The gaps around your doors and windows allow air to enter your home. If you want to prevent this, install a good weather strip. Installing a window film is also a great way to keep your home warm during the winter months. This saran-like material does a great job insulating your windows so that cold air can't seep into your living space. Other materials you can use to keep you warm are door draft stoppers/blockers, insulated curtains, and door snakes.