Solid Wood Floor Underlay

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I'm laying an 18mm solid oak floor ontop of old floorboards on the 1st floor of my home. Do I still need an underlay like is used with engineering boards? If so, what thickness of underlay is preferable?
 
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Depends on your installation method and on the direction of your new boards.
 
I will probably be nailing the floor to the old floor boards. How do the direction of the boards come into play when deciding upon the need to an underlay.
 
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.
 
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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.

i.e. If the case is that I'll be installing the new boards in the opposite direction (i.e. 90 degrees) to the old boards. Does it make a difference if they're nailed or not in this case?
 
If nailed you don't need underlayment, if installed floating you need foam underlayment (minimum 3mm) without any DPM.
 
If nailed you don't need underlayment, if installed floating you need foam underlayment (minimum 3mm) without any DPM.

So let me see if I got this right:

a1 ) If new boards are NAILED in the same direction as the old boards, underlayment is needed in the form of 6mm ply.

a2 ) If new boards are FLOATED in the same direction as the old boards, underlayment is needed in the form of 6mm ply & 3mm foam underlayment.

b1 ) If new boards are NAILED at 90 degrees to the old boards, underlayment is unecessary.

b2 ) If new boards are FLOATED at 90 degrees to the old boards, underlayment is necessary in the form of 3mm foam.

Apologies for the confusion...

EDIT: Edited errors.
 
So let me see if I got this right:
Yes, you got it right except

a2 ) If new boards are FLOATED in the same direction as the old boards, underlayment is needed in the form of 6mm ply.
plus 3mm foam underlayment as sound-insulation

b2 ) If new boards are FLOATED at 90 degrees to the old boards, underlayment is unecessary in the form of 3mm foam.
Mistype here I think: Necessary instead of unnecessary
 
a2 ) If new boards are FLOATED in the same direction as the old boards, underlayment is needed in the form of 6mm ply.
plus 3mm foam underlayment as sound-insulation

Why is sound-insulation unecessary when the new boards are NAILED in the same direction as the old boards? But is necessary when the new boards are FLOATED in the same direction as the old boards?
 
If you nail the boards (in which ever direction) tightly to other boards what do you think will happen to the 3mm foam?
 
Well it will be compressed... But is it not better to have it (for insulation purposes) compressed than not at all?

Or may this cause the boards to 'bow' out in the middle where the foam isn't as compressed as at the edges where the nails are pushing the boards into the floor?

Finally, I know this is a 'up to you' type question. But would you personally nail the boards to the floor or float them?
 
It will be compressed and loose it's insulation character - so waste of money, materials and time.

We install 85% of all over floors with the floating method - but like you say: it's up to you :)
 
Why only 85%? What's the deciding factor for nailing the other 15%? What stops the floor from 'moving around' if it is not secured to the ground?

Also, how is the foam insulation attached to the floor?
 
The other 15% are either nailed directly onto joists - no floating there - or glued down when underfloor heating is involved and when we install a design parquet floor.

The weight of the floor stops it moving around. And to be honest, when a wooden floors wants to move due to changes in humidity no matter which installation method you use it moves!

Foam lays loose on the underfloor.
 
One final question (sorry!). The floor we purchased was from an elderly chap who's had it lying in his shed (still as new directly from the supplier) for around 1 year. It's unfinshed and will require a sand then laquer/oiling.

Is 2 days enough for it to adjust to our home prior to laying it?
 

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