Before going out of town for five days over Thanksgiving I placed masking tape over two little holes in the floor of our laundry room. The holes were originally drilled for pipes on our hot water heater. One hole was no longer used so I taped over it completely. The other hole is still used so I just taped around the pipe. I should probably use some of that insulation foam at some point in the future though. Thanks for the tip Ethan. Anyway, I thought the holes were way to small for a rat to get in through but we noticed absolutely no droppings when we got back from our trip. Score one for the humans! Of course, the rat is now eating the tomato plant we have growing outside so there's still some work to be done but at least it's out of the house.