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Suggested wall thickness of elastomeric foam insulation to prevent surface condensation (R-value of approximately 7.0 oF•hr•ft2/Btu per inch of wall thickness). normal conditions 85oF / 70% RH mild conditions 80oF / 50% RH severe conditions 90oF / 80% RH relative humidity dry bulb air temperature nominal pipe size 3/8" ≤ d ≤ 1.25" 1.25" < d ≤ 2" 2" < d ≤ 2.5" 2.5" < d ≤ 6" 3/8" ≤ d ≤ 2.5" 2.5" < d ≤ 6" 3/8" ≤ d ≤ 1.25" 1.25" < d ≤ 3.5" 3.55" < d ≤ 6" 50 oF 3/8" 3/8" 3/8" 1/2" 3/8" 1/2" 3/4" 3/4" 3/4" 35 oF vapor permeability with a continuous jacket that provides a vapor barrier. If such measures are not taken, the vapor pressure differential between the air surrounding the insulation and the cooler air near the pipe wall will cause vapor diffusion through the insulation. This vapor will condense on the pipe surface or within the insulation, and it could eventually saturate it with liquid water, leading to dripping, mold and significant loss of thermal resistance. Closed-cell elastomeric foam insulations typically have low vapor permeability, as does cellular glass insulation. Acceptable vapor barriers for fiberglass or open-cell foam insulation include aluminum foil, PVC or composites using layers of paper, fiberglass scrim and aluminum foil. Care should be taken to maintain the integrity of the insulation and its vapor barrier at all joints, as well as components such as valves, circulators, buffer tanks, unions and other pipe fittings. This often requires Figure 7-31a 1 1 1 3 /2" /2" /2" /4" 3/8" 1/2" 1" 1" 1" The accumulating condensate will eventually drip from the piping and possibly damage whatever is under it. Such damage includes stained ceilings, deteriorating materials and eventual growth of mold and mildew. To prevent condensation, the outer surface of the pipe insulation must be maintained above the dewpoint of the surrounding air. Insulation manufacturers typically provide tables, such as shown in Figure 7-30, that relate the minimum insulation wall thickness required to prevent surface condensation to the temperature of the fluid in the tube, and the air conditions surrounding the insulation, including dry bulb temperature and relative humidity. It is also important to use insulation that has a low vapor permeability, or to wrap insulation materials, such as fiberglass, that have relatively high Figure 7-31b fluid temperature in pipe Figure 7-29 Figure 7-30 71 Courtesy of GrundfosPDF Image | Heat Pump Systems 2020
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