I’m hoping for code and energy sanity check on my make-up air plan.

I plan to install a 1200 CFM range hood with 8″ duct and an 8″ make-up air damper connected to the cold air return through a panned & sealed 9 1/4 x 16″ floor joist bay.  This is a 1978 house so certainly not air-tight – maybe I could get away with a 6″ damper though not sure how I’d make that case to an inspector without a blower door test.  The existing wall is 2×4 framing + 1/2″ sheathing + old school WRB (tar paper?) + cedar channel siding.

I also understand an un-insulated floor joist bay isn’t ideal from an energy standpoint but alternatives will cost $3K-ish more in my situation and reduce ceiling height in a basement bedroom, and this penetration is not even close to the weakest link in my thermal envelope (that honor goes to a cathedral ceiling in my office with, I’m guessing, R19 insulation…)

My plan is to screw / mastic the termination (“wall vent hood”) onto the damper so pulling out the termination will also remove the damper, with enough slack in the switching wiring to pull the entire assembly clear.  I would also run the damper through 3″ of XPS foam.  The penetration would be sealed against the cedar with foam and/or silicone. 

Two questions:

1) CODE: my understanding for Kirkland, WA is that code requires make-up air dampers to be accessible for inspection, service and replacement “without removing any permanent construction”.  Is there anything in IRC that would clearly define the silicone/foam seal as permanent, or is “permanence” an inspector’s judgement call?

2) From an energy & bugs perspective, am I missing a simpler or better  solution than silicone or foam that would not be considered permanent?