Ok major cock up on my part, I have installed a 3/4", pre finished oak t&g floor over my old floor boards, which sit over a crawl space which is well vented by air bricks and not what id consider to be very damp.
Anyway the floor way layed last december, yes in a rush for xmas and not done as well as it could have been to say the least....
I face screwed round the edges and only blind nailed every 7 rows, as you can imagine the floor is raising in places, spongy and starting to twist in places.
I have accepted this needs to be re-laid and nailed every row, I was going to also put a sub floor down of 1/4 exterior ply to ensure level along with a vapour barrier between the existing floor and ply.
Would is also be advisable to place building paper on top of the ply?
Does all this sound the correct way of installing?
Also wont it be a problem if the nails puncture the vapour barrier and building paper?
Finally can anyone advise what most likely caused the problem, my poor instalation or maybe moisture?

poor instalation , Did you run the new wood in the same direction as the old ?
i would use at least 1/2 " ply .. Blind nail 2 " in from the ends and every 8 to 10 " down the board .. You can use felt , or aquabar B for the moisture barier
the crawl space needs to be covered with a 6 mil polly .. dont know why you are screwing the last boards .. you can glue the last two rows with some pl 400 and face nail the last on with a 15 ga finish nail .. i am sure there are other pros on here that will chime in and help you further ..

Thanks for that, so just normal roofing felt and ok if the blind nail pierces it?
And yes I have ran the boards the same way as the old ones, I presume this is part of the problem?

That and not nailing every board are a few .. good luck ..

Thanks again.
I presume if I lay min 12mm ply, runing the boards the same way as the existing ones is ok though?
Also will the nails piercing the buliders paper or felt be ok?

if 12 mm equals 1/2 ' you should be fine .. also the moisture in the wood should be no more then 2 to 4 % difference in the sub floor .. ideal Rh aroud 55% .. do the crawl space first .. 6 mil = retarder , 15 mil = barrier ..