26x40 3 car garage with radiant floor heat in the concrete.  47460 area code climate zone 4 southern line of 5

Nice quality garage doors R-value of R-16, Radiant heat in the floor, 2 3x6 windows and 2x6 exterior walls.  I am wanting to maximize the radiant heat to assist me with keeping the bonus room above comfortable.  Gargae is nothing special framing wise standard 2x6 with 1/2 inch sheathing and Tyvek.

Should I be super worried about airsealing the stud cavities?  I don't want to end up with a sauna in the garage with humidity issues since I plan to keep it fairly warm in there through the winter.  I could easily flash and batt the exterior walls on my own with a foam DIY kit but i'm not sure it's worth the extra money to do that simply because it's a garage.  I plan to do wet sprayed cellulose in the rest of the house but I have exterior rigid foam on the main part of the house, I did not do the poly iso on the garage.

Flash and Batt or flash and fill? or just go with the cellulose and it'll work fine?

Air sealing the walls makes the insulation work better and will reduce the amount of heat you'll need.

I knew you would say that! I'm going to make it as tight as I can get it without breaking the bank. This energy efficiency build is not MONEY efficient that's for sure.

It is after you move in.

I hear you Bob.

I am thinking about having the wet sprayed cellulose done in the studs...need to figure out the easiest most cost effective way to get the darn walls air tight though...would it be a problem to cut and cobble the poly iso into the stud cavities then blow the cellulose in there on top?

Did you do ZIP sheathing with taped seams? That helps tighten the house. One of the issues with cut & cobble is that it is very time consuming to do well. If you use the canisters of spray foam (froth pak), or a pneumatic caulk gun with good caulking, that would take care of most of it, but a blower door would tell you if you need to bother. There are also branded foam sprays for air sealing, typically used with fiberglass insulation. The garage is hard to air seal anyway due to the doors.

Fill the walls with cellulose. If you want more insulation than that, then add interior side rigid foam. I wouldn't consider cut & cobble.

I'm seriously considering the cellulose....I am also considering R-19 batts with an interior vapor barrier. Only if I can get the walls nice and tight first though.

If you have a garage door that seals well enough to make the walls an air infiltration concern, I'd like to know more about it (mine is very poor).

I understand that 3 large holds on one side of the garage and extra money spent air sealing on the other side of the garage seems stupid. Just hard to not apply the same level of detail and thought to all areas.
Would it be against all the standard ways of doing things to just install batts (R-23 or R-19) and hang a poly vapor barrier on the garage walls??? This could be done for 50 cents a ft vise 80 cents a foot for the cellulose

we just did an attached garage & occasional workshop with 6" FG batts and 9" in the ceiling; fine for occasional short duration heating. Lose the poly, though so it can dry to the interior or exterior. Making the building tight never hurts. Built a camp up north years ago - no insulation except two layers of boards separated by a layer of 15# felt, so the building is pretty tight & heats up easily with our woodstove.

My tyvek seams are taped and I have caulked all my wood to wood contact points as well as the sill plate to concrete area. My plan is to blow cellulose in the walls and be done with it. I understand that airtight is best but wouldn't the poly vapor barrier be sufficient since it's on the inside of the wall. It would at least give the wall a chance ot dry to the outside right? Especially since the room will be warm from the radiant floor heat???