Everything I've read about green building talks about the importance of insulating a slab when building.  Is this the case in south Florida, where the ground temps run 70 to 72 degrees?  I'm wondering if you aren't better off without insulation, since the slab temperature would moderate the outside climate year-round. - cool in the summer and warm in the cool spells.
   Am I wrong?

Roger

Roger;

You are right, the benefit of slab insulation in Florida is minimal

More of an issue in Florida is the humidity. When a structure is built without air leaks the interior becomes vulnerable to humidity.

buck3647;

That is true, When you build a tight building in Florida it is imperative that the HVAC system is not oversized. In a old conventional building it was common practice to upsize by another ton of AC just cause it was cheap insurance.
In a tight building envelope, the HVAC system MUST be properly sized so it will run long enough to get the humidity out, it is also necessary to bring in controlled fresh air.

Roger, Comments regarding insulation are correct and applicable in several areas of the US and not just Florida! More important than insulation is a Vapor barrier for your slab.

Consider proper sizing of the HVAC system that has been mentioned! This is a REAL problem, and unfortunately not understood by some owners/ builders! An unscroupoluos AC contractor will up sell to a larger ton unit [he makes more money] without telling of the temperature - humidity relationship. If you walk into a cool/cold house in the morning and notice you have condensation on the inside of the windows. You've been oversold!!

Back to the question: A vapor barrier under the slab is subject to tears, penetrations and being compromised during construction. This allows moisture to wick through a slab and seek the low humidity interior of a house. Especially important in high water table environments or where rain accumulates after a hard rain. When the AC does it's job and removes interior humidity, Moisture in the slab is drawn into the occupied space.

Most common are flooring issues where this moisture collects on a slab top surface and Under a Floor Covering. This moisture causes mastics/glues and adhesives to loose their bond. In some cases bubbles are observed under a coating or a bubble will appear where sheet goods have delaminated from a floor and are raised above floor grade. A razor knife to to open one of these areas will reveal a floor covering that is disbonded from the substrate and may actually have ponded water under the covering.

Tons of information out there about MVER and the deletrious affects! Suffice to say -- pay attention to protecting your slab from Moisture Vapor Transfer, or you'll pay for it many times over after you are COed.

Roger,

      You are right, you don't have to worry about insulating you concrete slab, due to your location. The ground won't be subject to frost heaving, in the cool months. I am sure you are well aware, but I think it is important to note that good insulation for your walls and roof are still very important, even in Florida. Insulation will do two things, firstly it will keep the cold out in cool climates, and it will keep the cool inside in warm climates. If you live in Florida and run AC 10 months of the year, your insulation is just as important as someone that heats their home 10 months of the year. Just something to keep in mind.

Cheers.