The Collaborative International Dictionary of English v.0.48:

Porousness \Por"ous*ness\, n.
   1. The quality of being porous.
      [1913 Webster]

   2. The open parts; the interstices of anything. [R.]
      [1913 Webster]

            They will forcibly get into the porousness of it.
                                                  --Sir K.
                                                  Digby.
      [1913 Webster]

WordNet (r) 3.0 (2006):

porousness
    n 1: the property of being porous; being able to absorb fluids
         [syn: porosity, porousness] [ant: solidity,
         solidness]