What is the out-of-plane thickness for?

I have a very simple system as depicted in the picture, I wish to simulate Joule heating of a Pt heater sitting on a SiN membrane. The thickness of the Pt heater will be 20 nm and the SiN membrane will be about 150 nm thick. I've been reading up on the documentation that the out of plane thickness is indeed as the name describes but I don't know how whatever thickness I put in there is used by comsol.

So far, using the geometry attached and using FR4 and Pt from the material library I can get a Joule heating simulation whose temperature changes drastically when I change the out-of-plane thickness from 1e-6m to 1e-7m. At 1e-6m the temperature distribution is essentially constant at 293.16K and at 1e-7m I reach a high temperature of ~550K.

I'm not adding any of the other cooling mechanisms such as convective cooling and surface to ambient radiation yet because I can't make sense out of the current results. However I do have specified one of the edges of the SiN (or FR4) membrane as being at room temperature. I have my terminal and my ground at opposite sides of the wire such that it heats up as expected, but I'm dumbfounded by the radical temperature changes I see upon changing the out of plane thickness by just one order of magnitude.