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I plan to use 1/2" Rigid XPS covered with 1/2" plywood sheathing on the floor.
Reflectix is 5/16" thick, and would fill the low spots under the XPS perfectly.
My question is, would it be better to fill the low spots with Reflectix, or leave it open for air flow?
 

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Leave them open to dry if ever needed.
BTW on your walls switch to Polyisocyanurate not XPS
 

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I 3rd that.
The reflectix won't provide any support and the insulation value is so low, it's not worth the effort to cut it into strips, not to mention the horrible cost to R value ratio.
The best bang for buck with that stuff is to use it for window shades to reflect sunlight.
 

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Leave them open to dry if ever needed.
I think that this is an under valued concept. It is very easy to have rain come in or spill water in a vehicle. It has to go somewhere and that will be to the lowest spot.

I could be tempted to suggest just using 3/4 or 1 inch baltic birch on the floor and seal it. Maybe carve in an electric floor heater grid and throw some carpet on top that can be removed for cleaning.
 

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I plan to use 1/2" Rigid XPS covered with 1/2" plywood sheathing on the floor.
Reflectix is 5/16" thick, and would fill the low spots under the XPS perfectly.
My question is, would it be better to fill the low spots with Reflectix, or leave it open for air flow?
No to filling in the floor valleys (drainage paths).

Per All above, best not to fill it in (gravity low spot incase of water spill or moisture accumulation). Treat your plywood with a few coats of mould killing paint like Kilz.

Per RD best to use polyiso in walls & ceilings (Foil Faced)

I prefer XPS on the floor & if it matters to you “high load”. Just because it is xps does not make it all the same material characteristics. You can get specifications for xps off the DOW website for the different xps they manufacture.
 

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I think that this is an under valued concept. It is very easy to have rain come in or spill water in a vehicle. It has to go somewhere and that will be to the lowest spot.

I could be tempted to suggest just using 3/4 or 1 inch baltic birch on the floor and seal it. Maybe carve in an electric floor heater grid and throw some carpet on top that can be removed for cleaning.
@HarryN

Pretty much what I did; 3/4 BB on top of 1” high load XPS & a mat with rubber backing ($12 Costco). More info on my build thread;

62116
 

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I don't see that the Reflectex would do anything for you insulation wise as there would be no airspaces, and the XPS with half inch plywood does not need support in the floor troughs -- so, I'd say no.

Gary
Just completed floor with 3/4 xps covered with finest 1/2 Murrieta
Grade marine.ply epoxy finished so do not need finish floor
All glued together one piece solid
Nothing under xps
I considered 1/2 xps ..could.not find
3/4 was thinnest and worked fine
 

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Just completed floor with 3/4 xps covered with finest 1/2 Murrieta
Grade marine.ply epoxy finished so do not need finish floor
All glued together one piece solid
Nothing under xps
I considered 1/2 xps ..could.not find
3/4 was thinnest and worked fine
Pls excuse misspelling
1/2 floor is meranti plywood
Very expensive $90 / sheet
But no flaws
Epoxied both sides
 
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