Foam board insulation and vibration?

Just use structural insulation board, such as this:
FOAMULAR XPS
You really don't need the 600 as the 150 (cheapest) is able to support 15 psi and when you consider the weight is dispersed by the plywood...
Links to the various strengths, compression and otherwise...

It should be available at any big box store.
Actually, any of the rigid foam board would be fine as the compression value is more than you think, however I would up the thickness of the plywood to at least 3/8"...

As a note, most closed cell rigid is a vapor barrier, just read the specs in the links that I provided.

Quicksilver has a foam sandwich floor. Ribbed 16-gauge steel with 20 gauge flat factory spot welded. The original floor. 2"x1" teak stringers were used full length as longitudinals. Every four feet cripples were laid. Nothing but Franklin Tite Bond was used for the longitudinals and cripples. 3M spray adhesive was used to prevent noise squeaking of the insulation when flexing was borne by the foam. Then half-inch laminated plywood teak (marine tongue & groove) flooring was laid. Adhered to the stringers with Tite Bond and to the foam with adhesive spray. Concrete cinder blocks were used for compression clamping during the 2 day adhesive "cure". Salvaged marine through-fittings were used to pass pipe tubing and electrical.

Naio,

I'm going from memory here ... but I'm pretty sure we used 1/2 exterior plywood for the floor in our van camper build. It had to be strong enough for our motorcycle to travel on - held down with straps and some rings that would fold down into openings I cut into the plywood flooring - whenever we weren't using the van for camping. We glued vinyl down onto the plywood for the final flooring surface.

Foam board insulation under the plywood should provide excellent floor insulation ... but I recommend 1/2 exterior plywood on top of the foam paneling. BTW, how are you going to secure the plywood - long screws down through the plywood and foam paneling into the van's steel floor? If so, you might want to use stainless steel screws.

She did Not say anything about styrofoam

If this was my project, I would use Owens Corning FOAMULAR 250 XPS 1 in. R-5 Tongue and Groove Insulation Sheathing covered by 1/2" OSB. The FOAMULAR 250 has a compressive strength of 25 psi (3,600 # per square foot) compared the white styrofoam which is only 16 psi. Unlike styrofoam, XPS is virtually waterproof so it doesn't support mold growth. The 1/2" OSB provides a more solid base to attach furniture to and better weight distribution so you're less likely to get soft spots in foot traffic areas near entrance doors.

I would put 1x2 sleepers down 12" OC then cut your foam board to fill in between the sleepers 3/8 plywood over that. Your foam wont break down and you'll have support for anything sitting on it. The 1x2's are nominal 3/4x 1 1/2 they should fit between the ribs in the van bed which would for a 1/2 inch foam if you want thicker go with 2x2's and go with 1" foam.

MrWizard wrote:
with a complete sub floor, the fridge is on the plywood, should not be any problem
Although I would be tempted to go with a heavier grade plywood

x2 My truck camper weighs over 3k and sits on a 4x8 piece of 2 inch foam board with nothing on top. It has held up for many many thousands of miles. I would go with a 1/2 plywood though over the whole floor. I would also probably glue with the proper glue the foam board to the plywood. That would give you one solid floor.

Unless you see yourself removing the floor at times for some reason. Then no glue. Either way I see no problem. JMHO

I'd make a sandwich

plastic 6 or 12 mil or Tvek.

I would top that with some plywood intended for below grade (underground) construction.

Then top that with the closed cell foam.

Finally 1/2 inch of plywood.

Then choose the lino you wish to have.

The foam would be lying on the corrugated steel floor of the van.

I suspect it is important to preserve the air space between the corrugations. It has multiple large holes as part of its design, and I think it collects condensation from behind the walls and allows it to drain and/or evaporate.

So I want a floor that is relatively, but not completely, impermeable that sits on top of that.

As I said upthread, I am talking about the pink or blue 4x8 sheets of rigid foam used in s&b insulation.

Hi Naio,

1/2 inch would be considerably better than 1/4 inch.

What will be supporting the foam? (and of course the loads above the plywood)

Do make sure to use closed cell foam. I'd use the foam that has silver "tin foil" on both sides.