This is a question I get a lot, and it’s a fair one. 

The short version is yes, vulcanized fiberboard can absorb moisture. That’s part of the nature of 
the material. Vulcanized fiber is a cellulose-based electrical insulating material, and published 
manufacturer data and standards describe it as a material used for electrical insulation, with 
good mechanical and dielectric properties, but also moisture sensitivity compared with modern glass-epoxy laminates.

That said, “can absorb moisture” is not the same thing as “it automatically causes problems in every amp.” 

Vulcanized fiber has been used for a long time in electrical applications because it’s tough, 
electrically insulating, and easy to punch and form into useful parts. Manufacturers still sell it 
today for things like insulating plates, arc barriers, and other electrical components.  

So the real answer is more practical than dramatic. 

If an amp lives in a damp basement, an unconditioned outbuilding, or anywhere that stays 
humid for long stretches, fiberboard can take on moisture over time. That’s not internet folklore. 
It’s consistent with the real life behavior of the material.  

For most normal indoor builds and restorations, though, fiberboard is still a workable and 
familiar material. It holds eyelets well, it’s easy to fabricate cleanly, and it fits the kind of 
traditional amp work these boards are meant for.  

My own take is simple: humidity is real, but it’s not a reason to panic. It’s just one of the things to think through when choosing materials. 

If you’re building for a very humid environment and that’s a major concern, it may push you toward a different material. If the amp is going to live in more typical indoor conditions, 
fiberboard is still a practical choice and has been for a long time. 

The goal here isn’t to pretend fiberboard is perfect. It’s to be honest about what it is, where it works well, and what to keep in mind.