Through to centre and spray water in to damped. Then foam. It will pack the gap between joist and boards.Cracks on edge of plasterboard on joist.
I was going to use expanding foam. Do I drill 8mm holes through the plasterboard at the edge of the joist ?
Best way to use expanded foam is to spray some on hand, lick fingers and push it into holes. to clean hands rub through your hair. PS post phots, love to see finished work.
That reminds me of the bird that (allegedly) had concrete put in her arse.