You may be surprised to hear that full-foam insulation around a hot tub won’t necessarily insulate it far better.
First off, most of the heat in a hot tub escapes from the top and bottom, not through the sides. Much of the heat either seeps into the cold ground below where there’s typically the least insulation, or escapes through the cover on top. And ten inches of foam will not give you 10x times better R-value than one inch, because you get diminishing returns after the first inch of insulation.

Brands that fully foam their tubs love to brag about it as being superior. The truth is, for south-western BC weather, full foam will not make as big of a difference in your heating bill as you might think. Things such as exterior temperature, exposure to wind and frequency of use make way more of a difference than how thickly the tub is insulated. If you really want full-foam, you can always stuff removable insulation bats in the cavities rather than committing to full foam.

What is good about fully-foamed hot tubs is that they sometimes leak less. The foam holds the hoses in place so that they don’t move when the motor kicks on and off. This also helps prevent the plumbing from sagging, which over the years creates a subtle stretch at the bottom of the hose and a compression at the top. It also seems that the airtight foam preserves the plastic, which keeps the plumbing more resilient and less brittle, thus less prone to spring leaks.

However, the drawback of full foam is that it can lead to more serious trouble when they do leak. I talk more about this next.