Not positive that it works for rats, but I know that moth balls work for skunks, snakes, squirrels and such. When I had a raccoon under my old house, I took a sling shot and fired a few under mothballs under there and within a couple days no more raccoon. There is a product called snake away that is nothing but ground up moth balls that says if you sprinkler it in a line around a barn it will keep the snakes away. One word of caution, I have a buddy that moved into a house out in the country several years ago and had a family of skunks move in under the house (1 momma and several babies). He and his wife were understandably unhappy about this situation, so him being the brilliant person that he is, originally decided to jump up and down on the floor and bang pots together to scare the skunks away......the only effect this had was to cause all of them to spray at once and make the house completely unliveable for a month, he then decided to try the moth balls, but decided that if a couple would work, 10 lbs would work better. It took more than 6 months for the moth ball smell to leave and it was unbearable for 2 of those months. So if you try the moth balls, use them sparingly. =)