A model for freeze–thaw cycling of the South Pole Station exterior sheathing

Abstract

An infrared survey [Phetteplace, G. 2006. An Infrared Survey of the South Pole Station Modernization Project, Final Project Report to National Science Foundation, US Army Cold Regions Research and Engineering Laboratory, Hanover, NH.] of the new South Pole Elevated Station revealed evidence of excessive air exfiltration in some locations. Despite the relatively low indoor humidity normal in the South Pole Station, typically less than 10% relative humidity (RH), there was concern that excessive exfiltration could result in moisture accumulation within the building walls. While the accumulation and freezing of moisture at any point in the wall section could be problematic, what was felt to be of greater concern was the potential for repeated freeze-thaw cycling near the exterior oriented strand board (OSB) sheathing due to the significant diurnal temperature cycling of the siding during the Antarctic summers. The intense solar radiation conditions on vertical surfaces at the low Antarctic summer sun angles results in very high solar fluxes, and surface temperatures can rise well above freezing even in ambient temperatures that are -20 °C or lower. Typical building energy analysis methods are not suited to analysis of this problem for a variety of reasons. Thus we constructed a finite difference heat transfer model of the wall section with a full compliment of realistic boundary conditions for the South Pole application. The base case of the planned wall section and various potential retrofits were evaluated with the model using actual weather data for six summer seasons. The results show that under the worse case summer season the base case wall section would see approximately 44 freeze-thaw cycles. That number would be reduced to only 12 with the addition of either 25 mm of extruded polystyrene foam insulation, or 19 mm of plywood and 13 mm of the same insulation. Over a 25 year period these same additions to the exterior of the building would reduce the number of freeze-thaw cycles by over a factor of six.