All it takes is one small entry point and wasps, bees, squirrels, or bats can take over this seldom-visited space. Then they build nests, chew through wiring or framing, or just make your home their final resting place. "Animals can get trapped and then die. After a few days, the stench can render a house uninhabitable," says Los Angeles–based architect Leigh Jerrard. "Make sure crawlspace and eave vents are securely screened with hardware cloth or welded-wire fabric," says Jerrard.
Tom Silva recommends that you do a visual inspection all around the exterior, both before you go away and after you get back. "Look for vulnerable spots—holes under the soffit, flashing lifting on the chimney, rot around the windows," he says. "A bat can climb into a hole the size of your fingertip. Wasps and yellow jackets can find their way in through a tiny crevice." Patch, caulk, and replace as needed.