Return block window/door opes

[JackofAll]

Hi,

Noticed on the latest draft of construction drawings that they have returned the block 200mm leaving 100mm gap between inner and outer leaf. Have not seen this detail on any wide cavities that I've seen online, seem to have box frames closing cavity instead or those pvc closers. Before I go back to engineer, is the return block OK to run with?

[Dave Jones]

cavity closer.

[JackofAll]

1 hour ago, Dave Jones said:

cavity closer.

As in cavalok or the likes and why are they better.