Haven Spas High R Value foil insulation. Sold by the foot.  Ideal for do it yourself, if you have a not full foam spa that needs more insulation, like a mass merchant spa. Huge difference in electric consumption.

Havenmade Super Foil Spa Insulation This can save you 60% or more on your electric bill in winter.  Easy to install with a stapler. Dont Forget to add a thermal blanket also

This is for the "do it yourself" people who want to properly
insulate their thermal pane spa.  This material has an effective
R-value of 14 when used with an air space. This is truely "space age"
insulation.