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Hi all - 
My tentative plan for installing my floor is as follows:

1. van body
2. silicone vapor barrier
3. FloorMuffler underlayment (polypropylene foam)
4. Reflectix
5. 1/2" or 3/4" marine ply subfloor
6. sheet vinyl 

I already have all the materials except for the plywood.  I know the underlayment is supposed to go between the plywood and the flooring, but I read Bob's guide to flooring and apparently Reflectix will not work very well if it's right up against the van body.  My thinking is, if I have the vapor barrier and underlayment between the Reflectix and the metal, it'll give it enough of a space that the Reflectix will work well and I don't need to do any extra insulation.  I know the underlayment is more for floating floors, but I figured the moisture prevention and sound barrier will make it worthwhile.

Does something else need to go in between the plywood and the sheet vinyl?  Or can I just glue it down?  

Thanks in advance, any help is much appreciated.
That won't insulate effectively. Maybe not such a big deal if you're only worried about heating in the cold, but for warm climates I would go for a bare minimum of 1/2" XPS.

Plywood's darn heavy, do you really need that strength?
The vapour barrier, IMO, is not only a waste of time and money but will trap any moisture between the van floor and the barrier thereby increasing the  chances of rust, not prevent it.

Reflectix is a radiant heat barrier with very low actual R factor even when applied correctly. To install correctly and have it be effective as a radiant heat barrier it requires an air gap not just distance from the metal. To install it directly above other solid materials will render it ineffective as a radiant heat barrier and the thermal insulation value is so low anyways that you wouldn't notice any benefit from it.

Using Marine grade plywood is also unnecessary IMO unless you're buying it cheaper than G1S plywood. You're protecting that plywood from spills and moisture by installing vinyl over top anyways.

If you really require insulation from the cold - because you'll be living in relatively cold environments full-time then consider laying down polyiso on the floor between the van floor and the plywood. There is a difference of opinion as to whether to lay the plywood directly over top of the polyiso or to build a frame work in which the polyiso is place such that the plywood doesn't rest on the insulation but rather is supported by the framework...your choice.

I laid 3/4" G1S plywood directly over the van floor. The only time my floor is cold is when it's below 40F like first thing in the morning. Warm socks and/or shoes take care of that. It can also be uncomfortably warm for about an hour after the engine is shut off due to residual heat from the exhaust system and when I park it on asphalt in really hot weather.

I just checked on the product spec sheet for the FloorMuffler. It's intended for indoor use for under various laminate and ceramic flooring. Not sure if it's of any value as a sound deadener in automotive uses.

The thermal rating is quoted as something I'm not familiar with - 3 ft²-F-hr/BTU-in . Someone else with more thermal resistance knowledge than I will have to interpret it for you but at only a 2mm thickness, I can pretty much guarantee that it's not a whole lot.
Thanks for the advice!  I like polyiso, if I were to lay the plywood directly over the polyiso without building a framework, would I need to screw it down?  Or would adhesive do the job?

Do you think I need something between the sheet vinyl and the plywood, or is it ok to just glue it down?
I didn't lay in polyiso, it's impossible to obtain it in less than 2" thickness in Ontario because no one needs that low a R value product.

Some people bolt the plywood down through the floor. I personally think that anything that goes through the floor leaves room for moisture penetration even if you're careful and use something like butyl tape to seal the hole.

I laid 3/4" plywood straight on to the floor. My plywood layout was done with a view to having all the seams under various pieces of cabinetry in such a fashion that the plywood can't move. The first piece of plywood was a full sheet that runs from the front passenger side. That left 2' x 8 down the drivers side and a 2' by xx across the back. The kitchen cabinets and the bed uprights span the seams and all of the cabinetry is screwed in to the plywood forming one continuous and large object. It was also pressure fitted in place along both sides of the van  (as in I had to hammer it in place Big Grin ). Simply put, it ain't moving.

As to how to lay the vinyl, I haven't done that in years. Peel and stick tiles are so much easier for s&B applications and for the van I used vinyl laminate flooring. I'd be hesitant to start putting down glue on vinyl unless you're very experienced at laying the stuff. Cutting in and around all the notches in a van with rolled flooring can be a PITA from what I remember when a couple of us tried to use it years ago in the show vans.
My van bed has recessed 'groves' ? front to back. I used 1x2 glued into the 'groves' ? that are over the frame member under the deck. I then cut and fit 1/2 inch foam faced poly insulating board between the 1x2 strips and the strips and sides. Then cut and fit 3/4 plywood screwed to the 1x2 strips. My furniture and other build ins are then screwed to the plywood. The one mistake I would do over. I should have had large battery cables from left side to right side under the floor. I'm thinking 2/0 red and black. Such a pain to run over head in a window van. I did coat the 1x2 strips and metal floor with brushed on Rustolium grade paint.
GREAT info from Almost There. Too much misinformation and misconception floating around.

The question as to a flooring framework or not when laying ply over any type of foam is if the repeated flex will break down the insulation. My consideration for this is van height. If you have a high top and are truly walking around I would opt for a frame. In a low top, you spend less time moving around in the van and are often just scooting so it's probably not needed. Either way, the thicker the ply the less flex. That said, I think 3/4" is overboard for just about any van build short of under-floor storage setups.

If you want to add some radiant barrier without dealing with thickness issues you can laminate the underside of the ply with mylar. You gain radiant barrier from the foam side which probably helps more with heat than cold but it is cheap, easy, and adds only a human hairs worth of thickness. 3M spray adhesive work fine for the laminating.
Mylar and reflectix only reduce infrared radiation transfer, no insulation at all.

If the foam lies flush and flat, no flexing, it has good compression resistance, no framing needed. A heavy thick floor is heavy, better IMO to bolt tie-downs through the steel (rivnuts) and treat the floor covering as that, no need for it to be structural.
While building out and modifying, I had many needs to change things and re-fasten to deck. I am glad I did not have to have all those holes in the metal floor. And it is sometimes difficult to align all those rivnuts with the needed anchoring points.