Thermal resistance of air gap for roof structure

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Hi Fellas! Currently, I need to simulate the shop building with copper roof construction. The layering of roof construction shows a 20 mm thickness of the air gap right under the copper tile and above the bitumen layer. The air gap is not fully enclosed with the roof but divided by bamboo structure. I notice that there will be air leakage from copper tile to the air gap. Should I define the 20 mm air gap and involve it in the roof construction? What is the best way to define the thermal resistance for that kind of air gap? This illustration of the roof cross section below explains clearer. Thank you.