I am looking into the required U-values for an extension I am doing. I am planning to clad the exterior with cedral fibre cement boards. These are attached to batons on the exterior of the wall allowing an air gap for natural ventilation. The cedral website gives a U value for the cedral boards (click in this case) how do I or do I need to account for the 30mm (minimum 10 mm at top and bottom ) air gap? And what value do I allocate for this if that is the case?

Cheers
C