Thanks for the thoughtful replies. The thermostat probe is currently taped on the underside of the Flexwatt (therefore surrounded on most sides by the variable temperature air). This is my only tub and I really don't have room for another one to put the probe in, although that sounds like a good idea. My substrate is two layers of the Sunday paper.

Would it be better if I put the probe in the tub and make sure it is secured very well with the aluminum tape? I guess that would mean covering it fully with the tape. Or between the Flexwatt and the tub?

I will get some Reflectix (I'm glad I know now what it is called!) for the bottom of the tub. Maybe I should cover the top as well, as long as I leave the holes for humidity clear.

Any more input?

Thanks - you guys are awesome.