Most plastics are water absorbent by nature. That means all plastic, epoxy, acrylic, urethane, silicone, etc. will absorb water, generally up to about 2% by mass. Some more or less but never less than about 1.5%. The type of plastic will change dimension very, very slightly depending on water content. It is hard to predict what dimension will change on a plastic part without just taking careful measurements of a finished part.
Some material data sheets will show a plastics / epoxies with a low absorption rate - for instance .01% per 24 hours - but this is NOT the same thing as absorption proof. It'll still absorb water, but take longer to do so. Always.
Most spray-on / dipped conformal coatings will do a great job at really reducing most humidity effects but aren't 100% humidity proof. They will really toughen up a PCB against dirt and abuse though, and some coatings are essential for high voltage applications.
The only PCB conformal coatings I know of that are really humidty-proof is a PTFE (Teflon) or Parylene coating applied by a vacuum vapor-deposition process. This is not a hobbyist project - you send completed, assembled boards out to a coatings lab. That's what I do for critical projects. Pricing is not for faint of heart if you're doing one board. But when you get the boards back they will have very high humidity tolerance. They will also not be repairable unless you scrape off the coating, and sometimes that's not easy or impossible. You really can't make any changes to a PCB board with this coating in place, but it's chemically tough. That's why it's popular for industrial, military and space-flight applications.
The other method is a good air-tight enclosure with seals. That works very well also and is an EMI / RF shield too. Less expensive than some coating processes, and your board is repairable.
HOWEVER please note that 3458a's have been built for decades with plastic packages around the LTZ reference, and generally even the film resistors may or may not be in a hermetic package - and it doesn't make a lot of difference. You still see very low drift rates year per year in almost every case regardless of how the components are packaged..
Why? Because if the inside of your instrument package is always kept warmer (keep it turned on) it will by definition have a much lower, moderated humidity inside the case than outside - in general humidity effects are reduced or non-existent. This also makes for much longer lifetimes in general of your IC's also without the thermal cycling of the chip package bonding leads (inside the package) , and keeps your Vrefs at a lower stress level.
Look at Fluke 732's - you keep these always powered on. Part of the reason is for the Vrefs crystal substrate and resistor stability, part of the reason is to keep temperature constant and humidity in-consequential. If you power cycle these, you're looking at a long de-stress time due to a variety of factors if you want to get it back to very low yearly drift rate.
My advice: In general - Keep your precision equipment powered on always, if it designed for that (most are). The warmer cabinet interior will do a lot to reduce humidity effects, and result in more stable devices. The more you power cycle, the more you're adding stress to the components inside. The same goes for any type of stable resistor, especially for precision DC - you really want that power dissipation inside the resistor to come to thermal equilibrium with the surrounding ambient conditions and stabilize - and that holds true for any PWW or Vishay Magical Hermetic or Diffused resistors or
.
It can take hours or days for any resistor to become truly stable under bias - and that IS NOT the same thing as doing a quick measure on a DMM.
NOTE: To measure resistor at different humidity, you place them (under bias) in various saturated salt solution atmospheres. It's easy to do but important to take your measurement under various actual current flow thru the resistor, with approximately the same thermal flow to ambient temp as your application. Again, just measuring resistance value (with DMM) while it a certain humidity level won't tell you much, because you need to add the effect of resistor self heating at a realistic current flow - whatever that is per application. That will give you the TOTAL effect of power dissipation and dimensional change due to humidity. The problem with some "hermetic" resistors is yes - they have a can and maybe oil - but with even small current flows the power dissipation still causes stress problems on the substrate that you see as noise or resistance fluctuation. This is why they sometimes
don't give you 80X more performance than a $1 resistor - at least not in every case.
The humidity effect is usually the very least of your drift problems - although I have certainly seen this problem and you need to be aware of it. Tempco, good tight ratio matching, low noise, repeatability and good aging characteristics I have found to be usually more important in the long run. That's why PWW is a very good choice for DC and low freq - and go to some of the metal films and exotics for higher freq.
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