Fibreglass/foam/plywood box

That is a good suggestion, setting a circular saw to the ply thickness.

 

Just for the record, we did this and applied the luan to the fiberglass with contact glue...I don't know if the foam will cooperate with the contact glue. I would probably encapsulate the foam with a neat coat of epoxy at 2 oz per yard.

 

I think strong contact will attack polystyrene, but builder's adhesive cartridges don't, but test check.

 

I’d be looking for a trowel on adhesive to apply with a notched trowel.
Maybe a couple coats of sealer on the backside of the new plywood would help prevent regrow the of mold/mildew?

 

Carpet adhesive would work. It sticks to eberything and you get a minute to move things.

And the mold will come back.

luan has special abilities to feed mold!

 

Hello Gandolftheblack. I have over 25 years experience as an Industrial Hygienist (something like an industrial doctor). Most varieties of Mold and fungus are harmless. With regard to taste and smell, Mold usually will have a musty smell or a smell associated with rotting vegetables, depending on the type of mold and what the mold is consuming. The long and short of it is that Mold and Fungus, in general, will not kill you. That said, mold and fungus spores can act as a respiratory irritant and cause you respiratory problems. Considering we live in COVID-19 times, and the fact that persons with respiratory problems are 'at risk', better to be safe than sorry. In that respect, the simplest measure is appropriate PPE (Personal Protective Equipment - a.k.a. respirator). You mentioned you are using a "mask". If using a simple surgical mask (like you see on ER), this is not sufficient. For mechanical processes such as sanding that generate large volumes of dust, best to purchase a basic HEPA filtered 1/2 mask respirator, just look for the Acronym 'HEPA' on the package. Additionally, you can also treat the wood prior to sanding to kill or reduce the amount of mold. Mix up a 10% solution of household bleach and water. Spray the surface liberally- let sit a few minutes- then scrub off with a green scrub pad- rinse and repeat as necessary, finally let dry. This may not remove all the mold , but will certainly reduce the amount of mold. As a recap, provided you do not have any pre existing conditions that are aggravated by exposure to aerosols, dusts, mists, fumes, etc mold won't harm you. It may have an unpleasant smell, but that's about the extent of it- the "killer" 'black mold' story's are not necessarily documented by clinical research. That said, better to be safe than sorry- buy a 1/2 face HEPA filtered mask and use it! Additionally, remember for work involving epoxy and resins, this type of mask is also not sufficient. For these applications, use "P100" cartridges that protect against dust, organic vapors, and solvent vapors. Fiberglass and the resins involved are nasty in long-term exposure scenarios. They WILL harm you! Here is a simple safety link: https://trojanfibreglass.com.au/wp-...03/Fibreglass-and-resin-safety-fact-sheet.pdf. Simple Rule-of-Thumb, go on the internet, look up the epoxy name you are using, and 'google' the Material Safety Data Sheet (MSDS) for the epoxy. Good Luck!

 

Mold was given as the reason for my Aunt's copd and death. I don't like chasing the rabbit hole here, but just saying.

 

There's an elephant in the room here, which is the source of moisture causing the rot in the first place. Unless this is eliminated, any replacement ply will rot again.
In the absence of any apparent leaks, it is more than likely a condensation issue. With an impermeable skin (the fibreglass, and, as stated, the foam insulation) condensation is always going to be a concern. The amount of moisture generated in camper life (cooking, washing, breathing, sweating) will quickly saturate the relatively small volume of air in the box. Even with the foam, the impermeable internal surface of the foam may be cool enough for dewpoint - and the moisture in the air condenses to form liquid water behind or in the plywood.

Whether the camper is lived in for extended periods, or just occasionally, is a factor.

I would consider:
1) improved ventilation - perhaps mechanical. (obviously a concern for CO generation from any combustion anyway)
2) better/thicker insulation over all inside surfaces of the body. This alone still might not eliminate condensation in all circumstances.
3) an internal facing that both can't rot, and which forms an impermeable vapour barrier. Care with sealing all joints is most important.