underfloor insulation

[gary67]

Will the water reach the insulation? If it doesn't in your situation I would probably go with the wool as at least the damp can drain out of it which it can't do with polystyrene

[plod]

I recommend fixing the water running under the house problem. Divert it, drain it, whatever is the issue that it gets there in the first place.
Hot air rises, putting plastic, polystyrene or whatever under your floor is a silly idea, and yes it does hide the pipes and stuff, a right PITA to have your plumber get at things, and yes, will cause water to puddle.

Solve the problem, not the symptom.

Doesn't go under floor it,as on the ground. The plumbers I know love because it's nice and clean in the crawl space

[BBCmicro]

Have to agree with PCtech about fixing water problems. We got our present house very cheap and I think dampness was one of the reasons. After buying I found that a downpipe was cracked and sending water under the house

Also agree with others that polythene on the ground + good underfloor ventilation is the way to go. In addition we have foil-backed woollen batts between the joists as underfloor insulation (and some polystyrene, which is hard to install so I changed to the wool)

[tweak'e]

Mould created from moisture pooling on the polythene especially round the edges of the house can and has rotted the ends of joists, plates etc.

In one place I fixed they had a water leak which they didn't know about for ages, this was in Canada (no water meters) the water couldn't escape into the ground.

interesting.
was polythene the actual cause tho? i've been under plenty of homes with standing water under and none of them where rotten. (not much fun running cables under those ones)
being rotten around ends of joists and bottom plate i would have suspected cladding issues.

but pooling of water is a good point.

[notechyet]

Mould created from moisture pooling on the polythene especially round the edges of the house can and has rotted the ends of joists, plates etc.

In one place I fixed they had a water leak which they didn't know about for ages, this was in Canada (no water meters) the water couldn't escape into the ground.

Gary, the problem seems not to be a typical one for most places. As said in other posts, keep water away, certainly not letting running under any house.
Again, keep rising moisture away with the DPM and venting.