Insulation around a window revieal - large airgap to bridge.

[bobbyb]

Hello everyone 

 

I am in the middle of a Diy garage conversion - it's not a super high-end conversion, we just want a nice insulated box to use as a workshop/office - The total cost is going to be around 3k, so not high-end in the slightest

 

I've erected stud work all around and on one wall the air gap between the stud work and the single brick wall is pretty large - it's like this wall has some larger brick supports that protrude out to around 100mm into the room. This means my stud work is approx 125mm away from the windows...and that's a pretty big gap

 

My issue really is how best to do those - I could obviously just fill the gap with (insulated) plasterboard, but I do worry about the size that its got to cover and worry about how little support it might have. But perhaps im overthinking it?

Anyway, I've attached a diagram of how I might plan todo this, and I just wanted to do to hear some thoughts really. 

 

I