Air leaks around windows and doors cost you money. It makes perfect sense. If air is escaping, then your air conditioner / heater has to work that much harder to keep the inside of your house at your desired temperature.
The problem is that these air leaks aren’t always readily apparent. The best way to check for air leaks is to light a candle or an incense stick and walk around the house placing it around the frames of doors and windows (it helps if it is windy outside rather than a calm day). If the candle flame starts to flicker (or smoke from the incense gets sucked out), you know that you have an air leak and can determine exactly where it is. If you don’t have a candle or incense handy at the moment, you can use a piece of tissue (you want to separate it so that it is as thin and light as possible) although this may not detect small air leaks.
Take your stroll around the house with your candle/tissue in hand. It shouldn’t take more than a few minutes. When you come to a window or door, slowly move it around the edges watching to see if the candle flickers.
Once you have determined where the air leaks are, caulking them (you can purchase the caulk at any hardware or home improvement store. Ask an assistant there for help if you are not sure – the bigger discount home improvement stores should have “how to” brochures that will show you exactly how to make the repair. Once you have the caulk, all you do is spread it over the cracks so that they seal up and no longer are able to leak air) will be well worth the investment. If you have leaks under doors, you can purchase a door sweep which will attach to the bottom and help block the air flow. Repairing air leaks can save up to $100 in energy costs depending on the severity of the leaks.