Source : Webster's Revised Unabridged Dictionary (1913)

Impermeability \Im*per`me*a*bil"i*ty\, n. [Pref. im- not +
   permeability: cf. F. imperm['e]abilit['e].]
   The quality of being impermeable.

Source : WordNet®

impermeability
     n : the property of something that cannot be pervaded by a
         liquid [syn: {impermeableness}] [ant: {permeability}]