Ruling 188
If the sun shines on one side of an impure wall and as a result the other side of the wall – on which the sun did not shine – also becomes dry, both sides become pure. However, if one day the sun dries the exterior of a wall or some earth and another day its interior, then only its exterior becomes pure.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)