For a properly installed system, the insulation should be between the ground and slab eliminating any heat transfer to the colder soil directing the heat upward.
Bonding of the upper finish layer of concrete and the lower existing layer is not as important on an interior application where the slab is contained.
There are a few items which you should consider in making your decision prior to proceeding.
1. Vapor barrier. If the existing slab is not insulated, then does it have the proper vapor barrier?
- If you were to proceed in installing a slab atop your existing slab with no vapor barrier you could be creating an area which will trap moisture and promote mold and mildew growth.
- To test for a vapor barrier, proceed by taping a 12"x12" piece of clear visqueen to the floor with duct tape. Allow it to remain there for 48-72 hours.
- If water droplets develop, you do not have a vapor barrier and I would not recommend proceeding with the over pour.
2. Gypcrete. Most hydronic heating systems are installed in 1.5" of light weight gypcrete. If you were to install this over insulation the thickness would not allow for proper support of foot traffic and furniture and would result in crumbling of the upper pour.
3. Vertical clearance. Do you have the room in height to install at least 2" (R-10) insulation and 4" of concrete without affecting every door, counter top and plumbing fixture in the home?
If not this approach of installing a hydronic heating system will not work.
If you find that a vapor barrier does exist, look into alternative systems which do not require the installation of gypcrete poured slab as part of the heating system.
For more information:
Read "Can radiant heating be installed under our existing flooring?" a Q&A answered by Brad Hubbell.