What would happen if I fiberglassed a bag of desiccant inside a sheet of plywood?

If I sealed up a bag of desiccant inside of a plywood bulkhead, what would happen?

 

The desiccant will draw up some nearby moisture , but not necessarily all the moisture in the wood.
Once it is saturated, it no longer works, so sealing it in the wood is futile.
If the wood is wet, it must be dried out or replaced, and more importantly, the source of the water intrusion must be sealed.

 

Years ago, I read that once you sealed a piece of wood, it started to rot from the trapped moisture. This was in reference to furniture and the way it was explained made sense at the time but now I think about it, most furniture would rot away if that were true. Now I’m wondering if that only applied when using green lumber.

 

Pretty likely it would attract all available moisture and get the wood nearby wet and help it rot.

 

Moisture moves through wood. Even with plywood or encapsulation it seems to find a way.
How long is the boat expected to live? Is it cheap and dirty? A gold plater? Somewhere in between?

 

No boat yet, just a question I had rattling around in my head.

 

A truly sealed wood compartment will move and work the seams with air pressure changes.

 

Nothing out of the ordinary depending on how big the bulkhead was and how small the "bag" of desiccant was.

Why do you ask?

 

Any answer to the question depends on the permeability of the vapor barrier, the temperature ranges, desiccant used and humidity variations over time. You might start with looking at permeability testing using the ASTM E96 desiccant test.

All coatings have some degree of permeability, so all they can do is slow the process. The roofing industry has done some research in this area, because heating and cooling buildings leads to condensation and rot. Sometimes a high permeability on one side of a panel allows it to dry, so most designs depend on permeability as opposed to trying to prevent it. It might be better to try to determine which side of the panel is the source of moisture and coat it with a low permeability material and apply a more permeable coating on the dry side.

Salts have an equilibrium humidity that varies with salt and temperature, but there is a limit as to how much moisture a desiccant can absorb, and after that, it can't remove any more moisture. A deliquescent desiccant like CaCl can absorb enough water vapor to dissolve and form a liquid solution. Silica gel will absorb a considerable amount of water but will remain solid once saturated. The perm rating of a coating will affect the time it takes for wood to absorb and loose water vapor, but might only delay the end result rather than control it. Salts are used to control humidity in musical instrument cases to prevent humidity cycling from cracking the wood. They try to maintain 50% RH, which works for violins. The packets can only delay the time when the ambient humidity overcomes the salt packet. The process is reversible, so under dry conditions the salt releases moisture and when conditions ore humid they release moisture. If the periods of high or low humidity are too long, the packets need to be replaced or regenerated. If you have a cigar collection and want to keep the case at 75% RH, look at Table 2 in the link for the salt that is closest to 75% which is sodium chloride. A bowl of wet rock salt in the case will work nicely. Decant the excess liquid when the ambient humidity is too high and add water when it is dry.