Moisture control guidelines for indoor ski parks
|This special report is the final 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
• Ice rinks
• Cold storage facilities
• Indoor ski parks
The five articles that precede this article discussed the unique design parameters associated with high humidity and low temperature buildings such as swimming pools and ice rinks. These buildings are much less common than offices, condominiums, or retail spaces, but the design strategies required to make these buildings function properly are well understood and thoroughly documented within the design community and building industry. Problems in these buildings typically stem from designers’ lack of awareness and training rather than from the lack of collective knowledge and experience within the fields of architecture and engineering.
Pushing the limits of design challenges, one could imagine a building type with which only a handful of designers have experience; one with extreme interior conditions unlike anything found in typical or even atypical buildings. Perhaps a building where it rains on the inside as well as on the outside. Such a building does exist, although the interior precipitation turns out to be snow instead of rain—an indoor snow park.
Most indoor snow parks are located overseas, with several in Europe, Russia, and one, oddly enough, in the United Arab Emirates, where the exterior temperature exceeds 100ºF for nearly 10% of the year. The first U.S. park will be located at the Meadowlands sports complex in East Rutherford, N.J.
These unique buildings represent the absolute extreme in terms of interior/exterior design conditions, and with only a few dozen in the world, pose many challenges that few designers have faced. Design of these facilities must take into account problems associated with both low temperature (see my previous articles on ice rinks and cold storage facilities) and high humidity buildings (see my article on indoor swimming pools). This article presents the unique challenges associated with these buildings as well as design strategies for addressing those challenges.
|Photo 1. Typical floor slabs are completely elevated to provide appropriate slope for the interior space.|
|Photo 2. Interior columns penetrating the field of the snow and floor slab|
|Figure 1. 2D thermal analysis of floor-to-wall intersection, including effects of cooling pipes|
|Figure 2. 3D thermal analysis of column penetration to assess risk of interior snowmelt|
|Figure 3. 3D thermal analysis of column penetration including cooling pipe layout|
|Photo 3. Insulated enclosure around interior column|
|Photo 4. Conveyor lift supported directly on floor slab|
|Photo 5. High density foam block (thermal break) installed between conveyor support and floor slab|
|Photo 6. Cross section of typical floor slab showing air/water/vapor barrier, insulation, and fluted deck for glycol piping|
|Figure 4. 3D thermal analysis of column penetration to assess risk of exterior condensation|
|Photo 7. High density foam block used to isolate roof davit from main building structure|
Heat loss and interior snowmelt
For most refrigerated buildings, thermal bridging through the building enclosure is primarily an energy-efficiency issue. Unintentional heat gain from the enclosure will add to the already-high cost of maintaining interior temperatures substantially lower than the exterior for most of the year.
When refrigerated buildings are intended to maintain up to two feet of snow cover over the floor, thermal bridging or heat gain from the walls or floors may be sufficient to cause melting of the interior snow. Under most summertime design conditions, even a heavily insulated floor slab may experience above-freezing temperatures on the interior surfaces. Part of the reason for this is the snow itself, which provides some insulation value (freshly fallen snow is a better insulator than packed snow) and “shields” the surface of the insulation from the low interior temperatures in the occupied space.
The typical building configuration of indoor ski slopes exacerbates the risk of snowmelt in the floor areas. Due to the difficulty of finding a project site with suitable topography (Photo 1), indoor slopes are typically constructed by elevating one end of the building. This results in a relatively complicated structure with multiple column penetrations through the floor slab that often occur within the conditioned space (Photo 2), as well as full exposure of the underside of the slab to exterior air. This exposure eliminates the risk of frost heave that is often associated with “cold” buildings on grade, but an elevated slab will experience greater temperature swings than a slab-on-grade structure, which benefits from relatively constant ground temperatures. These penetrating columns are unusual in refrigerated buildings, which are designed with few, if any, structural penetrations through the thermal envelope.
Snowmelt in the field of floor slabs is prevented by embedding cooling pipes in the floor system. The pipes circulate a cooling fluid, such as glycol, to regulate slab temperatures and maintain floor surfaces below freezing. This is shown schematically in Figure 1, which presents the results of a 2D heat-flow simulation of a floor-to-wall intersection. In this case, without active cooling pipes, the interior surface temperature at the slab-to-snow interface would exceed 40ºF during the winter. The “freezing point” on the wall panels above the snow cover, towards the upper right corner of Figure 1, is just barely outboard of the interior panel surface, even if the cooling pipes are inactive. As discussed above, this is due to the lack of snow (insulation) against the wall and the wall’s full exposure to the low interior temperatures.
More-complex 3D analyses are required to predict the risk of interior snowmelt around penetrations such as columns and other structural members. Figure 2 shows the results of a 3D analysis of a perimeter column penetration. In the analyzed condition, high (~ 40ºF) temperatures occur in the snow adjacent to the column near summertime design conditions. Figure 3 shows results from a similar analysis, but with a transparent snow layer to show the locations of cooling pipes in the floor system. Depending on the specific geometry of the penetration, insulation must often be added around the column to control heat flow and provide a physical separation between the snow and any above-freezing temperatures on the penetration (Photo 3).
Some interior components may need to be fastened to the surface of the floor slab rather than penetrate the slab itself to reduce thermal bridging. However, any component that penetrates the insulation layer could still cause problems with interior snowmelt, and must be carefully analyzed. Small supports for features such as conveyor lifts (Photo 4) may require some type of thermal separation between the floor slab and the support component to limit thermal bridging through the insulation. High-density polyurethane foam blocks, more commonly used as a thermal break below columns and other structural members, are generally sufficient to minimize the risk of snowmelt at these components (Photo 5).
Conditions within indoor snow parks can vary, but are typically between 20-30ºF for normal operation, with temperatures on the lower end of that range during snowmaking operations. At those temperatures, condensation on and within the building enclosure (due to the infiltration of warm, humid air) is a risk for most of the year, even in mild climates.
Similar to cold storage buildings, continuous air and vapor barriers are necessary to control the migration of exterior moisture into the spaces. The elevated floor slab of indoor slopes typically results in full exposure of the “floor” to exterior conditions. Consequently, an air/vapor barrier is also required on the floor, whereas slab-on-grade construction will rarely require an air barrier and, in some cases, may not require a vapor barrier.
Waterproofing is an additional design consideration on the slab, which would be required in the event of accidental melting of the snow (or intentional, for maintenance purposes). If the elevated slabs are placed over steel decking, any wetting of the concrete would require significant time to dry, could not dry towards the exterior, and could result in freeze-thaw damage if not sufficiently dried before the interior temperatures are lowered. Photo 6 shows a cross section of an insulated floor system. An air/vapor/water barrier (blue) is installed directly on the concrete slab, followed by insulation boards. The top insulation board is bonded to a corrugated steel panel, the flutes of which will eventually serve as “tracks” into which cooling pipes are laid.
P enetrations through the floor slab, such as columns, pose a risk for exterior condensation, even in climates such as the Northeast that are not considered hot and humid, but regularly experience exterior dew points above 70ºF during the summer months. As discussed above, 3D heat-flow modeling and analysis of cooling pipe layouts are required to assess condensation risk at most penetrations (Figure 4). Other penetrations, such as roof davits fastened to the structure, may require thermal breaks or additional insulation to limit heat loss and reduce condensation risk, although treatment of these components may be simpler than for heavy structural members (Photo 7).
An additional complication in indoor snow parks is that they are often constructed as part of a larger entertainment/sports complex and may abut conventional, heated spaces on one or more sides. Treatment of penetrations from the “cold” to the “warm” side of perimeter walls is critical, since the interior relative humidity on the warm side may be as high as 60-70% during the summer, even in air conditioned spaces. This is in contrast to typical buildings, where interior humidity levels are low when condensation risk is the highest on exterior walls (i.e., during the winter). The separation between cold and warm environments within the same building requires insulation and air/vapor barriers to prevent the long-term accumulation of moisture in these partitions.
Partition walls are often overlooked or improperly addressed in refrigerated buildings, as air and vapor barriers are rarely required in interior walls in typical buildings. In snow parks, the constant vapor drive towards the cold space could lead to damaging levels of moisture accumulating within months of start up. Glazed components such as windows and curtain walls, although not commonly used on exterior walls in refrigerated buildings, may be used in partition walls to provide occupants in the adjacent spaces with a view of the cold space. Due to the incidence of high summer RH levels on the warm side, glazing systems must be high-performance products, similar to systems that would be used in a museum or natatorium than to systems for traditional interior partitions.
Heat trace cables or other supplemental electric heating devices can be used to prevent “warm side” condensation on beams or penetrations in areas where insulation alone is not sufficient. However, this may not be a viable option on penetrations through floor slabs that extend through the interior snow cover. In this case, adding heat to the exterior may solve the problem of summertime condensation but exacerbate interior snowmelt. The use of heat trace may be suitable when addressing heat flow at through-the-snow penetrations, but insulation may be required to balance out the addition of exterior heat.
Energy use and ventilation
The large size of most indoor ski parks provides some insight into their potential energy consumption. Heavily insulated (R-20 or higher) floors, walls, and roofs are necessary not only for energy conservation purposes, but also to reduce the risks of condensation and interior snowmelt.
In addition to the primary cooling costs, ancillary systems such as electric heat trace may contribute significantly to overall energy use if they are not properly designed and controlled. Running 200-watt column heaters on a building with 100 exterior columns would require the energy equivalent of more than five tons of cooling. Given this potential, heat trace components need to be carefully designed to reduce unnecessary/excessive heating.
In addition, control systems that function on real-time monitoring, including exterior dew point measurement, should be designed to limit their use to critical portions of the year, rather than attempting to prescribe operation based on historical weather data.
An additional feature of indoor snow park mechanical systems, made necessary by the potentially high number of occupants, is the need for outside/ventilation air. Using New York City as an example, the total design cooling load for a single person (assuming 15 cubic feet per minute per person) is approximately 1,750 to 2,000 btu/hr, depending on whether cooling or dehumidification is the primary design criteria. By comparison, the heating load—typically the dominant load in the Northeast—for a single person in a conventional, heated building, such as an office, is approximately 900 btu/hr. Ventilation for refrigerated buildings is typically more energy intensive due to the need to significantly reduce both the temperature and the moisture level of the outside air before delivery to the cold space. For a building with 500 occupants, ventilation alone could account for more than 80 tons of cooling. This load is in addition to cooling required to offset conductive losses through the enclosure and provide low-temperature glycol for the floor cooling system.
Some less common, but still important, issues that designers may face when dealing with indoor snow parks include:
• Indoor storm water piping. Roof leaders that extend through the conditioned space will require heavy insulation and may require heat trace cables to prevent freezing.
• Sprinkler systems. if sprinklers are required by code, the constant freezing temperature within the occupied spaces makes dry-pipe sprinkler systems necessary. These systems need to be equipped with an air dryer to prevent any moisture from entering the pipes, as a slow flow of moisture into the system could lead to ice formation within the sprinkler lines that would interfere with operation. A regular check of the sprinkler system should be included in the maintenance plan for sprinklered facilities.
• Stratification issues. Large, open spaces such as atriums may experience significant temperature differentials due to stratification of the interior air. In a tall, refrigerated building, mechanical systems should be designed to counteract the natural tendency of cold air to sink towards the low point of the space, which could lead to the accumulation of warmer air at the top of the building.
There are likely to be many other design issues with indoor snow parks as these buildings become more common (or less uncommon, depending on your perspective). As mentioned at the beginning of this article, the relative lack of collective experience with indoor snow parks makes prediction of performance and development of design solutions a challenge.
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
Indoor ski park design guidelines
The following must be considered when designing and operating indoor snow parks:
• Design appropriate insulation, air, and vapor barrier systems for the exterior enclosure.
• Minimize penetrations through the insulation, especially on floor areas covered with snow. Analyze all penetrations through the snow to assess the risk of both interior snowmelt and exterior condensation.
• Include insulation, air barriers, and vapor barriers in partition walls between all warm and cold spaces. Any windows, doors, or curtain walls must be high-performance systems capable of resisting condensation under high humidity conditions on the warm side and low temperatures on the cold side.
• Design heat trace systems, if necessary for condensation control, to function only when warranted by exterior/interior conditions and to avoid excessive heat flows that could result in interior snowmelt.
• Coordinate mechanical systems, including the main floor cooling system, with enclosure design to maximize energy efficiency and minimize operating cost.
• Approach all design decisions carefully and avoid relying on “rules of thumb” that have been developed for other building types. Recognize the unique nature of indoor snow parks and provide conservative designs to the greatest extent possible.