Ah no - batts are the air gap they work by being a still air barrier - an air gap space is needed only with foil insulation as contact with the reflective surface will a) cause a thermal bridge and b) stop its emissivity which is how it works. As to the OP as others have said don't compress as that reduces the R-rating - that fluffy looseness is what creates the thermal barrier. Also it can distort sheeting and splitting it is the go - but it will reduce the R-value of course unlike what was said above - for 70mm of batt you can't get a better R-value with batts - 70mm of foam would, but the batts are cheap you said.
Originally Posted by r3nov8or
Advice from me on this forum is general and for guidance based on information given by the member posing the question. Not to be used in place of professional advice from people appropriately qualified in the relevant field. All structural work must be approved and constructed to the BCA or other relevant standards by suitably licensed persons. The person doing the work and reading my advice accepts responsibility for ensuring the work done accords with the applicable law.