Thermal Blanket - floats on the surface of the water while the tub is not in use.  Cut to fit the inside of your tub and floats on the surface of the water. Creates a dead air space between main cover and keeps steam from escaping the surface of the water.  Adds about R-4 insulation to your tub and reduces energy costs.  Made with 1/2 inch closed cell foam.  Very light weight.  Charcoal grey color.  Highly recommended.