Liquid DPM on concrete subfloor

[DPM confusion]

Hey guys, I have a job I'm doing that's a bit unusual, a client has an old cottage with a concrete floor, early 1900s house (I'm convinced the floor is newer, however), its about 10m2 that's having a bathroom put in, but, they want a timber suspended floor put in over the top as they want to insulate it and it just makes much more sense for the plumbing side of things. My question is, does the concrete floor which is dry with no signs of mould even behind the old kitchen cabinets need a liquid DPM, my initial thought was that it probably needs ventilation but due to the floating floor height this is pretty difficult to achieve. An outside source has said it should have a DPM put in but I'm now thinking along the lines of does it even need anything at all, it would save some mess. Any advice appreciated, cheers. 

[nod]

I’d put a pvc membrane down under the insulation Regardless of weather the floor is dry 

[markc]

If the floor will be suspended then a liquid dpm is pointless as no physical contact will be made. 
if the insulation would be sitting on the concrete then I would be doing as Nod says and just using a sheet membrane

Edited September 9, 2022 by markc

[DPM confusion]

2 minutes ago, markc said:

If the floor will be suspended then a dpm is pointless as no physical contact will be made. 

Sorry the suspended floor will be packed about 50mm which will be in contact with the floor but will be using plastic. I guess another question would be, would putting a DPM down even though it may not need it do any harm? 

[DPM confusion]

4 minutes ago, nod said:

I’d put a pvc membrane down under the insulation Regardless of weather the floor is dry 

The advice I was given was a liquid DPM to be put down on the concrete floor and then I will build the floor structure on top, it will be in contact with the floor via packers but the bottom layer as such will be a square of plastic to avoid possible transfer of moisture.

[markc]

Just now, DPM confusion said:

Sorry the suspended floor will be packed about 50mm which will be in contact with the floor but will be using plastic. I guess another question would be, would putting a DPM down even though it may not need it do any harm? 

I was in middle of editing my post as it was clearer in my head than on page, if packing then definitely put down a sheet membrane as it’s cheap, easy and peace of mind. Liquid is over the top and messy

[DPM confusion]

10 minutes ago, markc said:

I was in middle of editing my post as it was clearer in my head than on page, if packing then definitely put down a sheet membrane as it’s cheap, easy and peace of mind. Liquid is over the top and messy

Cheers, you mean a sheet of DPM loosely laid on the concrete and frame it all out as planned? 

[markc]

5 minutes ago, DPM confusion said:

Cheers, you mean a sheet of DPM loosely laid on the concrete and frame it all out as planned? 

Yes, visqueen or similar sheet material