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From what we have tested , when its below -10 degress celcius outside our perimeter insulation is drasticly more efficient . You are right to say that full foam is a little bit more efficient when its above -10 outside but just the fact to not see whats going on with your plumbing, the fact that a perimeter insulation reduce the hearing of the pump noise is 2 good reasons for me to choose that insulation indtead of foam . When you choose the right components and dont insert cheap material in your spa you should not have any problem with a perimeter insulation . Have you experienced any issue wihthat type of insulation?
fyi..........44000 btu's over 6 hours is equal to just over 2 kW per hour.