Modified bitumen moisture barriers are applied during the asphalt or bitumen modification processes by integrating polymer additives into the bitumen. This is typically done through a blending process where the polymers are mixed with heated bitumen to enhance its properties. The modified bitumen is then applied as a waterproofing layer, either by torching, hot mopping, or using self-adhesive methods, ensuring a robust moisture barrier that improves durability and resistance to environmental factors.