Windows represent the biggest potential holes in the building envelope. Think about it: the rough openings are just big holes — until you actually plug the opening with a window. But plugging the opening with a window unit doesn’t necessarily keep the weather out. Think about an inner tube with a tiny hole in it. If the tube is full of water, it drains slowly. If it is full of air, it goes flat almost immediately. Air squeezes through tiny holes a lot faster than water does. So sealing a window for air leaks can have a huge effect on the energy performance of a home.