Roofing felt may bleed the asphalt impregnation at high temperature and affect the electric wire insulation as it is often EPDM or other plastic. I prefer not to have the asphalt product with a potential odor inside the living space. The mud is usually spread directly on the wood subfloor or on cement board in bathrooms.
"Installation: Installation simply involves laying out cable to predetermined spacing on the floor to be heated. The cable is routinely stapled to the subfloor or tied to reinforcing wire. The cable is then covered with concrete or a typical mortar to make a bed approximately ½" to 2" thick. After the covering is installed and totally dry, the cables are connected to the thermostat, completing the installation."
Using at least a ¼" x ¼" square notch trowel, lay a coat of fresh mortar over the area to be covered by NUHEAT. Place NUHEAT onto the fresh mortar pressing firmly with grout float or lightweight roller. Create 100% contact between the heating unit, the mortar and the subfloor. Make sure that there are no air bubbles or curls and that the surface is flat to receive tile. If leaving overnight, place a skim coat of mortar over the surface of the NUHEAT mat to cover it completely. Level the areas of the floor that are not covered by the NUHEAT mat(s).