Dan,
Yes, it will help.
I would suggest something other than "Styrofoam" though as, if/when water gets up there, Styrofoam will tend to absorb it. I used about 2.5 inches of spun fiberglass insulation (easier to fit around wires and plumbing) held up with 2" thick foam made to go below grade. Long screws with fender washers hold up the foam and all joints are taped. Reflectix type insulation under the foam and, finally, the corplast. As recommended by another forum member, I put drains in the Reflectix/Corplast so, if/when water gets in there (it will), there is a way to drain it out. Radiant heater above the fresh water tank keeps the floor warm and, via conduction, the tank seems to stay "warm enough". Of course, your layout may not allow for this.
Might want to wrap any plumbing down there with heat tape and foam pipe insulation so, if needed, you have a way of thawing it out.
Hope this helps.
