Insulating with natural cork a 70's house wall air gap was a challenging project saving thousands of Euro's. Our contractor suggested rebuilding all the interior walls!
Step 1: Get the Kork
Unless you have a cork tree, buy it from the spanish or portuguese dealers or collect wine bottle cap's
Step 2: How to Fill the Air Gap
You may drill 10cm holes in the wall and fill 'by hand' or use an air blower with some PVC pipes (10cm diameter)...
The key issue when using an air blower is the 'venturi' effect that helps the cork bits to be blown into the wall thanks to the main air stream.
Step 4: YEEESSS!!!!!
Insulation improved confort and heating need reducing thermal conductivity down to 1/2... so CO2 not generated!!! It also improved the house acoustic isolation
In the attached infrared image you can appreciate 10cm against 14cm insulation... the lower the temperature, the fewer the looses.