[perchancetodream]

I'm using 1/4" cork underlayment on the ribs before installing plywood panels. Got the 5-pack of sheets from Home Depot online. Will cut into strips to match width of rib.

https://www.homedepot.com/p/QEP-2-ft-x-3-ft-x-1-4-in-Cork-Underlayment-Sheet-30-sq-ft-5-Pack-72005Q/203110563

Also available in rolls for larger quantity.


QEP tells me the underlayment is bare cork without vapor barrier membrane.

 

[perchancetodream]

Thanks for the input everyone!
@perchancetodream are you using 3m 90 adhesive for cork too?

I'm putting in a lot of plus nuts ahead of the cork. Spray adhesive would get into the threads, so I'm going to use Weldwood contact cement and a brush for better control.

I'll use 1/4" sharpened studs to locate the threaded holes on the backside of the cork. Then use a hollow punch to make oversize holes in the cork to maintain access to the plus nuts. Brush contact cement on the ribs, keeping clear of the plus nuts. Brush on the cork. Use the studs in the plus nuts to help align cork holes to plus nuts. Roll with 2" Marshaltown rubber roller.

I'll use same sharpened studs to locate holes for bolting 1x3 furring over the cork. L-track attaches to plus nuts and wood, depending on spacing. Paneling attaches to wood on either side of L-track with wood screws. I think this will be easier than trying to attach the panels directly over the cork at the plus nuts.

I've considered filling the space between the 1x3's with rigid foam. But I've been concerned about squeaking and vapor permeability. As far as I can tell, polyisocyanurate doesn't squeak like XPS or EPS foam. But its aluminized facing is completely impermeable to water vapor. Two vapor barriers not good. The wall cavity has to be able to dry to the van interior. The facing peels off pretty easily, but iso is fragile without it. I would use a spiked carpet seam roller to make a bazillion holes in the facing. Remove some of the star wheels and substitute washers to avoid totally shredding the facing. Not sure it's worth the trouble....

https://www.amazon.com/Seaming-Roller-Spiked-Grooming-Carpet/dp/B009KJXV6W

 

[perchancetodream]

Discovered today that Baltic Birch plywood comes standard in 5' x 5' panels at hardwood suppliers. Game changer for paneling short / low roof, as only 5 panels needed, with only 12% drop and just two joints, compared to over 33% drop with 4' x 8' sheets and a lot more joints. Price around $17 - $22 / sheet for 6mm (1/4") 5-ply B/BB. Very good quality material. With this size panel, I can attach directly through the cork thermal break rather use furring to handle the multiple joints that would otherwise be needed.

 

[perchancetodream]

I did not use the cork as thermal break in the end. Went to 1x3 furring on the ribs and horizontally over the body instead. Filled in with frictiin fit and caulked 1/2” R-Max reflective polyiso board insulation, after perforating the foil with a dough docker. Cork ended up installed as an interior finish on the steel safety partition. Looks good. Absorbs sound perhaps.