You used Platon, XPS, and OSB??
Platon is suppossed to be a moisture barrier, so, you'de need to seal the edges with something designed for the same.
Silicone caulk maybe?

The only way I've ever done anything like this over a slab was simply one layer of 3/4" marine grade plywood. Never had a dampness problem or a smelly problem, but, the moisture barrier was UNDER the concrete.
And, These were on grade. No basements.

Unfortunately, putting together two excellent basement subfloor treatments doesn't mean you've got twice as good a system.

Platon membrane and 3/4" of XPS (extruded polystyrene) are each semi-permeable to the movement of water vapour. Platon is rated 3.3, 3/4" XPS somewhat less than the 1 rating of 1" XPS. Each one of them alone permits the controlled movement of water vapour into the basement space, so it doesn't stay trapped under the Platon or the XPS and condense into a liquid, i.e., water.

The relatively high permeability of the Platon explains, I think, why the manufacturer doesn't advise sealing the wall to membrane intersection. The membrane is designed to allow vapour to permeate through the membrane at a relatively high rate. By contrast, Delta FL has a perm rating of 0.3, allowing for a much slower movement of water vapour into the living area and I think the point of the sealing is to keep the permeation rate uniform.

What may be happening in your situation is that vapour permeating through the 3.3 Perm Platon is then trapped by the lower perm XPS. Or perhaps the combined Perm rating of the two materials is creating too high a Perm to allow sufficient moisture through, so it's accumulating and feeding mould growth under the Platon.

Since I've never come across such a combination before, maybe you should contact the source of this idea and ask them what they were thinking.

Thanks for both of the replies. Worthy, your ideas about why this may be happening is really appreciated, though I was never given the impression by the platon manufacturer that there was any vapor escaping THROUGH the membrane. In fact, one of their big selling points is that you can choose ANY floor covering you want, carpet, vinyl, hardwood, etc. They do not seem to think that any vapor will need to escape from under the platon. This is why we thought we were able to raise the R value of our slab with a small layer of XPS. Building Science has also endorsed this idea on their figure 3 diagram found in their 'concrete floor problems' publication. And if there is any company I am likely to trust, its Building Science. I have tried to contact both Building Science and Certainteed (the makers of platon) to no avail.
Thanks again for all your thoughts.

I'd like to somewhat EAT CROW. (recant my prev statement of no problems).
As coincidence would have it, I had carpet laid in my latest remodel. They finished it last Tuesday. Whole house. With ONE EXCEPTION.
All the crap that was all over the house has been stored in a sun room. This one room is concrete slab, and has one layer of 3/4" ply.
This room has been "unconditioned" for 3yrs, nothing but the old carpet padding on the floor. Piled ceiling high with...stuff. Sealed off from the rest of the house, unconditioned, humid environment, vast temperature swings, and...the walls are nothing but jalousy glass.
Emptied the last of the stuff out of this Sunroom last night so the carpet guys could come finish the house.
I found one warped/bubbled at the edge 3/4" piece of plywood. I went ahead and pulled it up. Smelled musty underneath.
There was a layer of tar paper in between ply and concrete.
No mold growth bad enough to justify removing anything else. I fully expected it to be worse considering what I'd found.

I think I found the cause though. This piece of ply is against the backside of a chimney. Appears water has been leaking around the chimney, running down the chimney, and then out underneath the plywood floor. Not a river, but, enough to cause this piece to warp.

I can't complain too much though, that ply has been there almost 40yrs.
Given the floor heights in between this room, and the rest of the house, I really have no choice but to replace it with what was there, and in the same manner. (correct? anybody?)
AND..FIX THE LEAK!
(seems it's difficult to find a roofers now days that know how to "flash" a chimney with something that doesn't come in a tube...lol)

The Perm Rating as mentioned for Platon above as listed of 3.3 I believe is incorrect. It is an Armtec product and they state the perm rating fpr Platon as .05 in their specs. Both Platon and Delta FL are made of the same material HDPE, so I believe that they both have a perm of less than 1.

Correct or incorrect the thread is 4 years old.

4 years old, 10 years old, 20 years old, someone may find the correct information useful, the reason the information is retained over time is online is for reference purposes, if you did not know that, now you do