ᐅ Insulating an attic space with 160mm of insulation at a thermal conductivity of 0.035 W/(m·K): which vapor retarder should be used?

Hello everyone, I want to insulate the attic space of our house built in 2017. The ceiling between the top floor and the attic is already insulated. Since our little one now uses the attic to play and it gets very cold there in winter and very warm in summer (temperatures are almost like outside), I would like to insulate the attic with 160mm (6.3 inches) mineral wool batts, thermal conductivity 0.035 W/(m·K). Now I’m wondering which vapor barrier membrane I should use and what SD value it should have: SD value 2 or SD value 100?