What is commonly used to prevent heat transfer when heating pipes are installed underground?

When heating pipes are installed underground, a common method to prevent heat transfer is the use of materials that act as barriers to heat loss. In this case, 6 mil polyethylene is used effectively due to its properties as a vapor barrier. It helps reduce heat loss by minimizing moisture ingress, which can lead to heat transfer and thermal conductivity through the surrounding soil.

The use of 6 mil polyethylene provides not only a degree of insulation but also protects the pipes from moisture and corrosive elements in the soil. By limiting moisture exposure, it contributes to maintaining the temperature of the fluid inside the pipes, ensuring that the heating efficiency is preserved.

Other materials listed, such as insulating foam or fiberglass insulation, may not be practical for underground applications where water exposure is a concern. Concrete encasement is typically used for structural reasons and does not effectively insulate against heat transfer. Therefore, 6 mil polyethylene stands out as the best choice for preventing heat transfer when heating pipes are installed underground.