Installing Durock for Shower Wall

[MikeG]

Hi everyone. I'm new to the forum and it seems there is a wealth of great information here.

I'm currently working on a masterbath remodel and I'm ready to install the cement backer (Durock) for the shower walls. It is a corner shower. The interior wall will be insulated with r-11 unfaced insulation. The exterior wall has the same insulation. My question is should I put a vapor barrier up between the insulation on the exterior or interior walls? When I called USG they seemed a little vague stating that the vapor could cause mold build up if water got behind the Durock but it was common to install the vapor barrier.

The Durock will go up about 6' from the shower floor. I'm using green board for the last 2' of the wall on both walls. I am not installing a vapor barrier behind the green board.

Thanks for your advice,
Mike

[jimbo]

As I understand construction, a vapor barrier should be on the inside walls, to protect the insulation from moisture. Another way this is often stated is that if you are using foil-faced insulation, the foil faces the heated space. For sure, a moisture barrier material is always recommended under backerboard in the shower walls. On exterior walls, they use "house-wrap" such as tyvek, which 'breathes': it keeps moisture from entering from outside, but it allows any moisture from the inside to breathe out.

[SeattleSoxFan]

The cement backer board is porous and, though unharmed by water, will not prevent it from passing through. Tiles and grout is also more porous than you might think, so you should always put something behind. I'd suggest using 15# tar paper and overlap the lip of the tub (if one exists) to assure proper drainage should moisture collect back there.

You can use other types of waterproof membranes (check out tile stores/books for suggestions), but tar paper is generally the easiest and cheapest Some people like to "glue" the joints with roofing cement, but if you overlap the paper properly it's probably unnecessary.

You might check out johnbridge.com -- a frequent suggestion from this site -- for more tile oriented questions/answers.

Oh -- you might also want to make sure you use a good PVA primer on the greenboard. It is a good idea to not put a barrier behind it, especially in a bathroom (greenboard vs. "gray" board has virtually no difference on mold). The PVA will create a vapor barrier on the interior side of the drywall -- which is the best place to keep it! Just make sure you have a decent fan and your install should stay dry last a long time.

Good luck!

[SeattleSoxFan]

You posted as I was writing my own response!

Just have a few points/additions:

Outside moisture is best kept outside and inside moisture best kept inside and any moisture in the walls needs to be able to escape.

The trend for smaller builders around Seattle (a very wet environment) is actually backing away from Tyvek wrap-types back to tar paper because it allows greater breatability, and less precise installation with regards to taping, penetrations, etc. The big-guys still use it for sure, and when used correctly tend to do ok -- though all the buildings here being torn up b/c of mold problems are less than 5 years old and are all house-wrapped.

And for interior walls, because of the newer, tighter homes, the old way of putting up faced batts or unfaced with poly causes moisture to hit and collect on the back face of the drywall, which usually just soaks it up and eventually molds... thus the benefit of PVA primers -- it moves the vapor barrier to the inside of the room, so the drywall stays dry.

But there are countless opinions on both those issues I'd guess -- but I would definitely NOT count on faced batts as a "good-enough" vapor barrier behind the backerboard.

There are some tile-backers that don't require (and in fact it voids their warrantees) such as dens-shield, which have a fiberous core and a vapor barrier coating, though it's still a good measure to paint a coat of membrane atop them.

Just my opinion!

[jadnashua]

Where you have the insulation with a vapor barrier, take a razor and put some slices in it, then put some tar paper or plastic on the studs prior to installing the cbu. The goal is to keep moisture from the studs, and, if you messed with the insulation's barrier, add one back. This will prevent any moisture from being permanently trapped between the two layers. My unprofessional opinion.

[MikeG]

Thanks for the response everyone. Here's my plan... I will put 15# felt behind the Durock and not the green board. I was planning on taking the Durock up about 6' from the shower lip and green board the rest.

Just to clarify the insulation is not faced so there is no vapor barrior there.

Mike

[jimbo]

SeattleSox: Thanks for your input. Sometimes we tend to forget that conditions vary from place to place. It is so dry here (San Diego) that mold problems almost always stem from a plumbing leak, not building conditions. Your info about mold under Tyvek and mold on the backside of drywall is very interesting, and puts a whole new perspective on home construction.

Thanks,
Jim

[johnfrwhipple]

Vapour and Mold are nasty realities for many people and many builders around North America.

Seattle like Vancouver is a harsh climate.

Rain. Clouds. More Rain. More clouds. A couple of months of sun and then some freezing rain. Days and weeks of clouds.

For this weather the side affects are for some reason we live longer than the rest of the continent. Some say it's the rain. Some say its the fish. Some say we are more active in the Pacific North West.

One thing for sure is that we have a wack of leaky condos. Poor rain screen designs and are fair share of dodgy builders.

I am mixed in my opinion on vapour barriers and house wraps. I like the concept of the rain screen and the value it adds to drying out a home. I do not think a home should be wrapped up so tight it can't breath but feel it should breath a little bit just to stay healthy.

I have never seen on an exterior wall the level of mold that comes from a failing shower. Never. Often an exterior wall may be bad but to some degree it does breathe from the outside. An interior wall that is getting soaked with no airflow and a constant supply of water makes for a mold pit and nasty business.

Build your showers better. If you think you have mold - check. It's your home and you and your family breathe there every day and every night.

These points brought up above are very useful to deciding the routes to take. How I build an exterior wall here in Vancouver will differ from Jimbo in San Diego, or Tym in Winnipeg or Angus in Chicago.

There is a science to these homes we live in and one building material or system can affect the entire chemistry of your home.

JW