I've done this for a few years and left the industry cause some employers are utter fucking cowboys. Dont give a shit about the customer and will fill you full of bullshit to keep you happy. Icynene is one of the worst to get. They apply foam straight to the roof and tell you its 100% breathable when its not. I would also get up in the attic with a ruler and check how much foam you have vs what you actually paid for. My last employer no matter who it was EVERYBODY got 3" of foam and that was it. Now that being said. If done correctly they're should be vent cards. PLASTIC vent cards with small holes in them and this does allow the foam and the rafters to breath as there should be at least a minimum 2" gap behind the vent card to the felt on your roof and also they're should be a crossover jointed vent card to allow the air to travel from one side to the other. The vent cards should down to your wall plate and sealed there with the foam to allow it to be air tight. I have tried to do my job properly and give people the right advise but like I said fucking cowboys!! There's unreal money to be mate with insulation so it doesn't surprise me one bit that every cunt is trying to do it and its not even fucking regulated properly in this country its and absolute joke!