The Collaborative International Dictionary of English v.0.48:

Impermeable \Im*per"me*a*ble\, a. [Pref. im- not + permeable:
   cf. F. imperm['e]able, L. impermeabilis.]
   Not permeable; not permitting passage, as of a fluid. through
   its substance; impervious; impenetrable; as, India rubber is
   impermeable to water and to air. -- Im*per"me*a*ble*ness,
   n. -- Im*per"me*a*bly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

impermeableness
    n 1: the property of something that cannot be pervaded by a
         liquid [syn: impermeability, impermeableness] [ant:
         permeability, permeableness]