I have an existing 2-car garage which I'm in the process of
insulating. There is no living space above and it will not be a
living space when complete. This will become my workshop.

The gable roof framing is 2x4 struts, not joists and rafters as
normal. Because of the maze of strut members, it will be very
difficult to retrofit a vapour barrier inside the attic. One
possibility would be to take down all the plasterboard in the ceiling,
install the vapour barrier and resheath but I don't want to do this.

If I insulate the garage attic to R-26 with Rockwool batting (two R-13
batt layers layed perpendicular to each other), can I avoid installing
any vapour barrier and still retain the R-value of the insulation?