What is the best way to close a cavity at the wallplate level in a concrete build? It seems bad to just put a block on the flat.
I know special insulating closers can be used around windows and doors.
Bed in a course of plain fibre cement slates (like an undercloak) then lay your block on the flat. The slates will prevent mortar on blockwork joints from falling into the cavity.
No, the block closer performs a number of functions:
1. Closes off the cavity to prevent ingress of moisture, vermin etc.
2. Reduces heat losses and helps prevent "thermal cycling" around the insulation.
3. Forms a barrier to the spread of fire, this is a requirement of building regulations.
4. Ties both leaves of blockwork together structurally and helps to spread the roof load to both leaves, internal and external. A cavity wall would be considerable weaker if it were not closed at the top.
Why don't you want to close the cavity with a block?