Airflow on the warm side of flat roof?

[thesheep]

We have a flat roof, which is over a single storey extension at the back of our house, and is currently covered in felt with no insulation.

Local roofing company has suggested removing the boards, then adding an extra layer of counter-battens to provide airflow. Then place 18mm board, then adding 120mm of insulation board, then putting new felt on top.

There is no room to add insulation on the inside, between the rafters.

Right now I’m just trying to understand why it might be a good idea to have my airflow on the warm side of the insulation on a warm roof. Just seemed weird to me because I know that on our pitched roof the air gap is on the cold side of the insulation. The roofing company assures me this is the right thing to do to prevent condensation, and he has been advised to do this by the manufacturer of the materials he uses. But I want to make sure I’m not unnecessarily losing a lot of heat through the airflow.

Does it make sense?

 

[catlad]

doesn't make any sense to me either.

 

[chappers]

nor me, make up should be; roof joists, firing pieces, roof deck, VCL, insulation, secondary deck(optional depending on final covering and insulation used) and then final weathering cover . VCL should be turned back up over the insulation and back down over the gaps at joist ends which should also be packed with the same insulation at the perimeter.
Having a gap ventilated to the outside would introduce a layer of cold air between the deck and the insulation which would encourage condensation on the underside of the roof deck.
I would ask to see evidence of what he is saying and probably be finding another roofer.

 

[thesheep]

OK thanks for that.

I've been studying the installation guidance from Kingspan and a couple of other manufacturers of flat roof insulation, and I don't see any mention there of a need for ventilation on the warm side either. They seem to advise more like what chappers says.