How do I install a vapor barrier on the exterior of an existing wall?

Question

In this answer, it states quite emphatically that the vapor barrier should be on the exterior side of the wall, outside the insulation. In this particular question, the OP is in Michigan. I'm just a bit south of there in Indiana, so I presume that the advice is the same, particularly since this is a bathroom/closet installation.

Since the exterior of the house is buttoned up (inside to outside: studs, OSB sheathing, Tyvek™ house wrap, vinyl siding), how should I go about installing a vapor barrier on the outside envelope of the bathroom, leaving the drywall directly touching the insulation on the inside?

Would I staple the plastic up to a stud, then wrap it tightly inside the bay, staple it to the OSB, cross the bay, staple it down again, then up and over the next stud?

Answer 1

Vapor barriers belong on the warm side of the house. The answer stating that vapor barriers belong on the outside only because air conditioners is assuming that everyone lives in a warm climate and has an air conditioner, which very much of Michigan certainly does not, and does not need.

In the middle of the country, especially in areas that spend roughly equal amounts of time heating and humidifying, cooling and dehumidifying, and doing nothing, the exact position (or presence) of a vapor retarder becomes less relevant. You don't want to create a sandwich of moisture in the wall, but you don't want all of your humidity to move through the wall willy nilly, or you'll have lots of condensation. That's why the more accurate term is vapor retarder. Many things are vapor retarders, including drywall and paint.

In a bathroom in Indiana, kraft paper faced insulation, drywall, and paint should be just fine as a vapor retarder, and you shouldn't need to worry about changing anything on the exterior of the house.

primary source: https://www.energy.gov/energysaver/vapor-barriers-or-vapor-retarders

Answer 2

I don't know that it's going to be a bad thing if your vapor barrier is on the inside of the wall. It's preferable it be on the outside, but it's easy to overthink it here as well.

Generally, when putting a wall together, you have (outside to inside)

1. Exterior material (brick, siding, etc.)
2. Waterproofing material (tar paper, Tyvek, etc.)
3. OSB or other non-exposed siding
4. Studs
5. Interior wall material (drywall)

Your insulation goes in between the studs but, as you'll note, there's other material doing the water-resistance. Even if you mount your kraft batts with the tarpaper facing outside, the batts are simply butted together and stapled to the stud. If moisture gets there, it can still penetrate the studs (and it's unlikely the kraft paper extends down the the floor).

So why have kraft batts at all? You want to limit how much humidity reaches your interior side. If water reaches your drywall, all sorts of bad things can happen (chief among them mold). But that benefit is marginal at best for most people. If lots of water is making it inside your home, you'll have other issues, none of them preventable by kraft paper.

I have put the kraft paper on the inside without issue. Other sites, as well, have suggested it doesn't matter

As you can see, the permeance of the kraft paper rises as the relative humidity rises and hits 10, the point at which we describe a material as vapor permeable, when the RH is 60%. The upshot here is that if you put the kraft paper on the wrong side and it gets wet, it won’t trap moisture. The wetter it gets, the better it dries. If you put it on the right side, where the humidity is, it’s not much of a vapor retarder, because that’s where it becomes vapor-permeable.

I still want it on the outside

I would just stuff the insulation in, with the kraft paper on the exterior side. You can't have it perfect (stapled tabs to the studs), but I doubt it will make any difference.