings). Keeping the building negative will increase air infiltration through any gaps in the air barrier. It will also prevent moist interior air from flowing out through those gaps and condensing within the envelope or migrating into adjacent spaces.

A good air barrier and negative operating pressure are critical to the performance of natatoriums, since the nearly unlimited source of moisture within the natatorium could lead to dangerous levels of moisture damage within the building’s first few years.
 
3. Corrosion and degradation of interior components
Bare steel will readily corrode in a condensing environment within natatoriums. Add airborne chlorine compounds (chloramines) to that wetting and you get a corrosive mixture that will degrade even some stainless steels (Photo 6).

For this reason, all exposed metal within the natatorium should be coated to prevent corrosion or made from materials with higher-than-average corrosion resistance.2  Materials such as gypsum board that are susceptible to moisture-related damage are generally unsuitable for use in natatoriums.

Materials should also be dimensionally stable under changing humidity conditions. For example, the sudden change in interior moisture levels caused by draining an indoor pool could cause shrinkage and distortion of wood components as those materials lose moisture.

Designers must choose durable materials, such as cement plaster, masonry, and plastics that can undergo occasional wetting and drying cycles without compromising their performance. 

Other concerns
Although not an envelope issue, natatoriums must provide a comfortable environment for swimmers and spectators. Mechanical systems should be designed to provide two separate zones, often part of the same general space, for the swimmers and the spectators. Windows and doors should be positioned sufficiently far from the pool area to prevent cold drafts on the pool deck and radiative heat loss from swimmers. 

Due the significant heating and cooling loads associated with natatoriums, conservation methods such as energy recovery ventilation and heat recovery from dehumidification equipment should be employed, and owners
               
 
  Photo 6. Corrosion on stainless steel door hardware
           
should be made aware of the potentially high operating costs associated with these buildings.
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.

References

1 O’Brien, S.M., "Finding a Better Measure of Fenestration Performance:  An Analysis of the AAMA Condensation Resistance Factor," RCI Interface, May 2005; available online at: http://www.buildingenvelopeforum.com

2 "Stainless Steel in Swimming Pool Buildings – A Guide to Selection and Use," Nickel Development Institute (NIDI); available online at: http://www.nickelinstitute.org (free site registration required)
 
         
 

RELATED ARTICLES FROM BD+C

Comments on: "Diagnosing and Solving Moisture Problems in Natatoriums"