Insulation, Radiant and Vapor Barriers - cold floors/Inaccessible,not insulated

I have a ground level bathroom floor that is not insulated and thus very cold. The bathroom sits on concrete block foundation. How can I insulate the floors without damaging the current flooring? What type of insulation is the best since there is nothing but air between the bottom of the floor and the ground. I think there is about 18" of space between the floor and the ground.

It's almost impossible to insulate that floor with that amount of clearance. This crawl space underneath is very susceptible to termites and carpenter ants. This is because of the moisture that usually accompanies bathrooms. You should have this area inspected regularly. You should do this before preceeding.

If the ground is dirt, first cover it with at least 6 mil plastic. Then using one inch fiberglass rigid board insulation cut one inch wider than the height of the crawl space wall, slide one end up against the crawl space wall. Stand it up and wedge the insulation against the wall and between the crawl space floor and bathroom floor. The reason for the extra one inch width is because it will give and hold the insulatiion against the wall. You probably don't have to actually get into the crawl space. You can probably install the plastic and insulation from the access entrance to the crawl space and use a 2 x 4 board to move them into place. You should also insulate the crawl space door. You must check this area regularly for moisture.

Though I don't normally agree with this type of application, in this case, it is probably the best way to accomplish insulating this area. If you would like more insulating value to this area, you can install another one inch board against the board you already installed and so on. You can even put some over the plastic if you want.