Foam board insulation and vibration?

Is there anything underneath the foam?

If you made a plywood and foam sandwich, plywood bread and foam filling, and had it well glued ("laminated") together, you would make a surprisingly stiff and lightweight floor. This works because the stresses in a beam are concentrated at the surfaces; the middle does relatively little work. (I-beams work on the same basic principle, with the middle web serving mainly to keep the upper and lower chords in place.) Laminated floors of this general description are not uncommonly used in RVs. Instead of the lower plywood, it would also work well if you could attach the foam to the floor of your van securely all over, which might be practical if the floor is flat rather than having corrugations or whatever.

The foam at any rate is not subject to point loads, as the plywood spreads it out, so I would not expect it to break down from vibration etc. very quickly. If the foam is unsupported on the bottom, the plywood needs to be heavy enough to carry the weight on the floor on its own, and quarter inch plywood is probably inadequate for that.

That's a great idea, JaxDad!

I would skip the insulation underneath, put down 1/2" plywood, and use some sort of cushion material on top. I think the foam will break down under weight of normal use over time. The only thing I would use under the plywood is Reflectix, if anything. Put down some replaceable foam mats in areas you will be walking to keep the floor warmer.
If I were doing it, I would use these foam squares.

I don't know how big (small?) a fridge you're looking at using but the simple solution is to just cut some large (1"+) wood dowels the thickness of the foam board and then drill holes in the foam board roughly (does not need to be exact at all) where the fridge will stand.

The dowels will act like table legs under the plywood and will support the weight of the fridge.

As Wizard mentioned, the plywood will take most of the weight.

You could do the same anywhere a constant load will be applied and areas like walkways and door ways too.

Ah, I could cut a piece of yoga mat and put it under the fridge tray!

What would you be looking for, from thicker plywood, MrWiz?

Maybe felt under the cabinet feet would be some help. I will have a tray under the fridge, probably an aluminum baking sheet. That will contribute a bit of load spreading.

I don't want to have to rebuild the floor in a couple years!

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

To clarify, I am not talking about adding insulation to an existing RV. I am talking about building from scratch, with insulation under a 1/4" plywood subfloor.

I have not tried that for under the frig...
I don't think I would consider it.
The frig needs a hard solid place to sit on.
Also, there are screws on mine that anchor the frig to the bottom plywood.
I have used the foam board to add insulation to the sides of the compartment.
I put foam board where there was room to add it. Had to work around wires.
I have not noticed any vibration.
Pat