DPM beneath fibreboard and laminate?

[RJH]

I'm about to lay laminate flooring in a ground floor room. Room has a suspended timber floor with cellar beneath. The existing floorboards are tongue/groove in reasonable condition. The cellar isn't bone dry, but is well ventilated.

I'm going to lay the laminate over fibreboard, as the existing floor isn't snooker table flat, but well within 3mm/1m. Should I use a polythene sheet dpm between the fibreboard and the existing floor? I can't fully get my head round whether it's better to allow an element of 'breathing' (no dpm) and risk damage to the fibreboard/laminate. Or protect the fibreboard and risk damage to the sub-floor. This thread sums it up:

So, DPM or not?!

[stuart noble]

As things stand, the moisture content of the joists will be around 20% average I would think, and the top side of the floorboards probably less than 10%. It's anyone's guess whether the laminate will be more affected by the cellar conditions or those above floor level. On balance I think I'd go with the dpm over the fibreboard, or toss a coin :-)

[RJH]

Thanks, yes, it all seems a bit imprecise! I think I'll do without and keep an eye on the floor from below.

[Brian Gaff]

There used to be some kind of stuff that allowed things to breathe without letting water through. I don't recall what it was and I understand its main drawback was that it did tend to degrade in 10 years or so, so maybe that is why one does not hear of such things any more. One assumes the Cellar is damp just because it lets some moisture in, not due to condensation? If it is the latter a non permiable sheet could make things worse. Brian

[Tim Watts]

On Sunday 29 December 2013 11:11 Brian Gaff wrote in uk.d-i-y:

That sounds like Tyvek and other breathable membranes. They are still very much in use - but I don't think they'd help for this application - you really do want to block *vapour* not liquid water.

OP: I would add the DPM. I did in a similar scenario - floor in flat over undercroft parking, not a dissimilar siutation to yours - ie damp could get up into the subfloor from underneath.

It worked, no problems.

[stuart noble]

And if it wasn't necessary, nobody knows (or cares)