Vapor barrier vs Moisture barrier & locale

[DavidTu]

We're in Seattle and doing a 3-wall alcove tub/shower combo on 2 exterior walls, one with a window. We are planning to use hardibacker and already placed 4 mil plastic on the studs down to the tub flange.

Today we stopped off at a tile place and they said they do NOT recommend a vapor barrier and that it had been part of code here in Seattle and subsequently removed b/c it was causing mold problems. They are suggesting NOTHING behind the hardibacker (except insulation).

Now in case you don't know, we get a LOT OF RAIN here in Seattle all year long (well, almost). So their argument is that most of the vapor barrier conventional wisdom is borne in a warmer (CA) climate and it just doesn't work here.

Well, what do y'all have to say about this? Also, I am not sure if they were suggesting no moisture barrier as well or not.

We need to make a quick decision and so now I am calling everything into question: 1) should we keep the 4 mil plastic in place on all three walls (or only on 2 exterior walls!?) OR 2) should we instead go with Redguard on the hardibacker? I think we have to get in there to remove some insulation facing that I didn't realize might create a vapor sandwich... so I guess we're going to do something, OR 3) is there anything to this idea that in Seattle we should do neither?

I'm also getting a little confused about vapor barrier vs moisture barrier in that it seems like all the conventional vapor barriers discussed (felt paper & plastic) are also moisture barriers and that the Redguard moisture barrier is also a vapor barrier. So... clarification on this would be helpful.

Thanks.

[cx]

Welcome, David.

A vapor barrier in the construction industry is any material with a permeance rating of 1.0 or less. The term being adopted more commonly by the industry is vapor retarder. I have a little problem with that because nearly everything will retard the passage of vapor somewhat. I want something to stop nearly all of it and call that a barrier. But don't nobody pay me no never mind.

At any rate, a "vapor retarder" or vapor barrier has a permeance of less than one.

To actually act as a vapor barrier, though, this material must be installed without penetrations. That rarely happens. Therefore, the code required vapor barrier in some jurisdictions is usually not a vapor barrier at all. But they do stop a lot of the moisture vapor, primarily by stopping the movement of air.

Shower construction (non-steam shower) requires a moisture containment of some kind to be lasting and functional. The use of a moisture containment material, such as poly sheeting or roofing felt over the studs behind the wall board, does that quite well and properly directs the moisture back to the drain. It does not a vapor barrier make.

Absent the use of such a containment, you can waterproof the interior of the shower using a direct-bonded waterproofing membrane, either sheet or liquid-applied. Some, but not all, of these also constitute a vapor barrier or retarder.

In your situation, I can't speak to your local codes, requirements, or building customs for treatment of exterior walls, but you still must provide one of the two moisture containment methods above for a proper shower installation.

My opinion; worth price charged.

[DavidTu]

We've decided to rip out our poly sheet from studs and instead go with a product that will apply on top of the hardibacker as moisture and vapor barrier. The only one I know of is Redguard. What are the recommendations for this product vs other options?

This is for an alcove tub/shower along two exterior walls which are insulated but not faced.

Thanks.

[jgleason]

One thread per project please. I merged your latest post with your previous thread.

[DavidTu]

OK Joe, but I think I've seen elsewhere instruction to only ask one question per thread... also my Redgard question is a general one, not specific to the topic. OK... in light of this I will add on another general question here, feel free to split it out though.

For mounting Hardibacker, HD has two different screws... one for Hardi and one for Wonderboard/Durock... trouble is the Hardi version only comes in 1 5/8" longest length, whereas the Wonder/Durock screws come in a 2 1/4" size. The screws pretty much look the same and the head structure looks identical. The Wonder/Durock version has a low/high threading scheme though.

So ... is this just a marketing thing or are these screws really materially different? Can we use the longer 2 1/4" screws with the Hardi? I would like to use a longer screw as there is a 3/4" firring on top of the framing that I would like to penetrate deeply through to get more bite on actual studs.

[Houston Remodeler]

David,

2- Use whatever screws you want. I like the backer-on screws with the square drive head installed with an impact driver. They countersink themselves nicely.

1- Redguard, Hydroban, Hydro Barrier, (all liquids) Kerdi, Nobleseal, Dalseal (all fabrics)

[cx]

Originally Posted by David

OK Joe, but I think I've seen elsewhere instruction to only ask one question per thread

That would not have been on this website, David.

We like our all-volunteer army of helpers to be able to see what a visitor is working on and what's been previously asked and answered. And we can tell if you've seen the responses to your previous questions.