LTZ1000 and buffer from 7 to 10V are on the same PCB, all resistor are top quality, lowest PPM/°C reasonable money can buy but molded, reason of my concern about humidity, enclosure will be hermetically sealed, option of argon is silica gel.
My target is 1PPM/year or better.
Again, you should at first create a clear target specification concerning humidity, temperature and time, then measure the as-is specification of time / temperature, and then calculate the influence of these three parameters, especially the humidity onto the stability of the circuit.
Humidity effects have been measured by Andreas and /or Branadic, I think.
It might turn out, that to achieve your target specifications, humidity effects can be neglected, but an oven might be needed as the only measure to achieve a certain T.C.
'1ppm/year or better' does not fit at all to temperature or humidity stabilization measures, that's dependent mostly on the quality / timely stability of your step-up resistors .
So if you did not use Vishays oil filled, hermetically sealed resistors in first place, you will definitely or very probably get >1ppm/year stability.
But again, don't confuse that with the temperature coefficient (stability)
It's a big disadvantage, if you can't separate LTZ1000 and boost circuit.
Frank
PS: I have built my 7 => 10V step up by 10 bulk metal foil resistors, green molded Z201, or so, and that 10V tracked its LTZ1000 voltage relatively within < 1ppm over many years.
Temperature or humidity effects I did not observe, or did not test it explicitely.
As an orientation, the left-over 49.99k Z201 I have, has a T.C. ~ 0.2ppm/°C, but I don't know about the 10 others.