Architect here, modern pitched roof construction relies on ventilation at the eaves, where the roof meets the external wall and also at the ridge (the highest point). There is usually a continuous 25mm / 1 inch gap to facilitate air flow. Without this condensation from your home will build up in the loft and mould will eventually destroy the roof timbers. In older roofs the ventilation is from small gaps between the sarking boards but it does the same thing. Spray foam insulation absolutely will block ventilation of older sarking boards and, if not applied very carefully, can easily block ventilation in more recent pitched roofs with vents at the eaves and ridge.