Setting three bug bombs off in the attic was a waste of time (and somewhat evil, seen in retrospect). We thought we scared them out so we closed up all the holes. Dum-te-dum-tum. No go. The squirrels stayed up there throughout the whole thing. I find this shocking but that's what happened.
So then I had dog knows how many squirrels penned up in my attic, breathing horrible gas fumes. In other words, the situation was even worse. Luckily, a friend of my sister's lent me a squirrel trap. I put some peanuts, still in their shells, into the trap and left it on top of a duct (that looked fun to walk on, if you were a squirrel).
Worked like a charm. Within a matter of hours, I trapped four squirrels (one at a time) and let them loose outside. There were three babies and one adult and all seemed fine despite the bug bombs. Phew. I'm glad for that.
In all, it was so easy that I'm not worried about any future squirrel-tenants. I'll just lure them into the pain-free trap with the food of the gods: peanuts. Never underestimate the power of a peanut.