I am about to sheetrock a bathroom (I will use CBU in the wet areas) and was going to use “greenrock” or MR drywall. I found a FH magazine in a pile of stuff on my desk from sept 2000. There was a section in “what’s the difference” about MR drywall. There is a section I am wondering about, and I quote – “Chapman (a national gypsum co. rep.) says MR board is often mistaken for a vapor barrier. But with a perm rating of between 24 and 25, he says, it isn’t(building materials need a perm rating of 1 or less to be classified as vapor-diffusion barriers). Builders have also been known to install tile-covered greenboard over a vapor-diffusion barrier, such as polyethelyne sheeting, on exterior walls. Wrong. According to Chapman, a wall of tile and mastic becomes its own vapor barrier. If you put a second barrier behind it, any moisture getting through the wall tile becomes trapped, degrading the drywall. Even kraft faced fiberglass batts should be avoided on outside walls covered with gypsum board and tile.” True or false? I have an outside wall that has kraft faced insulation that has had numerous slices through the paper and it is covered with a poly sheet. I am going to tile halfway up the wall and the rest will be painted. Do I remove the poly sheet from the bottom half of the wall? Remove it from the whole wall? Leave it alone? Do I still use poly sheeting behind the CBU in the shower area? Any thoughts? (sorry the quote was so long) Thanks.
Discussion Forum
Discussion Forum
Up Next
Video Shorts
Featured Story
Skim-coating with joint compound covers texture, renews old drywall and plaster, and leaves smooth surfaces ready to paint.
Featured Video
Highlights
"I have learned so much thanks to the searchable articles on the FHB website. I can confidently say that I expect to be a life-long subscriber." - M.K.
Replies
Yes, still use poly sheeting behind the CBU in the shower area.
To be in compliance with the board manufacturer's installation instructions, your info is correct, do not use a vapor barrier between the MR board and the framing if the MR board will be used as a tile backer. Obviously this is for dry areas.
To create a barrier, skim coat the green board with mastic, then tile. The skim coat of mastic will act as a barrier. You can then tile and use regular portland-based grouts.
Or tile without skim coating with mastic, or tile with portland-based thinset, and use a grout that doesn't allow vapor to permeate through the grout. Silicon grouts and epoxy grouts would suffice. In the latter case the tile and the grout will be your barrier.
I've never gotten a formal acknowledgment from the manufacturers, but tile adhered with portland thinset and grouted with portland grout has a perm rating of over 1, so it does not qualify as a vapor barrier. However, that assembly is not mentioned in their installation instructions. The one time I called National Gypsum years ago the tech guy I spoke with waffled all over that method. He agreed it wasn't a vapor barrier and said it wasn't disallowed as an installation method, but they wouldn't put in writing that it was allowed either.
I think the waffle factor stems from the size of the tile used. Small 4" squares or subways would have a perm rating of over 1, but perhaps there's a point with large format tiles where the perm rating goes below 1, thus the hesitancy on their part regarding clarity.
Personally, I skirt the issue by tiling over cement board and using thinset instead of mastic.
A couple of questions -
When you say skim coat the drywall with mastic do you mean coat the whole area and let it dry and then apply another layer with a notched trowel to tile?
In another bathroom I used thinset on the greenrock and then read that that is not good practice. 15 years later it still looks good so I guess it can't be that bad, but to play it safe should I only use mastic on drywall in the future? It is certainly easier to use.
Thanks.
Your use of the plastic on the interior face of the studs is determined by the overall design of the wall and climate and insulation, not by the use of any particular interior surfacing material. If the decision to place the plastic over the FG batt was right for the house then it is still right.
Sounds like you got your answers about vapor barriers.... Going back to the title of your thread, "Moisture resistant Drywall" something else for you to check into is mold resistant drywall which I guess is just MR drywall with some added resistance. Around here all the building supplies and drywall suppliers are stocking it and some send that out by default if you ask for MR. Some I got is purple, or some other wierd colors other than the regular "green board". Seems worthwhile to me.
I saw something about mold resistant drywall and I believe that it has a fiberglass face and it is hard to get a good painted surface without a skim coat.
I don't know if this is the same as the purple stuff you used.
I've seen (and used) the
I've seen (and used) the fiberglass faced stuff too but that is something different than the purple stuff I was refering to. Purple stuff is just paper faced.
I say he's smoking something.
I say he's smoking something. First, you should not use MR board as tile backer in a wet area (ie, shower). So (presuming you're not doing that) what you have is a regular wall with regular "moisture drive". If the insulation is adequate, the temperature of the drywall will be sufficient that there will be no condensation in it once "equilibrium" as been achieved. Moisture will not build up and degrade the drywall.
The old "double vapor barrier" bugaboo is mostly fiction. You can have five vapor barriers and they won't cause problems so long as they're on the warm side. And a single vapor barrier WILL cause problems if it's on the cold side.
As someone else said, if code in your area calls for a vapor barrier (because of your climate zone), install one -- don't depend on a layer of thinset or whatever.
I don't really want to call
I don't really want to call my building dept. as they are pretty tough to deal with and I am not getting a permit for the bathroom. where I live each bathroom adds a substantial tax burden to an already high property tax.
I live in New York, Long Island to be precise, so we have a temperate climate and range from high 90s, both heat and humidity, during the summer to near 0 with snow during the winter.
The room was designed as a bathroom but shown on the plans as a storage room. The outside wall, only one, has vinyl siding (sorry) over rigid foam over tyvek over 3/4" plywood. The stud cavities are filled with fiberglass batt insulation and the kraft facing has been sliced to render it a non vapor barrier. Over that is a poly sheet stapled to the studs and then sheetrock. I removed the sheetrock and poly and some insulation to do some electrical so I can easily do it the "right" way if I can figure out what that is.
So far it seems split between using the poly behind the tiled part of the wall and not using it.
How about a tiebreaker or a coin toss?
Thanks.
As an aside - why, after writing my reply, is there not "post" option. I only have "save" and "preview" options. I hit "save" and it seems to post. Am I missing something?
A.Green board is Mildew Resistant not Moisture Resistant. It should not be used in wet areas and never on ceilings - the green chemical weakens the surface of the board according to USG.
B. The "purple" board others are referring to might be Blue Board, intended for plaster veneer applications.
C. Vapor barrier must be on the warm side of the wall before the insulation. Insulation must be kept dry.
D. A vinyl mastic can be considered a vapor barrier. Is this what you're using or are you going use a mudset?
E. We use Hardibacker for ALL the wall bathroom walls. This is because we're tired of the architect changing the specs after sheathing, but also because we have had zero failures using HB. We tape and skim where paint will be applied. Yes, the skimming is an additional step but it saves us money over the year.
F. For the vapor barrier, we use 4 - 6 mil plastic sheets, but I prefer 15lb felt. Old school, sure, but it works and is easier to apply. Vapor barrier is full height, regardless of materials. All wet and exterior walls receive a vapor barrier.
G. Leave existing insulation. Apply your own vapor barrier as you are liable for the new wall performance.
Hope this helps,
Frankie
As far as using hardi on the walls - does that come in bigger than 3x5 sheets? The outside wall is about 10ft long x 8ft high and the studs are placed for 16"oc sheets.
What do you use to skim coat the Hardi? I am not going to tackle that myself but I want to make sure the material is correct.
It is my own house so I want it done right and the cost is not that much of a factor since it is only an 8x10 room and I figure the right material can't add much to the job.
I was going to use mastic since I understand that is what to use on drywall. If I go the hardi route I would use thinset.
The wet areas will all be some sort of CBU with thinset and will be tiled floor to ceiling.
The other, dry parts, will be tiled to about 4ft from the floor and above that will be painted.
Thanks.
I buy HB in 4x8 sheets, but not from HD. This I get from a lumber yard. Where in LI do you live? I know Marjam carries it.
I skim with joint compound, same as on gypsum wallboard. We skim because we want the entire wall surface to look the same. HB has a texture which is different than a taped joint.
Go with HB. If the wall is dead flat and plumb and does not require any adjustment at time of tile install, use mastic. Then it's a no-brainer. Just remember, mastic is not a vapor barrier and is still soluble until it cures. I would wait one week for mastic behind tile to cure.
HB is cool for wet areas.
Just so we're on the same page - HB is considered a/ same as CBU.
Oh, one last thing. If you are tiling to 4' then don't have the sheathing joint at 4'. Adjust the horizontal joint so it is a minimum of 6" away from the tile edge. Behind the tile is preferable. Since the room is 8' tall, you may want to install the sheathing vertically if using 4x8 sheets.
Frankie