Originally Posted by Derek.ca
Thanks for the advice
For now (going camping in two weeks) I am not installing insulation behind the 4mm marine plywood panels. The ply is varnish on both sides so it will not absorb water.
This winter (after my camping trip) I will remove the interior completely and address the flooring, any structure and insulation issues.
I am not concerned with R value, condensation is my main concern. I am considering using the underlaid used for laminate flooring. it is thin, (R value is almost "0") it does not absorb water or support bacterial/mold/fungus growth (close cell foam), and its thrifty.
The plan is to glue it to the walls and screw the panels back in place.
Of coarse the best laid plans.......
The trick to avoiding condensation getting behind the walls is to have a complete vapor barrier seal that does not allow moisture to get behind the walls.
I also put blocking inside of my fiberglass shell and put in insulation and then topped it with thin plywood paneling. I put in sheets of foam insulation that has a film on it. The insulation panels are pressure fit between my blocking but I also used insulation tape (in my case aluminum duct tape) to seal the foam to the blocking which meant no interior air could creep behind the insulation.
Then I used a high quality, mold resistant paint
(marine varnish is OK too) on both the inside and outside surfaces of the wood paneling to water proof it.
I place thin weather stripping foam on the back of the paneling so that when I screwed it down there was no way for moisture to get behind the paneling and into the insulated cavity.
My panel joins are all covered by wood trim over them so I also taped all of those joins with the aluminum duct tape before applying the trim.
It is overkill perhaps but I won't be getting mold on the inside of the trailer wall behind that insulation or on the backside of the paneling.