dpm foam underlay or fibre board?

[shardy]

Hi all,
We are having a laminate floor fitted in a new loft conversion bedroom. There is only 18 mm clearance between french doors and green chipboard T+G floor so we were quite limited in our choice of flooring.
I've ordered duralay gold silentfloor underlay 4.2 mm thick, as I read good reviews about it and it is supposedly very quiet.
For the laminate I ordered Quickstep IM flooring which is 8 mm thick and supposedly water proof. If the french doors or Velux are open and there is a sudden downpour, the floor can get quite wet.

The fitter thinks the fibre board would be a better choice of underlay as there will be less movement, but I was told the fire board with laminate can create a clacking noise when you walk on it. I wondered if we should have ordered fibre board?
also the foam underlay has a dpm. Is this a good idea in a loft bedroom?
Some say it is a good choice, others say it could trap moisture in the floor and cause problems?

thanks in advance for any advice

 

[chappers]

All of that, Duralay is my preferred underlay too. If the floor is in any way uneven you are correct fibre board can move as you step on the floor, imagine a slight see sawing effect. Don't see why you should have any moisture problems there, any excess moisture in the air would be more likely to condense on the underside of the subfloor.