I haven't ever heard the "rats in the attic" phrase and I would interpret it more as having his own problems/ dirty business rather than being crazy.
Mostly words rather than phrases are used in reference to weird/crazy people
1. Cuckoo (sounds like kookoo)
2. Psycho
3. Weirdo
he/she "has a couple of screws lose" "not right in the head" "Mad as hatter"
"Lost his/her marbles" "stores open but nobody's shopping"
Hope this helps