Fiberglass insulation that doesn't stay wet or attract mice?

The only insulation that will not hold water is CLOSED CELL FOAM.

Insulation actually gets it insulative property by having tiny air pockets. If those pockets are open, they will hold moisture. Then only way to remove that moisture is to place the material in a vacuum and lightly warm it.

As for keeping rodents away, liberal amounts of cayenne pepper.

profdant139 wrote:
Some great tips! magicbus, I had never heard of encapsulated batts -- I will definitely look into that. John, I could not determine whether Rockwool sheds moisture -- have you used it in similar applications?

And Steve, they say that EPS closed cell extruded polystyrene panels don't retain moisture -- is that not true? For example, a styrofoam ice chest seems to be perfectly dry after the ice water is poured out.

Agesilaus, I was also not aware of the alternatives to Liquid Nails. I like the idea of cementing the foam into place. The only downside is if I have to remove the stuff in order to fix a problem with the fresh water tank.

I know that the pontoons on pontoon boats are made out of closed cell poly that is exposed 24 hr/day to water and they do not become water logged. I used foam panels when I built my house and the panels were stacked outside in Florida weather for months and showed no sign of water absorption. So I am very doubtful of that claim.

Based on a little googling, it looks like the encapsulated batts are not really moisture-proof -- they have a moisture retardant barrier.

But that gives me an idea -- what about stuffing fiberglass batts into heavy-duty bags, sealing the bags, and then stuffing the bags into the empty spaces around the tank?

If properly sealed (a big if), the bags would completely exclude all moisture and would protect the fiberglass.

The fact that I have never heard of this technique makes me doubt that it would work . . . .

Some great tips! magicbus, I had never heard of encapsulated batts -- I will definitely look into that. John, I could not determine whether Rockwool sheds moisture -- have you used it in similar applications?

And Steve, they say that EPS closed cell extruded polystyrene panels don't retain moisture -- is that not true? For example, a styrofoam ice chest seems to be perfectly dry after the ice water is poured out.

Agesilaus, I was also not aware of the alternatives to Liquid Nails. I like the idea of cementing the foam into place. The only downside is if I have to remove the stuff in order to fix a problem with the fresh water tank.

I would suggest you go to Home Depot and look for the white plastic encapsulated fiberglass batts. I bought a package for the outside wall of a bathroom rehab on one of our homes and had half left over. I used the rest in a high dampness application where the previous insulation was removed due to mold. Five years later completely dry behind the insulation.

Dave

Would Rockwool work?

Ten years ago I used to use Polyester-based insulation. It is itch free and no fiberglass dust to inhale. It does not retain moisture and does not mold. However, SafeTouch was discontinued by Dow in spring 2011 due to poor sales. It was a big hit with DIYers, but most installations are done by those who do not make material buying decisions.

I still find it, but I have to get it as pillows from Walmart. Here is the last job I did with it:

https://www.rv.net/forum/index.cfm/fuseaction/thread/tid/29684694/gotomsg/29685117.cfm#29685117

Liquid Nails type cement work fine with foam board, there is a type out now that is quick grab called power grab. Not as fast as spray on adhesive but works well.

I assume you know this, but Styrofoam (the white stuff) also absorbs water.....and it takes a very long time to dry out (much longer than spun glass insulation). Of course, if it doesn't get (and stay) wet, you'll probably be fine.
Why not use thicker foam (or multiple pieces) where you have the room and thinner foam under your tanks?
Under my tanks, I used a piece of 1/2" camping mat with a piece of 1/2" plywood underneath to help support the tanks (the plywood is supported with tubular steel that bolted up to the original tank supports).
I also cut lots of small foam pieces with my bandsaw to fit as needed. Glued them in with spray adhesive. Note that most spray adhesives will eat Stroyfoam, but are fine for Blue Board (or similar products designed for below grade use).
Don't forget low point drains to get any water out that will eventually find it's way in.....esp. if you tow when it's raining.
Great project and I suspect you'll find the floors warmer on your toes when done.