What part is this a replacement for? The Lay-Z-Spa Milan AirJet Plus inflatable lid is a direct replacement for the inflatable part of the lid. Why do i need an inflatable lid? An inflatable lid is essential to providing insulation to your Lay-Z-Spa. This helps to maintain water temperature when the spa has been switched off. It also helps to speed up the heating of your Lay-Z-Spa. It's important to note that the lid should be placed inside the cover before inflating.