The Collaborative International Dictionary of English v.0.48:

Undurable \Undurable\
   See durable.

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "undurable":
   brittle, capricious, changeable, corruptible, deciduous, dying,
   ephemeral, evanescent, fading, fickle, fleeting, flitting,
   fly-by-night, flying, fragile, frail, fugacious, fugitive,
   impermanent, impetuous, impulsive, inconstant, insubstantial,
   momentary, mortal, mutable, nondurable, nonpermanent, passing,
   perishable, short-lived, temporal, temporary, transient,
   transitive, transitory, unenduring, unstable, volatile