Flexfoil

I can't speak to the overheating issue, but there are three different types of heat transfer to consider: Conduction occurs when two things are touching. The hot water will touch the tub the tub will touch the insulation etc. This is measured in R-Value, the bigger the number the better. Mass insulation like styrofoam, fiberglass batts, Rock Wool, or polyurethane foams are generally used to slow down this transfer. Convection is when there is an airflow, so if the pump is vented it will actually cause heat to move because of the air that it blows. The way to stop this transfer is with a thin membrane so that the airflow is blocked. Any kind of film will do, but in a spa, I don't think that this is a big issue. Radiant heat transfer is an electromagnetic transfer. All things that are warm emit this type of heat. An example is your microwave, the sun or our bodies. This type of heat travels in a straight line and travels at a speed of, you guessed it, light. The way to block this type of transfer is with a reflective surface like aluminum. It doesn't matter what you use behind the foil, so bubble, fiberglass, OSB etc will all work just as effectively. Foil should be facing towards the heat. Some manufactures will use foil on both sides (marketing gimmick), but you only need it on one side. It should also be taped with foil tape, if you are taping it on the foil side, but you can use a Tuck tape if you are on the opposite side. I am going to guess that about 75% of all of the heat loss in a spa is due to radiant heat transfer, so it is important to address this issue first. Mass insulation will only help with the conductive heat transfer. Heat always goes to cold, contrary to the popular belief that heat rises. So if the ground is cold, you had better insulate the bottom.

Add reflective insulation to the walls and the floor, foil facing inwards. Very inexpensive and foil will reflect 97% of all radiant heat as long as there is an airspace in front of it.

Reflective insulation on the floor will block most of the radiant heat that the spa will generate. Pure aluminum foil is supposed to reflect 97% of that heat. Just make sure that there is an airspace in front of it. Also, facing the aluminum down is a waste of time, because you don't live on a volcano (and the ground is not hot). Heat loss through the floor will be just as bad in the summer as the winter because the ground temperature is the same, relatively.

Don't mind questions at all - I love talking about hot tubs now (as my wife is surely sick of) haha I got the Roxul at a RONA store - a Canadian building supply chain, I called 3 other places locally and they all had it, I'm not sure if HD in the US has it but I'd suggest visiting here: http://www.roxul.com/sw34444.asp or calling a HD store. It comes in 16" and 24" widths as well as 4" and 6" thicknesses (to fit in standard construction framing) - I went with 24" x 4". The insulation doesn't have a shiny face, I think you might be seeing the foil bubble insulation that Gulf Coast uses. I didn't insulate the floor because my rationale was that heat rises, the floor already had foam insulation (albeit rather thin), and I wanted to leave some air space to allow an air pocket to exist inside. Maybe your tub has more room inside? I actually didn't insulate the center of the front panel where the electronics are - the same as you have planned. There wasn't really much room to do it and I didn't feel right about placing insulation in the comparment right against all those electrical pieces that generate heat.... I insulated up to the hard plastic compartment that houses those parts, but not inside it - wish I had taken a photo of that. Heat does not rise. It goes to cold. The reason why we believe that heat rises is because we see steam rising. While it is true that when air is hot, it is lighter than air that is cold and so the hot air does rise. But if you take a temperature reading of a boiling pot of water both at one inch above the boiling water and 18 inches above, you will find that the one inch level is much hotter than the 18. If the ground temperature is colder than the air temperature, that is where the heat will go. Foil on the ground facing upwards is a quick and easy answer.

Have you ever seen foil being used both as a radiant barrier and as a vapor barrier in this application?

Make sure that the walls and floor have reflective foil insulation. If it doesn't, you can buy the stuff at your local DIY retailer.