If you want to install your new boards in the same direction of the of the old boards any defects in your existing boards (like cupped, uneven) will have an effect on your end-result.
If you install in the same direction you first have to install plywood 6mm at least. That's a kind of 'underlayment'
If this is not the case and you want to secretly nail the new boards any type of underlayment is a waste of time, money and effort.
If nailed you don't need underlayment, if installed floating you need foam underlayment (minimum 3mm) without any DPM.
Yes, you got it right exceptSo let me see if I got this right:
plus 3mm foam underlayment as sound-insulationa2 ) If new boards are FLOATED in the same direction as the old boards, underlayment is needed in the form of 6mm ply.
Mistype here I think: Necessary instead of unnecessaryb2 ) If new boards are FLOATED at 90 degrees to the old boards, underlayment is unecessary in the form of 3mm foam.
plus 3mm foam underlayment as sound-insulationa2 ) If new boards are FLOATED in the same direction as the old boards, underlayment is needed in the form of 6mm ply.
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