15 hours ago, Johnny Jekyll said:

May I ask how you properly seal the cavity at the top?

 

You can use a fireproof barrier like Rockwool cavity sock.  Stuff it in before you get the wall plate and trusses on.  Get your roof insulation to meet with it so you don't have any gaps around the eaves.

Quality of workmanship it a problem in building. But as said by someone before. Your concern is thermal by pass, where warm air escapes around your fancy insulation via voids to a cold void. So long as the boards are tight fitting (doubtful) there should be no issue. The best you can do is to make sure the cavity walls are effective sealed at all openings and tops and services to limit the movement of warm air to cold spaces. Obviously thermal dry lining will improve what ever u value you would have had without it.

My house has the same insulation, my was tight in the cavity. But I am still worried about whether it could be sitting on wall ties and  not pushed down together horizontally Although I did stand on the top of the insulation to force it down before the roof timbers went on. (All to late now, as at first fix stage). I have happened seal the inside of the cavity see photo taken at time buy screwing it  too the wall plate at the top. 

Very helpful photo thanks farm boy. Really appreciate it.

What are those yellow clips you're using to help screw to the wall plate? If you don't mind, where did you get them?

Thanks.