Sealing windows into timber frame

[Arthur]

Timber frame is up. Just been advised of significant delay end Sept for my window frames 🙁   So my dilemma is should I also delay the brick work (once the roof is on in a couple of weeks) until the frames arrive or press ahead with the brick work? 

 

WIndows installer has advised not to start work on bricks. For when the window is installed it needs to be sealed to timber from inside of the cavity as well as to timber on from room side and to brickwork on the outside to maximise air tightness.  

 

Apparently sealing to the window frame to the timber frame  from inside the cavity is very difficult for the installer once the walls are in place.

 

My builder however has proposed building some temporary mimic frames to build the brickwork against so build is not delayed.
 

Just concerned that I might not be able to seal the frames to the timber from inside the cavity once the walls are in place when they replace these mimics later.  Is this impossible though? Is there a technique or tool that I  could recommend the windows installers considers to achieve this 3rd seal even with the bricks in place? 

 

Has anyone has experience of adding the windows after the bricks are up and still achieving good air tightness? 

 

The financial impact of having to wait for months for the windows to arrive, delay trades, delay the build and risk damage to the external waterproof membrane does not sit well. Nor does the prospect of having to pay for the mimic frames to be built and installed temporarily!!