Maxwell has recently mentioned the trials and tribulations of combatting rat visitation (infestation, depending upon perspective). We've fortunately do not have rodent roommates. But we once did have cockroaches appear in our kitchen and stairway when we first moved in. Of course, the best defense against roaches is to simply keep a very clean home. But keeping them out in the first place is equally important. I can happily and wholeheartedly endorse DAPtex Insulating Foam Sealant, which has kept our apartment vermin free since spraying all the gaps/crevices exposed by plumbing and an aged interior (it's also a great way to prevent energy loss and drafts). Paintable, toolable, cleanable with water, and extremely easy to apply, this stuff rules school. Available at retailers like Target and Home Depot for about $6.