Thanks for the replies. I think I need to clarify. Originally, both
houses had a small basement with an attached garage. The two garages
are side by side. Before we moved in, the basement was refinished. I
pulled off some of the panelling and can feel cold air from behind the
insulation. It's got to be coming from the garage next door, since
the door to that garage is basicly open to the outside.
I guess the only real way to see what's going on is to pull off
some panelling, insulation, and drop ceiling.
We did have a problem with fumes a few weeks ago. The renter has a
large pickup that he had warming up outside of his garage. Even
though the garage door was shut and he was outside, the fumes went
right through the broken panels into his garage, then into our
basement. I had a talk with him and he happily agreed to not run it.