Cowboy job on extension?

[ivixor]

You'll never manage to get a telescopic air vent in there now. Assuming there are others correctly installed, i'd probably bodge the bodge and foam that lot up completely.

 

[Leofric]

Might have missed something but , as I asked earlier ,how high is your finished ground floor level above the external ground level ?
With a new build suspended timber ground floor the normal construction would be to have top of oversite concrete at around external ground level ,then with honeycomb sleeper walls ,dpc ,timber plate ,100mm deep joists , chipboard floor finish, the finished floor level can be about 375mm above ground level . Air bricks would be 150mm above ground level on the dpc and below the floor joists so insulation between the joists doesn't obstruct cross ventilation. In that situation telescopic vents are not required.