Insulation, Radiant and Vapor Barriers - Room Addition on Conc. Slab Insulation HELP!

I have a room on the back of my house that was added on (8) years ago by the previous owners. The addition was built on top of a concrete slab. The only access I have to see under the addition is a little 24" wide air duct supply cut out. My floor in absolutely freezing in the winter time and I want to add insulation to the underside of the floor but need some suggestions. Is it possible to have someone blow insulation under the floor through holes cut into the sheathing or does the floor need to be riped up some how in order for me to put down a new vapor barrier and insulation? Any suggestios?

You might not believe me, but even in cold climates, ground heat (heat that rises up from tectonic activity, and the source for geothermal heating systems) can be sufficient to heat your slab, so long as you provide the means for it to accumulate. I know I know, poppycock, right?

Well right now, ground heat is easily conducted to the perimeter and washed away by wind & air circulation. This is why man invented perimeter insulation. Insulating the slab at the perimeter is likely a great first step. In order to get this to work, you would probably want to seal the supply duct and install a new one elsewhere.

If the addition has high enough ceilings, a raised floor may be a good idea, as it would not require you to remove the supply duct.