Not a good plan there, you can't use Durock over a slab. To use Ditra the tiles would have to be secure and scuffed up, which you can't do with an asbestos tile. If I were in the same situation I would consider floating everything, including the half bath using a good quality laminate such as Mannington's NatureForms, switching to a tile pattern for the bath. Done correctly it would work just fine but you have to follow the instructions they specify for bathroom installations.

Solid over any concrete, is very risky. TEST the moisture vapor emissions... Mandatory.

We are having the exact same problem with a new slab which is below grade on one side. It is my understanding that this makes it "below grade". The calcium chloride test read 5.9 -- I don't know what that means except the flooring people said that was too high. The installer's hand-held reader comes out at at least 20%. I know the foundation has a French drain system as well as another 6 inch pipe all around. In 4 places I duct taped down 2 foot squares of plastic and after 3 days there is no moisture under the plastic. The flooring store says you can't glue down or float over this much dampness. They don't even think laminate would do. We are at a loss as to what to do. Can anyone help? (My dream floor was tile around the outside of the wooden floor. Naturally, the tile is already laid.) carole

The short story is we chose radiant because it worked the best with the floorplan / material choices we made. The long story is how we got there. We had the choice of a basement, crawl space, or slab. Basements are common in my area because of the required 3' footers - it doesn't cost much to go deeper and have a basement. Financially they make sense, but I don't care for them. They are cold, dark, and low ceilings feel oppressive. Crawl spaces are nice to allow access, but the floors were crazy cold in winter and the amount of humidity could also turn into a mold problem. We chose a slab foundation before we had land, and it worked out since the land we built on would not have supported a basement. Our slab has xps foam board under and along the edges for insulation. Our floor plan is completely custom that I did in autocad myself. All the common areas are on the south side and open. The bedrooms and bathrooms are on the north side. With an open floor plan (some of it 17' tall rooms) and concrete floors it makes forced air ducting more difficult. My experience with forced air is that it blows dust and curtains, feels like a cold draft across your neck when it first kicks on and forces the cold air out before the heated air, and is noisy enough to have to raise the TV or conversation volume when its on. Choosing radiant floor heat with separate zones for the common and private areas seemed like the perfect choice for heating. But we do use our air conditioner for 6-8 weeks and the floorplan layout was critical in being able to use a wall unit to comfortably cool the whole house. I was planning on finishing the concrete with coloring and waxing of some kind, but my husband became friends with a person who polishes commercial concrete floors and we had him polish our floors. It looks wonderful, and is easy to clean. The radiant heat is quiet and even heating with about a 2 hour lag time. I also thought it would be warm on my toes, but I learned that it's not - even in the middle of winter, and want to make sure others are aware of that so they are not disappointed. It's not cold, but its not warm either. Our upstairs is a quality pad and carpeted which feels nicer to my bare feet than the cool concrete. Each house and owner is different so I hope you find the best solution for your situation.