Hi. I have foam from just Foamking. It works really well. It's not flammable and it doesn't contain any toxic agents like sealant foam does. You don't need a ventilation gap because the foam doesn't absorb moisture and functions as a "closed system." For example, in our home, it's foamed from the floor up to the ridge and down to the floor again.
We have made a small ventilation gap, just so that if we need to replace the outer roof, the foam doesn't have to come off with it.