The reason this part is optional is that some fitters prefer to save money and find other solutions (e.g. make their own out of 7mm Hardiebacker concrete board).
Stove Fitter's Warehouse offer a full Twin Wall Design Service with all designs prepared within our Twin Wall Design Studio. Contact us for further details (firstname.lastname@example.org).
Duraflue Twin Wall was launched to the market during 2014 and, due to its high quality and innovative design, is already a favourite amongst professional stove fitters.