Most spas are kept outdoors,Fortunately, much of the heat that would usually escape is trapped inside by the cover. Spa Covers Keep The Heat In

The spa cover forms a seal with the spa simply by resting on the hot tub's surface.  This seal against airflow out of the spa is very important.  Improperly sealing spa covers will not perform nearly as well properly sealing ones.  As the spa water is heated upwards of 104F, the heat will begin to try to escape to the cooler outside.  A well-fitted and expertly-constructed spa cover will keep most of that heat in.  The heat will flow through the air underneath to the bottom of the cover.  Once the heat hits the bottom of the cover, it must travel (or conduct) through the spa cover mass.  Whereas most of the cover's insulating power lies in its EPS foam cores, the layers surrounding the foam also have some insulating abilities adding to the cover's overall R-value.  The cover's R-value, or resistance to heat flow, will determine how much heat will escape through the cover in a given amount of time.  The higher the R-value, or more insulating ability that is inherent into the cover's design, the less heat will be able to conduct through the cover in a given amount of time.

insulator cover