Insulation for small attached unheated mudroom addition Zone 6A – GreenBuildingAdvisor

I’ve designed an 8’x8′ cathedral ceiling mudroom for clients in Kingston NY – Zone 6A. Although the addition is unheated and will be constructed on pier footings (not a full foundation), it is connected to the heated space of the house with a 32″ door that may be left open often.

Please comment on the insulation design:

Walls: 2×4’s – R13 mineral wool batt cavity insulation +R9 Zip High R sheathing

Floor: 2×8’s – R23 mineral wool batt cavity insulation installed from underneath, and held tight to subfloor. 1.5″ foil faced xps rigid insulation at underside of joists (R9) Cover with weather tight/rodent tight sheathing.

Roof: 2×8’s – (3) layers of 2″ foil faced polyiso insulation – R38. Stagger and tape seams.

Does this look viable? I’m trying to not overdo this little addition, but I don’t want to create condensation problems. The existing house is old and drafty – am I overbuilding? Thanks for your feedback.