DensShield backerboard – does it need waterproofing membrane?
lucyna99
| Posted in General Questions on
From what I read in the posts on backerboards in GBA I see several comments on how well RedGard membrane is performing for the shower and tub areas. I also read that DensShield backerboard has a waterproof layer over gypsum core. We chose DensShield for its better environmental attributes. Do we need to apply RedGard over it or would it be an overkill? If we don’t use RedGard, what should be done to the seams and corners…?
Thanks,
Lucy
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Replies
Lucy,
While it's true that Dens-Shield backerboard has a waterproof facer, the reason for the facer is to protect the core of the backerboard, which is made of gypsum.
Any backerboard, including Dens-Shield, needs a separate waterproofing system if you plan to use it in a shower or similar wet location. [P.S. It turns out this isn't always true. See comments by Malcolm Taylor.]
Lucy,
Dens-shield, unlike cement-based backer-boards, is designed as a waterproof substrate. It is warrantied for use on the walls of showers without additional membranes. The details are available in their online installation guide.
Malcolm, would applying the RedGard around the seams of the DensShield be advisable..?
Lucyna,
It has a lifetime warranty, so your best bet is to follow their installation instructions. They cover joints with a fibreglass mesh. Would RedGard add an extra "insurance" layer? Sure.
As with all shower installations, if it is on an exterior wall it's worth thinking through how it fits into your strategy for allowing the wall to dry.
Yes, we have one shower which is on the exterior wall. Per Energy Star guidelines we taped a sheet of poly vb over this area, with insulation and the studs behind the poly. Hence, DenseShield would come directly against the poly. As I read the instructions for DenShield, it says, do not install vapor barriers directly behind it. We still have time to act. Should we remove the poly..? Put something else or nothing?
thanks,
Lucy
Malcolm,
I stand corrected. I've written two misleading answers this morning... I'm not doing so well. I think I'll stop answering questions for a while.
Thanks.
Martin And Lucyna,
I think my answer may have only been half-correct. Waterproofing systems for wet areas are something you need to be comfortable with too. I've used Dens-shield and share Martin's reservations about relying on a coated gypsum product (We used to use green-board as shower backing, and that sure ended up well...) I think a lot depends on what it is used under. Large tiles with small grout lines are probably fine. Perhaps something more complex, and therefore more risky, would be better done with a membrane based system.
Lucy,
Q. "Per Energy Star guidelines we taped a sheet of poly vb over this area, with insulation and the studs behind the poly."
A. This is the first time that I've heard that Energy Star recommends the use of a polyethylene vapor barrier. Could you please tell me where this recommendation can be found?
Q. "As I read the instructions for Dens-Shield, it says, do not install vapor barriers directly behind it. We still have time to act. Should we remove the poly? Put something else or nothing?"
A. I would follow the recommendations made by Dens-Shield. Since Dens-Shield has a waterproof facing, there is a very good chance that Dens-Shield is already a vapor barrier.
I looked through, and think it is the section 3.1 of this rater's guide for Energy Star homes that we believed that poly barrier had to be installed behind shower wall, as it is on the exterior wall. I recall this being rather clearly stated. Maybe it was verbal communication with raters? https://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/Energy_Star_v3_TERC_Guidebook.pdf talks about air barriers behind showers. Please see section 3.1
We are lucky in that our tiles installed over DensShield are large, so we'll probably go only for the RedGard in the seams/corners as additional precaution.
Lucy,
I think what's happened is that someone on your team has confused air barriers with vapor barriers. Energy Star requires an air barrier at this location -- not a vapor barrier.
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Could you suggest what to do with the poly sheet we have there taped to studs? Remove? Our house has a continuous air barrier / the plywood, seams sealed, so this is a bit confusing, but maybe we do need to act due to DensShield instructions not to install poly vb behind it.
thanks,
Lucy
Lucy,
I would remove the polyethylene if I were you. Properly detailed, the Dens-Shield will provide an air barrier and a vapor retarder.
The Energy Star program requires both an interior and an exterior air barrier; the main reason for this requirement is to address the deficiencies of fiberglass batt insulation. If you are using an insulation that restricts air movement (for example, spray foam insulation), these additional air barriers are less important (and the Energy Star program will consider the spray foam insulation to be an adequate air barrier).
If this is a shower, what is the pan made of, and how will you seam that walls to the pan? The tilesetters I work with are all using 100% liquid-applied waterproofing in their showers. The walls are usually backerboard of some type, often Denshield, the pans are usually mortar, and everything is completely waterproofed with a liquid-applied product.
Items 24 and 28 answer both questions.