Mechanical pipe insulation systems are commonly applied to cold piping surfaces in most industrial and commercial buildings in order to limit the heat losses and prevent water vapor condensation on the pipe exterior surfaces. Due to the fact that the surface temperature of these pipelines is normally below the ambient dew point temperature, water vapor diffuses inside the pipe insulation systems and often condenses when it reaches the pipe exterior surfaces. The water droplets accumulated in the pipe insulation system increase its overall thermal conductivity by thermal bridging the cells or the fibers of the insulation material. The moisture ingress into pipe insulation threatens the thermal performance and the overall efficiency of the building mechanical system. This phenomenon is also responsible for the mold growth inside occupied spaces and causes the pipelines to be more vulnerable to corrosion. Although a wide range of vapor barriers are used for preventing water vapor penetration into pipe insulation, common experience in the field shows that water vapor will inevitably ingress into the insulation materials from the end joints or from the cracks created during insulation installation. How to account for the moisture ingress on pipe insulation service life and thermal performance is still an open question. Thermal conductivity is one of the most important properties for evaluating the thermal performance of the pipe insulation systems. Using a new test apparatus, the thermal conductivity of pipe insulation systems below ambient temperature and in wet conditions with moisture ingress was measured. Fiberglass and phenolic pipe insulation were tested to investigate the moisture effects on the material thermal conductivity. The data showed that these two types of pipe insulation systems had quite different water absorption rates due to different characteristics of the material and its structure. A serious degradation of fiberglass pipe insulation thermal performance was observed and the thermal conductivity increased by as much as 3 times when the moisture content was about 12 percent in volume. Tested at a different condition, the thermal conductivity of phenolic pipe insulation increased to 1.6 times of the original value and the moisture content was 5% in volume. Considering the gravity effect, the moisture content on the top and bottom C-shells were separately measured and discussed in this paper
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