Oh yeah, mine could be detected, I have added insulation to all exterior walls with Mylar, but thats more to give me a warm fuzzy than anything. I am getting it more secure each week tho. They can't use FLIR here without a warrant. And I understand FLIR, and know it's limitations.
You'd be in good shape there with the dead air space, the attic is generally the warmest part of the house. I would think that a dead air space would be enough to deter FLIR (IF they are even using it to detect small grows) It's the same principle as dual pane glass.