Should I put vapor barrier only on exterior wall of a bathroom?

Question

I've been searching around the site and haven't found an answer for my question on where to place a vapor barrier so I'm hoping to get a little more clarity here.

Basically I'm in the home stretch of a small full bathroom gut/remodel. Took everything down to studs and even pulled the subfloor because of rot. Getting everything back together and have used the Roxul stone/mineral insulation for the walls (comfort on one exterior wall and safe 'n' sound for the inside walls) and good ol' Pink Panther faced for the ceiling. According to the Roxul site, a vapor barrier is only required for external walls. What I'm trying to figure out is whether I need to do just the exterior wall or if I need to do all around the tub/shower area as well. 1 of the 3 walls for the tub is exterior and the other 2 are interior. Thoughts or input?

Bathroom is about 7' x 8', is on the first floor and there is a room above it (1940's bungalow). Here in Detroit, I believe we're Zone 5. I installed an 80 CFM fan above the tub/shower area to vent out moisture and am planning on putting up the Sheetrock Mold Tough drywall. Original bathroom had no insulation and no vapor barrier but had about an inch of rock lath (don't know if that makes a difference).

Answer 1

The TL;DR -- vapor barriers belong on the outside only

You should only place a vapor barrier on the exterior side of the exterior wall, allowing the assembly to dry back to the inside if water does get in past the tub surround. I would recommend paperless drywall over cement board for the tub surround, as well -- gypsum board of all types is vapor open (around 50 perms) while serving as an air barrier, allowing drying to the inside without letting a ton of humid air into the wall cavity, while cement board is a fairly severe vapor retarder at 4-8 perms, which puts more pressure on the ability of your tub surround to keep water out of the wall to begin with.

What happens when you try to stick one on the inside?

When you stick a vapor barrier on the inside of a building, all will seem well at first, until you go and turn the air conditioner on. Then, the moisture in the warm, humid outside air that fills the wall cavity will start to condense on the cold exterior surface of the vapor barrier (assuming an insulated assembly), and you get mold as a result.

A similar problem will happen if you try to insert two vapor barriers in the same assembly -- the inside one will still generate condensation despite the efforts of the outside one to keep all that humidity at bay, and now there's utterly nowhere for the moisture to go!

Of course, there's no point in putting a vapor barrier on an interior wall -- the airspace of a partition should never communicate with that of an outside wall!

More info on why vapor barriers are like badgers can be found in BSI-073, aka "Macbeth Does Vapor Barriers."

Popsicles are for sucking, bathrooms aren't

One other mistake that can be made in a modern, tight house is to slap a "fart fan" in the bathroom and call bathroom ventilation done. Doing that just means that your makeup air will come from all sorts of random leaks that have no business providing indoor air to people -- especially if one of them happens to be the range hood, or the exhaust of some appliance.

The easiest way to solve this problem is to provide transfer air to the bathroom from a zone that's being blown on, say the basement, if your basement is a suitable transfer air supply, as not all are! (You'll need to provide, oh, 10% extra makeup air as this is a transfer air setup, as well.)

Of course, if the OP's house leaks more air than a SR-71 on the ground leaks JP-7, an old-fashioned bathroom exhaust fan like the one the OP proposes will serve him fine.

More on ventilation in general can be found in BSI-070, "First Deal with the Manure and Then Don't Suck", by the way.

Answer 2

You should install a vapor barrier on the exterior wall. The vapor barrier should be on the "inside" (between the insulation and drywall). There is no need to vapor barrier the non-exterior walls.

If the tub shares this exterior wall I would put vapor barrier behind it (if you can) as the plastic tub surround will not give you a "vapor barrier" (I don't think). If you have access it should be cheap and easy to just do the whole exterior wall.

Answer 3

If you are working on a bathroom you should have already placed a vapor barrier around the tub/shower area in the form of concrete board if you're tiling or if it is a one piece plastic deal. An additional barrier probably isnt necessary.

Answer 4

The only reason to not install a vapor barrier is, if doing so, you would create a "vapor barrier sandwich" in which condensed water vapor gets trapped between two vapor barriers. Trapped water = mold.

Does your acrylic/fiberglass shower enclosure reach the ceiling? If not, that's good, and you should install a vapor barrier. It will prevent vapor getting into the insulated wall and the area above the enclosure will allow the sheetrock to breathe. If the enclosure reaches the ceiling then it's a judgement call to determine if there is enough air circulation behind the enclosure to clear any condensed vapor. If that's the case I'd call in somebody who can advise on the right wall construction for your region.

Answer 5

Of course you should. Then be sure you have or put you in a fart fan to remove the moisture and humidity from the restroom. Make sure the fart fan is vented thru the roof and not into your attic or soffit vents.