There are also plenty of less-obvious places where air, heat, and humidity can pass through your walls: external stove and dryer vents, plumbing supply and drain lines, and even HVAC ducting can allow cool air to escape or hot air in. Caulk, foil tape, and spray foam (like Great Stuff) can be used to close these gaps, reducing the amount of cool air that leaks out of your home. A note on the spray foam—it’s effective, but it’s an expandable foam that hardens to a beige blob, and it usually doesn’t come out looking all that great, so save it for less visible areas like under-sink cabinetry.