I use 6 mill on the outside and a second layer on the inside od 4 mills. The 6 Mill I bought at Lowes fro 100 by 20 6 mill for around $70. Wal Marr has 6 mill in small rolls for like $15. its 20 by 15 I think. Watch what you get because some of the clear is more like a white clear and will not let as much light through! The second layer is important since it saves lots of heat! my solar greenhouse would not work without the second layer!
I enjoy fishing ,gardening and a solar greenhouse! carpet installation repair and sales for over 45 years! I am the inventor of the Bobber With A Brain - Fishing Bobber!