I have an unventilated cathedral ceiling, insulated with 7 cms of rigid foam. I have tested it and found a 1" airspace between the top of the foam and the backs of the slates. Would it be OK to blow air in at the ridge and force it out at the (internal) eaves, to circulate any entrapped heat ?

I have a good mechanical ventilation system, but I still don't want to risk introducing humidity into the ceiling !

I would then install a siphon near the ridge, and pump the hot air down to the basement.

Grateful for any professional wiews on this !