travertine over gypcrete

No, who the heck recommended Gypcrete for the job? Oh well, you can still install tile but need to use primers and or certain membranes. Didn't you ask about liquid membrane vs. Ditra? You need to keep all Q's about this project in one thread.

Jaz

travertine over gypcrete

what do you suggest instead of gypcrete it's not a go yet I'm still negotiating

I assume the manufacturer of the radiant heating system recommended the gypsum concrete so go with their recommendations. I also assume that these same people have confirmed that your porch CAN accept tiles like this and that all codes are followed, as well as having assessed that your subfloor system (whether wood or concrete) are suitable substrates for their product and can therefore warranty its installation.

That's alot of 'ifs' - but again assuming that has been done and you have the green light, then you're pretty much in their hands. This is not a piecemeal type of installation - everything must be considered - but I can tell you that you'll need a sealer over the gypsum concrete and under the uncoupling membrane but then again, the manufacturer will tell you which one, as well as indicate which thinset, typically modified will be needed as bond coat.

Which heating system are you putting in? did they recommend gypsum because it is lighter? just guessing.