First of all, I need to state that I usually have no issue regarding spray foam insulation. It definitely has advantages and disadvantages as every insulation material.
I´m just concerned about statements of some spray foam companies and manufacturers regarding the airtightness of their product.
The photo above shows a pitched roof area where spray foam has been installed in between the rafters and insulated boards were screwed on top. A membrane wasn´t used, because of the "airtight statement" coming from the spray foam installer. I do agree that spray foam might can be airtight as long as it will not processed afterwards. But mostly all installers have to trim the spray foam because it´s impossible to create a smooth insulation layer without overhang just by spraying. Unfortunately this trimming work weakens the cell structure of the foam and can create cracking between the rafter and the spray foam filling.
That´s the main reason why I believe, that spray foam insulation, without using a membrane on the inside, is not permanent airtight.
And there´s still another important fact for using a membrane in addition to the spray foam insulation, which is called vapour control. Without having a membrane on the warm side of the roof installed, you will gamble with the risk of condensation in your insulated structure each winter.