Vapour barrier needed?

[Tim Castle]

Are there any cob houses where the walls (without an interior vapour
barrier) are subjected to the effects of frost every year?
  Checking back in the archives... last august there was some discussion
about "Cob in really cold climates" where the issue was R-value of
cob.    What about moisture and frost?
  Where I live on the Canadian prairies, conventional stick frame houses
are built with a vapour barrier just inside the sheet-rock. We have
about 100 frost free days, and usually below freezing from November to
March, with stretches for a week or so when we're happy if it warms up
to -30 deg. C. during the day... but in the summer, can have weeks of
daytime +30 C and warmer.  In spite of these temperature extremes, it
remains relatively dry, and that's part of the problem, especially in
the winter. Indoor living spaces tend to be humid relative to the
extremely dry frosty outdoors.  
  What does that moisture, and hence frost, do to an earthen wall?  I
notice that one is cautioned against not building an earthen wall if it
will not dry before freezing...
  (Did I miss this topic in a later archive?)