[salty]

If you want it to code you will have to check your area. Some areas are mandatory fire barrier such as drywall.

Yes you need attic ventilation whether simple venting, 'whirly bird' or automatic. Besides heat exchange also for mold.

A basic 2 car garage is 24x24 feet. 3 walls - garage door is 8 feet x 24 x 3=576 sq/feet. Prices here, 2 by 4 walls is R12 @ $31 a bundle for 88 sq/feet into 576 equals $217 for walls.

Same 576 sq/feet for ceiling rafters but 2 by 6 studs, R20. $30 a bundle for 49 sq/feet for a total of $360.

$360 plus $217 plus $70 for vapour barrier = $647. You could go with the cheaper heavy poly for about $25. You will also need staple gun and tape.

As you can see I'm not doing much today.