[offering to spray closed-cell (polyurethane?) foam between my roof rafters.]

The roof 'draughts' I call ventilation, and are needed to dry out condensation etc up there.

This summer I'm finishing taking all the slates off my roof, replacing battens and Tyvek, and putting them back on. If they were sprayed with foam this wouldbe impossible and all the slates or tiles would have to be thrown away.

George