Purity: Thickness and Transparency of the Shroud (Kafan)
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 566:
The recommended precaution is that each of the three pieces of the kafan must not be so thin that the corpse can be seen through them. However, if the pieces are such that all three of them together prevent the corpse from being seen, it will suffice.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)