As a result of a deep-energy retrofit/remodel, my exterior walls are now 1/2" sheetrock backed by 2" of polyiso foam against the studs. We are now trying to install trim (casing, crown, and baseboard plus some other decorative elements) to our walls. Normally I use an 18ga brad nail to attach interior trim, but, it doesn't seem that nailers exist in either 16ga or 18ga that can handle nails with enough penetration to reach the wood. Plus, even if the gun existed, I can't find nails in that length.
While I can use a wood backer on the crown that is screwed into the studs / joists, this wouldn't work for the rest of the trim without removing sheetrock. Removing sheetrock reduces one of the layers of air sealing, so that's a no-go.
Other than gluing the trim onto the sheetrock, which I hate for repairability reasons, is there another good strategy for attachment?