Heating Help

INSULATION FOR UNDER RADIANT

I want to install 1/2" or 3/4" insulation on a radiant job so the heat goes up into the room and not down to the floor below.
It will be over 3/4" plywood (glueboard) on the second and third floors of a residential home.
It will be covered by 1 1/2" of gypcrete.
I remember reading about a company a few years ago on this site, but I can't remember what the brand was.
Thanks
Paul

might as well

just use rigid foam for that.

could use "the barrier" if you want, it's the only thing thin that is also still foam.

do not use reflective insulation.

also it's more typical to insulate the joist cavity, if you can. saves height.