Which of the following types of material and thickness is required to retard the flow of soil gases - radon - into a building?

The correct choice is 6-mil polyethylene, as it is specifically designed to create an effective barrier against soil gases, including radon, which can be hazardous to health. In construction, especially in areas known for high radon levels, a thicker polyethylene membrane provides better resistance to gas intrusion due to its durability and ability to maintain a tighter seal.

Using 6-mil polyethylene as a vapor barrier not only helps in managing moisture but also works efficiently to impede the movement of harmful gases. The thickness is significant because thinner materials may allow for easier penetration of gases, diminishing the effectiveness of the barrier. Therefore, 6-mil polyethylene is the recommended standard for radon mitigation strategies in building construction to ensure a safer indoor environment.