Which filaments actually do need to be stored in a drybox? As I started to learn about 3D printing, the gist I learned was &quot;it's better to store the filaments in a drybox.&quot; As I rechecked these notes, they were to a good degree from an era when PLA was rather new to the market and ABS was the goTo.
Then I learned &quot;PLA is not really hygroscopic and can be stored freely.&quot;1
Now, I know some materials are pretty hygroscopic, but not all. So, let's try to pin it down:
Which materials are hygroscopic enough to demand a drybox?
I know that it is good practice to store all filament in a somewhat dehumidified or airtight box, but there are some materials out there that become unprintable and need drying before printing if improperly stored. This question is to point out these &quot;special storage mandatory&quot; filaments only. If a material can't be printed without dry storage it belongs here. If it is a nice to have, it doesn't.
This is a Back It Up question: answers need to provide one of two:

explain personal experience, marked as such.
provide an authoritative source (scientific paper/videos, manufacturer papers, quote from an experienced maker).


1 - For some time (month?). I do store my PLA in a closed but unsealed IKEA container with all the desiccant bags I can find as it is clearly benefitial.
PVA
From experience I can tell that PVA filaments need to be stored with silica beads in a plastic bag or in a specific dry-box. PVA is soluble in water and is very hygroscopic. With moisture it gets soft and swells. My Ultimaker came with an open spool of PVA which popped when heated (steam bubbles popping) resulting in very poor quality supports and clogging of the nozzle. A newly bought spool which was properly packed did not have these issues.
Nylon
Some Nylon filament brands require to be stored dry or need to be dried before printing. I have a spool of Nylon that has taken up some moisture although carefully packed, the only time it was out is when it was being printed. This experience applies to Ultimaker Nylon; I've learned now that not all Nylon filaments are behaving the same, it is suggested to look-up the specifics from the manufacturer or from reviews prior to buying.