I've pulled up the corner molding to lay down the new laminate
flooring and have noticed that there is a nice gap between the floor and
the sill. This in both '29 part of the house and the 70's addition.
I'm guessing that is deliberate and is the way hardwood was laid.

Since I have the molding off here, it seems like thing to do is fill
it with some of that low expansion foam. I've pushed the underlayment
vapor barrier into the cracks. But I wonder what to do with the rest of
the house. Is this a significant air leak? Should I stuff fiberglass up
to the sill from underneath, or is there another course of action?

Jeff