Thank-you for the feedback. I agree that 2" styrofoam isn't quite thick enough, but it does insulate and keep most of the heat in. Like I said on an average night the Hot Tub would lower by about 3 degrees. The foam I used has reflective silver backing on one side so it repels water and moisture. The lid is approx. 50 lbs, I can move it by myself but, It would be a good idea to have someone to help remove it with you.
The trouble is you can't find 4" foam easily and it would be costly.
A hot tub without a cover would lose heat rapidly and would be costly, while my cover may not be as effective as a commercial cover it still offers much less heat loss then without a cover at all for a fraction of the price.