The 25mm air gap is "essential". If the foil is double sided, the 25mm gap each side will heat and cool, in reflecting both ways. The 25mm gaps should not be vented at all. There is great confusion out there at how these reflective barriers work. I have seen many of them just rammed up to adjacent surfaces. They will not work like this. The gap is "essential". Many houses have virtually no insulation if theses reflective barriers are not fitted properly.
If normal insulation is not fitted properly, at least it will perform reasonably well. If reflective foil is not fitted properly, it doesn't work at all. A 20mm bit of bubble insulation, between the shiny surfaces is neither here nor here in conventional insulation terms.