Glueing down solid wood floor onto another wood floor!

[The Surgeon]

For a (sort of) barn conversion I'm working on in France, I've bought some lovely T&G merbau to go into the bathroom. This room is on the first floor and so had floorboards down. These boards were in quite a poor state so they've been treated and then covered with chipboard. The store in France where I bought the new merbau flooring said that it was suitable for use in a bathroom (they are quite thickly lacquered) and should be glued down.
I guess I should put down some exterior or marine grade ply either over or instead of the chipboard before laying the merbau.
When glueing the floor down, though, how is this done? What concerns me is that with all the tapping into place of the T&G won't this break the glue bonds of the merbau onto the base? Am I better off nailing them down (invisibly of course)? The store in France sold glue that it said was suitable for use in bathrooms but this was very expensive - must I use this if I stick (arf arf) to glue?
Also, I'm think of putting underfloor electric heating under it as well. Anything to be aware of with this?
Many thanks!

 

[WoodYouLike]

The combination of solid wood and underfloor heating isn't recommended, but if you do (with narrow boards) it's best to glue them down.
And indeed, proper adhesive (flexible) is expensive, but that's quality for you

 

[The Surgeon]

Yes they are narrow boards (well around 3-4 inches from memory).

So presumably this flexible adhesive is flexible enough to withstand being bashed about as the new boards are laid?

 

[WoodYouLike]

All (proper) parquet adhesive is 'flexible' enough to allow 'bashing' about, the flexible one recommended is especially for moist rooms and for use on underfloor heating.

 

[The Surgeon]

Great - thanks very much for your helpful replies.

Final point(s) - no comment was made on putting down ply first. Presumably this is the right thing to do?
I guess I should allow an expansion gap around the edges of the room?

 

[WoodYouLike]

Yes, but also be aware that ply doesn't 'glue' that well with the adhesive as chipboard would (but in your circumstances ply is preferred). Allow for longer bonding time.