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2015 Promaster 3500 159 Ext gas silver
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Hope this is not a repeat of what others have done but I believe it is somewhat different. I just finished installing the floor in my camper conversion and it does not use ribs as seems to be the usual route. I wanted to have fully insulated floor decking which hopefully will provide both a reduction in heat transfer but also less transmission of vibration and road noise. What makes my system possible is the full width tongue & groove joints on the flooring.

First I installed plywood strips between the floor corrugations as most others have done (nominally 1/4" but actually about 5 mm). I bonded these to the floor using polyurethane construction adhesive (PL Premium). By buying it at Lowes in twelve tube quantity it comes to $5.65 a tube for the 28 oz size with my Lowes CC discount. The strips were straight enough that they generally stayed in place without weights. The adhesive expands somewhat upon drying and a heavy bead can fill a considerable gap.

Next I installed the 1" polyiso foam using at least one heavy bead of adhesive on each strip and corrugation, trying to get maximum bond area for the foam. I held the foam down while drying with pieces of 3/4" OSB and buckets of sand (about 70 lbs each). I left gaps around the edges and my runs of 3/8" radiator hose & 2/0 cable for house battery charging. I then used poly detail foam in these gaps before proceeding with the Advantech OSB 3/4" decking.

I used the OSB rather than plywood because it is very flat and stable plus it has T&G edges on the long dimension. I ran the decking in full length pieces side to side so it took a total of four pieces for my LWB Ext van. I started from the front, again leaving about a 1/2" gap at the ends. Using a heavy application of adhesive, I worked to the rear letting each piece dry overnight with the sand buckets holding it down (total of ten buckets). After running a bead of adhesive in the groove (of the T&G) I laid the panel down (with an assistant) and with the help of a block and sledge hammer engaged the T&G joint. This is where you really hope that joint will go together.

The finished floor feels very solid and so far I have no metal fasteners securing it, just adhesive. The foam is rated at about 15 psi so a square foot of the stiff decking should be good for over 700 lbs of load.
 

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Nice insulated floor!
The polyisocyranuate compressive strength can vary from 16-25 psi. The product I bought was 25. I floated mine on the top of the ribs without filling them and it has been fine for our walking on it on top of the the 1/2 inch floating plywood it supports. I know because afer 9 months of using it I pulled out the conversion and you couldn't even see the imprint of the ribs on the bottom of the insualtion. You cannot use a heal or kneel on the foam itself or it does show the imprint.
 

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Nice insulated floor!
The polyisocyranuate compressive strength can vary from 16-25 psi. The product I bought was 25. I floated mine on the top of the ribs without filling them and it has been fine for our walking on it on top of the the 1/2 inch floating plywood it supports. I know because afer 9 months of using it I pulled out the conversion and you couldn't even see the imprint of the ribs on the bottom of the insualtion. You cannot use a heal or kneel on the foam itself or it does show the imprint.
Thanks for that info, I'm thinking over options for my floor and have xps, polyiso, and Rubber membrane. First option is xps on the bare ribs, a layer of wood particle, then polyiso , and then a plywood floor until I figure on laminate or cork .... Thinks a spray adhesive should hold it for this year , 3m 77 or 98.
Now back to the wall panels.
 
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