I have a house built in 1995 with an attached 2 1/2 garage. I moved in a little less than two years ago at that time the garage walls were insulated with batt (R19 on shared walls, R11 on non-shared) and covered with OSB. The ceiling in the entire garage has drywall but has not been plastered or painted with about 6" of blown-in in the attic. I would like to use the garage a bit more (northern WI) in the winter for some projects am having some temperature issues.

I'm currently in the process of upgrading the electrical in the garage and as long as I've got every cleaned out of the garage I figured it is as good of a time as any to look at correcting the two other issues I have.

1) along some of the seams in the ceiling I'm seeing water staining, when checking in the attic there is no vapor barrier present, and when checking the roof only one of the 10 or spots was caused by an actual leak a year ago.

Do I need to drop the ceiling to put in a proper vapor barrier before adding more insulation? Or can I just add more blown-in?

2) The garage is a hot-box, there is absolutely no ventilation to speak of anywhere in the main area of the garage. There is a single window which is fixed unfortunately. The soffits are vented and there are inserts in the attic for general attic ventilation.

I'm planning on getting a small portable torpedo heater possibly for the winter, so I would need some ventilation at that point but in general I would I go about adding it in general?