Are vapor barriers still used in home construction?

I have a seldom used 1/2 bath. While trying to remove 3 layers of very ugly wallpaper, I discovered mold in the insulation in the outside wall. That wall has been stripped down to the studs, and we are putting it back together. The wall is currently insulated (R15 fiberglass on a paper backing.)

I was looking up the current recommendation for a vapor barrier inside of that. The little bit of construction I’ve ever done included stabling an impermeable plastic sheet inside of the insulation, below the sheet rock. So not totally sealed. Some of what I was reading now said that unless I can come close to fully sealing a vapor barrier, all the way around, that current practice says that this practice might be better skipped, as it could trap moisture inside the wall, exactly what I don’t want, and lead to mold.

The details:
The house is almost 50 years old. The outer siding is cedar siding that was painted at some point. There is not any kind of house wrap between the siding and the studs.
The room is a 1/2 bath, 4 1/2’ x 6 1/2’, containing a recently replaced casement window. There isn’t a shower or tub, and it gets used a few times a week.
The 1/2 bath has no outside venting, except the window.
I live in a Minnesota, a cold climate with winters that have several months below freezing.

So…
Am I better off using a vapor barrier or not? If yes, do I need to do something like double side tape all the edges, in addition to stapling?

Anything else?
Thanks.

Only a few building materials, such as glass and metal, can be regarded as impervious to the flow of water vapour, but a considerable number of materials will provide sufficient resistance to be used for vapour control purposes. To classify these as vapour barriers, an arbitrary standard has been established by which materials can be rated in terms of their water vapour permeability or water vapour permeance.

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Barring an expert’s experience who has personally viewed your specific situation, I’d skip the barrier.

You can use faced insulation with kraft paper backing. It’s used a lot in the South. The wall can still breath through the paper and there’s no mold.

It’s not nearly as effective in blocking cold air. But, it sounds like a sealed wall with vapor barrier won’t work in your house. Unless you did major renovations.

Here’s some good installation tips.

https://youtu.be/NpkXxHRp8U0

I’m no longer in the business, but yeah, skip it. You don’t have a bath or shower, so moisture issues will be nearly nil. If you do use plastic, you shouldn’t have moisture problems unless you install it on the exterior side of the insulation. As long as it’s installed on the interior side of the insulation where cold air doesn’t contact the warm surface, there is little risk of condensation, but why take the chance? If you’re really worried about it, install green board instead of sheetrock.

Thanks folks.

HUh??? Always put in the plastic vapour barrier on the outside wall.

If there is any leak letting inside air out then the room temperature normal humidity (cooking, breathing, drying towels) then over time this moisture escapes into the gap between the inner sheetrock wall and the outside wall, and condenses. This effect is most pronounced in houses where the outside temperature can get down well below room temperature (i.e. in North America).

Standard protocol: Get acoustic sealant. Seal the plastic vapour barrier to the studs and upper, lower plates on the inside of the wall, behind the sheetrock, with a continuous bead of sealant. staple into the acoustic sealant so the staples don’t leave pinholes for air to leak. If there is any moisture in the wall, it will evaporate to the outside. This is most critical in Canada, where outside temperatures can be quite low and modern houses have things like humidifiers; plus often, the house has positive pressure to prevent drafts. If there are outlets on the outside wall, cut a piece of plastic to wrap around behind the box and then tape (“tuck tape”) it securely to the sheet to minimize leakage.

yes, you are not building a sealed spacecraft, but any leakage you can minimize is worth it.

Nope. The air barrier - tyvek, house wrap, etc - goes on the outside in a cold climate. The vapour barrier goes on the inside, between the insulation and the wall board: