Water within thermal insulation can significantly affect the fluid and gas flow properties through the pipelines by altering the thermal properties of the insulation material. This paper reports on moisture absorption and thermal conductivity of some common pipeline insulation materials such as calcium silicate, cellular glass, mineral wool, perlite, high temperature perlite, polyisocyanurate, polyurethane, and aerogel at different water contents and test temperatures. The thermal conductivity was measured using a modified transient plane source probe. Each sample underwent isothermal water absorption using water bath submersion for a range of periods up to 30 days. After each submersion period, the water content and thermal conductivity were measured. Testing was performed for the following three water saturation levels: as-received, 6 hours, and 30 days immersion at 10°C (50°F), 20°C (68°F), and 35°C (95°F). Based on the results, the different testing temperatures did not seem to have a significant effect on the thermal conductivity, whereas the water content for each material showed a significant effect. It was also seen that the water absorption characteristics of the materials were clearly influenced by their cellular structure. For example, the granular and open-celled materials absorbed as much as 379% water content by weight with about 262% increase in thermal conductivity, whereas the closed-cell foams absorbed as little as 94% by weight with about 391% increase in thermal conductivity. The materials that absorbed a considerable amount of water showed a more linear increase in thermal conductivity than the materials that absorbed a relatively small amount.