How to insulate an old brick wall without sheeting or sidings?

I have a house built in 1960 with a un-heated garage. The 3 of the garage exterior walls are in brick attached to 2x4 structure with nails. So in order it looks like the following: bricks, air gap, 2x4, interior wall. I would like to insulate that wall from the interior because right now it is all stripped. I would like to keep the air gap between bricks and 2x4. My first idea was to seal all this with polyurethane but this would seal the bricks leaving no air circulation which I think is bad.

What are my other options ?

Thank you