Insulation-cold bridging

[johncf]

I have had built a Garden Room and am finishing the interior myself
It has conventional pitched slate roof and eaves to floor uPVC double glazing.
The roof is supported on a steel ring beam(150x75x18mm) which is totally within the building envelope.
The ring beam is supported on 6 steel posts(70x70x5mm SHS)These are also within the building except
where they go thro' the insulated concrete floor to a depth of 300mm.
I am going to clad the steelwork with 25mm thick sawn oak. 3 sides on the posts and inner and lower faces on the beam.
Will the oak be suitable insulation against cold bridging or will I have to use thin insulation under the oak or
perhaps a damp proop membrane

[welsh brickie]

drill the posts with a 10mm bit and fill with expanding foam,the foam is waterproof and will expand in the void preventing cold bridging and paint the steel internally with buitumin to stop any condensation occuring on the steel.