polystyrene under chipboard flooring

I think you would be better of with sheet material layed down(chipboard) rather than T&G but you will still have to knock in any protruding nails/screws.

-- Sir Benjamin Middlethwaite

Knock the nails in. Look at the sort of underboard solutions there are for laminated floors. Decide what you want the material for - the polystyrene may make most sense for insulation and be too expensive just to make it more even.

I think you would be better of with sheet material layed down(chipboard)

Actually, as long as they protrude less than the polystyrene is thick, they'll just punch into it, with no impact on the chipboard.

It may be better to remove the floorboards before laying the chipboard.

Christian.

This is how the ground floor of a mate's house is constructed, but over concrete rather than floorboards. So there's no reaction between the poly and chipboard. I doubt it will help much with sound insulation to you, but might help the transmission of footsteps downwards. It should help with heat insulation, though.

That's what he's using

Why? They'll just dig into the EPS.