Our home has a small addition (10'x12') at the rear of the house
which was not closed off at the base properly. Currently, the north
side of the addition is attached to the house and the south side sits
on a concrete foundation while the east and west sides are wide open
at the base (i.e. if you get on your knees you can see right under the
addition and through the other side)

Pics:
http://www.members.shaw.ca/pharmer1/Home%20Improvement/

The addition is cold in the winter (we live in Winnipeg, Manitoba
Canada where -30C or -22F is common) and looks like hell outside so I
would like to finish both sides properly.

My question is, what is the proper way to close the east and west
sides off? I was thinking about framing it with pressure treated
lumber, insulating and finishing it with stucco or vinyl. Does this
sound correct? Is it necessary to vent this space on both sides or is
one vent ok?

Any feedback is much appreciated.

TIA,
Curt