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rayf01 01-01-2010 09:48 AM

Densshield and Vapour Barrier
Hi everyone, I am new here and kind of in a pickle...

I have a bathroom renovation underwear, I gutted it entirely and installed a new tub. On the outside wall (which has a window) I installed a vapour barrier - poly, then put up densshield last night and taped all the seems then coated them with thinset. I plan on tiling everything. I read the instructions again today and did some more searching online and it says don't use vapour barrier behind it.... Am I hooped now, do I really need to tear it all down?

I read somwhere that Canadian codes still dictate you need vapour barrier no matter what on an outside wall. That was a ton of work last night and I'd hate for it to have to be all torn down and start from scratch again.

The other question I have is the window has an inset of about 9 inches in to the wall. So I will be tiling this area as well. From the bottom of the window to the inside shower wall there is a drop of about 0.5 to 0.75 inches over the course of the 9 inch recess. This should be enough of a drop in elevation to allow water that gets splashed up in to the window area to drain off and in to the shower area. With that do you think it would be wise to install a peice of kerdi over that lip (the transition between the bottom of the window well and the shower wall), or once its all tiled, with groat should it be much of an issue so there wont be any standing water there anyway because of the slight drop.

Thanks everyone.

ccarlisle 01-01-2010 10:34 AM

I think you'll have to be more precise in your use of "vapour barrier" so that we fully can answer a complex question; as for example, codes are usually suggestions and sometimes requirements but to the best of my knowledge are based on sound phyical evidence and common sense.

So it is for vapour barriers...what we up here call a "vapour barrier" is a 'vapour retarder' (vr) with less than 0.1 perms of vapour transmissivity, something like 6mil poly sheets. There are at least three Classes of vrs, we use Class I in the colder zones of our climate - but not everywhere. It is a very good starting suggestion but one would need a closer look at your house's envelope to really be able to suggest which Class of vr you'd need.

Don't know where you are but it would be afe to say you need a vr (of whatever Class) on the warm-in-winter side of the wall. That handles the vapour flow from in to out but doesn't do much for the wind and water infiltrations from out to in...So your house probably has an air barrier on the outside, depends when it was built. That handles air flow. Now your walls on the inside have drywall on them perhaps paint, so right there you have a vr on the inside too.

Now as far as I know 'Densshield' is a vapour retarder of Class 2, not a class I, much the way concrete board is. So it will let moisture from the shower through it somewhat, it isn't completely waterproof the way Kerdi is. So, some moisture gets through it and condenses on the outside of your wall. It might be slight but not good really...:(

Especially knowing that the state-of-the-art for showers is to build them moisture-tight using a waterproofing (i.e. a Class 1 vp) membrane. No moisture travel at all. So what you have is OK, not worth demolishing, but know that eventually mould will gather on your outside wall, where the insulation is and the air barrier is too. Now it may take 5 years maybe 10 but you haven't made a waterproof shower using Densshield no matter what GP says.

But again, I wouldn't tear it out just for that - after all, still most showers are built that way IMO.

rayf01 01-01-2010 11:54 AM

Yeah and I would assume there would be some moisture leakage eventually, which would probaly warrarnt remodeling the bathroom again in 10 years time or what ever. My concern is really GP saying no vapour barrier behind Densshield. I just am hoping there wont be a issue in 3 months with a mass mold or mildew growth between the poly and the densshield.

That said I am pretty meticulous and I made to sure to tape and thinset all the joints and screw holes and silocone the edges everywhere else to try ensure as much water as possible doesn't makes it through.

jaros bros. 01-01-2010 12:05 PM

In this type of situation where the manufacturer has made a recommendation that seems to conflict with the code you can simply call their customer service line and ask. They would be able to tell you the "why" of not including a vapor barrier. Companies spend time and money on the testing of their products and there is definitely a reason that they are not recommending a vapor barrier. I suspect that the reason has to do with moisture being trapped and not being able to dissipate.

Good companies are glad to answer your questions and have set up customer relations hotlines just for this purpose.

A quick google search help me locate toll free service numbers and confirmed my suspicions.

rayf01 01-01-2010 12:21 PM

bleh this sucks. Yeah I've seen the site before and it said dont install vapour barrier. They are closed today guess ill have to call tomorrow. This situation might be that you can't use denshield on an exterior wall here then if you cant use the poly I am guessing.. What a mess.

ccarlisle 01-01-2010 12:48 PM

Yeah, but remember the audience they are talking to...the average North American owns a home somewhere near Washington DC - not in a cold zone like us here.

So they print one brochure that doesn't make sense to a Mexican nor to a Canadian...most of the users are covered. That limits their liability anyway.

You just don't want moisture to condense on the cold outside insulation or wall covering...that means a vapour retarder just beneath the tiles up here.

Like I said, you MAY be redoing it in 10 years, so maybe in the next decade...a lot can be done in ten years and lots of people move.

ccarlisle 01-01-2010 12:55 PM

Oh BTW there is only one way to waterproof in-shower window sills and that is to put Kerdi everywhere. Including the 9" recess...No other way else: failure.

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