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]

Moby Thesaurus II by Grady Ward, 1.0:

46 Moby Thesaurus words for "impermeable":
   close, close-knit, close-textured, close-woven, closed, compact,
   compacted, compressed, concentrated, concrete, condensed,
   congested, consolidated, crammed, crammed full, crowded, dense,
   firm, gluey, hard, heavy, hermetic, holeproof, impassable,
   impenetrable, impervious, jam-packed, jammed, massive, nonporous,
   nonpuncturable, packed, punctureproof, sealed, serried, solid,
   substantial, thick, thick-growing, thickset, unpassable,
   unperforable, unpierceable, viscid, viscose, viscous