Cellular glass pipe insulation was tested unjacketed under flooded conditions to simulate underground water attacking an underground pipe in a district heating and cooling system. The pipe's fluid temperature was tested from 35°F (1.6°C) to 20°F (216°C). The surrounding water was maintained from 46°F (7.7°C) to 100°F (37.7°C) to simulate the possible conduit water temperatures when the system fails. Cellular glass pipe insulation of 2-inch, 3-inch, and 4-inch thicknesses was tested. The insulation demonstrated excellent moisture resistant performance when the pipe temperature was lower than the surrounding water temperature. Cracks developed in the insulation when the pipe temperature was higher than the water temperature, causing a deterioration in insulating performance.