This is an example of a gypsum overlay, it shows the streaking effect over the tubing. The  ability of the heat to spread out is related to the amount of resistance in the material above the tubing.  A wood subfloor has many times the resistance of the less than 1/2" of gypsum above the tubing. The aluminum plates used on top of the floor with some products does nothing to spread the heat out. The aluminum plates used under the floor do not contribute to spreading or pushing the heat up. In a staple up system it is best to keep the tubing close to the floor to shorten the response time.

Streaking; is the effect you get when the tubing is installed on top of the subfloor or in grooved floor panels..
The tubing is so close to the surface of the floor that a streak of high temperature can be felt. This is a problem for some floor coverings. Although tubing on top of the floor will heat the house and satisfy the thermostat it will leave noticeable hot and cool places in the floor when you are barefoot.

If you must use an overlay material, use only Levelrock® Quik-Top floor underlayment: 7,000-10,000 psi
http://www.usg.com/flooring/radiant-heat.html

AT 85F. SURFACE TEMPERATURE THE FLOOR CAN PUT OFF 30 BTU PER FOOT OF FLOOR SURFACE. MOST NEW CONSTRUCTION REQUIRES UNDER 20 BTU'S PER FOOT EVEN AT -20F.
NEW ADVANCED CONSTRUCTION METHODS ARE ATTAINING HEAT LOSSES UNDER 10 BTU'S PER SQ. FT.

AN 1 1/2" CONCRETE OVERLAY IS MUCH BETTER
THAN USING A GYPSUM OR A LIGHTWEIGHT CONCRETE  OVERLAY