Do I need DPM under insulated formed floor?

msft

We are hoping to use an insualted formed floor foundation like Viking, IsoQuick or Styrofoam systems.
I was just wondering if anybody knew if I need a DPM layer under the insualtion or is the EPS insualtion damp proof?

heinbloed

A damp proof course is strongly recommended.
This would have to be placed over the EPS, better between the layers of the EPS to protect it against puncturing.
Another method - faulty but frequently seen - is to put the DPC under the EPS, but this is problematic. It should be avoided to create a sink or pool under the EPS where water from direct ingress or caused by condensation would acumulate.

msft

Good point. Had never considered the collection of water which cannot escape thru the dpm. So if I use a 1200 gauge radon/dpm layer above the below the top layer of insualtion (100 insulation, dpm, 200 insulation), that should do the trick!
Can the radon and the dpm double up?

heinbloed

Had never considered the collection of water which cannot escape thru the dpm. So if I use a 1200 gauge radon/dpm layer above the below the top layer of insualtion (100 insulation, dpm, 200 insulation), that should do the trick!

Yes. This build-up would also very likely keep the dew-point beyond the habitated area, under the DPM. For a dew-point analysis contact your architect or builder or the like.

The DPC can act as a radon barrier, check the manufacturer's instructions.