Thanks for all the input on this. That was a good tip to insulate the
cold water pipes, so we've gone ahead and done that. About the danger
of leaking gas building up in the space above the ceiling, this won't
be an issue because only half of the basement is being finished, and
the other half is open all the way through the joists, so gas would
presumably leak out there. My main concern is having a future
homeowner attempt to blindly drill through the ceiling and hit either a
water pipe or gas line. I'm tempted to just put steel plates below the
lines where they pass under the joists to protect them. Any would-be
driller would attempt to find at least the joists first, right??
Again, we will put in access panels for the one gas valve, and will
make detailed maps of the whole ceiling configuration.