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