I have been using that exact size of Hydrofarm mat for two years--it has so far stayed consistently 8 to 10 degrees warmer on the soil surface of the 3-inch deep pots that I put on it (i.e. warmer than ambient temperature). I have it on a fitted piece of wood with a half-inch styrofoam on top and then the mat. Yes heat goes up but I prefer that the mat is on top of the least heat-conducting material I have lying around the house. Also, I have it in a thin paper box that also contribute to keeping the warmth within a small enclosed space.

Hope this helps.

Pagan

Hydrofarm used to be one of the only providers and a good brand. The last one I got failed within months. Have heard others having the same quality problems. I am thinking about the Root Radiance brand. My older Hydrofarms are over 10 years old, then they "improved" them :(

I have had one of these for a decade. It may be expensive but it is a heavy duty brute made of thick rubber. I definitely recommend a thermostat.http://www.hydrobarn.com/prod822.html