thick foam because the thinner sheets are more flexible there is not as much rollup compression and expansion as the thicker foam will experience and the finished surfaces of the foam material provides a stronger longer lasting cohesiveness of the closed cell foam this u00bd u2033 thickness of foam provides the minimum air space that the manufacturer of the reflective double bubble insulation recommends for optimum infrared heat refection back down towards the water in the spa these two sheets of foam are centered on top of the reflective insulation and temporarily fastened in place optionally final sheet of 45 mil or 60 mil edpm rubber membrane is cut to the exact same size as the layers of insulation and foam this layer of rubber is centered on top of the layers of insulation from the plan view there should be 12 inches of the bottom or very first layer of 60 mil ecowhite edpm rubber membrane exposed on all sides of the stack of insulation and rubber in the embodiment shown in fig1 and 11 this layer is substituted by an aluminized swim fabric which forms a further radiant barrier which is affixed as a continuous layer over the square tube support structure an air layer is formed between adjacent tubes beneath this aluminized barrier making for addition insulation the 12 inches of exposed rubber membrane is folded over.