All siding leaks under wind pressure- if the wall insulation is degradable from wetting, count on it getting wet- cement siding can store a significant slug of water. You really need a drain-plane between the siding and the insulation, and (installation instructions notwithstanding http://www.jameshardie.com/pdf/install/install_panel_west.pdf ), a air/drain gap between the Hardie siding and the drain-plane. Cement-based siding can soak up water in weather wetting events then release it more slowly- contact with the drain-plane materials or studs can lead to moisture damage at points of contact. Highly permeable housewraps used for drain-planes may be waterproof for intermittent events, but aren't designed to work in mostly-soaked environments.
Hardie Panels can exhibit warping under heavy wetting on one side. (see instructions about painting with spray methods.) A drain plane w/gap limits that risk from wind-driven rain or minor roof leaks.
You can probably get away with using permeable housewrap on the studs, then mounting the Hardie Panels to furring strips (pressure treated or plastic) forming a 1/2" gap between the housewrap and Hardie Panel. Whatever insulation you install has to not press the housewrap outward such that it makes contact with the siding.
In Seattle there's a code requirement that roof-decks be ventilated- you can't just jam insulation between the rafters up against the deck, not even spray foam. It'll be a P.I.T.A., but for maximum R-value for minimum space, cutting 2" foil-faced iso to fit, using pieces or strips of 1/2" iso or EPS as spacers away from the roof deck will probably be "good enough". If you want to be code-compliant, only use "fire rated" iso board (IIRC DOW makes some that doesn't require a thermal-barrier.
You do NOT want a vapor barrier of any kind on the exterior side of the studs, even in unheated space or the wood will likely accumulate moisture & rot. In unheated space that isn't likely to be super-tight in moderate-temp winter environments like Seattle you probably don't need to go hog-wild with vapor retarders on the interior either. Craft-faced batting is probably more than sufficient.
If there is a side to the garage that gets some sun (or "bright clouds", in Seattle winters) you may want to consider doing some passive solar heating by appropriately placed windows or skylights, with minimal glazing on the W/N/E sides. With even R12 insulation everywhere (including an insulated door) if it's not a totally leaky sieve you can get a surprising amount out of passive solar methods.
Last, not least: Sheathing is often used structurally to keep the wall from racking, and Hardie Panels don't provide that structure. Hopefully there is some diagonal timber in the framing to serve that purpose? If not, better add it- the weight of the siding itself could create some issues. Make sure you use fasteners that won't get chemically destroyed by the cement based material too.(hot-dip galvanized or stainless highly recommmended.)
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