Vapor Barrier or Not

[dvddiva]

I now this question has been asked a thousand times, but it seems
even on this site, some people say yes and some people say no.
Here is my specific situation. I live in a cold climate ( Canada). My home was built in 1954
and has siding on the outside.
I am going to put in a corner shower made by Maax,model- Azure ( 34" wx 42"l x81"h)
The 42" portion is on an outside wall. That outside wall is 8 feet long.
I plan on putting unfaced pink fiberglass insulation between the 2x4 studs the
entire length of that outside wall. Inside the shower stall itself I planned to
install DensShield drywall and cut it to size to fit with no seams except in the corner
where the other piece of DensShield will butt up against it. I will be using Laticrete
Hydro Barrier ( this product is similar to Redguard) on that corner joint.
I have been told NOT to put a vapor barrier behind the Dens Shield because it has
a vapor retardent in it. (On top of the Dens Shield I will be putting a Canadian product
called Duma Pan Panelling). Is this correct ?
The rest of the wall above the shower stall and beside the shower stall I planned
on using CGC 1/2" Sheetrock Mold Tough drywall gypsum 4'x8' panelling. This product
is described as ideal for bathrooms because it has been formulated for high humidity
areas and is moisture and mold resistant. I planned to NOT put a moisture barrier
behind that either. I will simply be painting over top of this product. Is this correct ?
There will be no tiles used anywhere in this bathroom. The bathroom is 8'x7' and 8'high and there
is a small window.There is no exhaust fan.( please do not suggest a fan ).
So, the bottom line is, I have been told NOT to put a vapor barrier anywhere on the
exterior wall. If there is any further information that I can supply please let me know.
Any help on this would be greatly appreciated.
Thanks.

 

[Dana]

What type of siding, what is the nearest city (for weather & climatedata).

For fiberglass insulation to function to spec it needs a tight air barrier on both sides, and have no large gaps or compressions where air can flow unimpeded. If there is any kind of air gap adjacent to the fiber there will be convective air exchange between the fiber & adjacent space, which speeds up the transfer of both heat & moisture, which can potentially load up the sheathing with moisture in winter. At the very least extend the dry wall to cover the framing and insulation completely, and air-seal the seams and edges. Missing wallboard behind tub surrounds is one of the most common HUGE air leaks in quickly built tract housing, a construction error you don't want to commit.

A layer of standard latex on the wallboard is a sufficient vapor retarder if A: the siding is inherently back ventilated (eg vinyl siding), and B: You are in a southern Ontario location. In colder areas "vapor barrier" latex may be a better idea, depending on the wall stackup. Vapor barrier latex runs about 0.5 perms- still about an order of magnitude more vapor open than 6 mil polyethylene, but an order of magnitude more vapor tight than standard latex paints. The wall assembly can still dry in either direction, but it won't pass very much of the intensely moisture drives that occur during showers onto the sheathing to create a mold problem. Since the shower runs a low duty-cycle, the semi-permeability of the v.b. latex doesn't much matter. But should the framing cavities ever get damp, it can still dry.

The kraft facers on batts run about 0.4 perms when dry, but about 2 perms when wet, but they are damned near impossible to make air tight, and air-tightness is far more important than vapor permeance. While you have the cavities open, prior to insulating caulk the framing to the sheathing (can I assume the sheathing is plywood, in 1954? If it's ship-lap or something, say so), as well as seam on the doubled top plate, and between the bottom plate and the subfloor with an acoustic-sealant type caulk (that stays flexible forever.) The tighter the air-barriers on both sides of the fiberglass, the less air-transported moisture ends up soaked into the wood inside the wall assembly.

 

[Sponsor]

 

[dvddiva]

Thanks Dana. I guess I wasn't that clear. The house is vinyl sided and behind that is plywood.
I live in southern Ontario in an area called Fort Erie. ( right at the border with Buffalo NY).
The entire exterior wall will be covered with the moisture retardent 1/2" sheetrock except in the shower
stall area where I will be using DensShield. The insulation does NOT have a backing on it i.e., kraft paper or tar paper,
or anything else attached to it. It is solely fiberglass insulation which will be placed along the entire exterior wall.
On the sheetrock I will be sealing the sheetrock joints with Laticrete Hydro Barrier compound and Laticrete 6" wide
fibreglass waterpfoofing membrane.
The DensShield will be cut to size to fit shower, so there will be no seams except where the 2 DensShield pieces meet
in that corner.On that corner joint I will be using the same Laticrete products as used on the sheetrock.
I totally forgot to mention that yes I will be using the acoustic sealant as you suggested.
So now that I have given you the complete picture, do I use a vapor barrier or NOT ?
Thank you again for any help.
P.S. I will be putting 2 coats of paint on the moisture resistant sheetrock.
P.P.S. If you are not familiar with the Duma Pan product ( Can.) that I am using on top of the DensShield you can google it.

 

[Dana]

If you were going to tile the DensSheild it would form a class-II vapor retarder, and would be more than sufficient protection for the plywood sheathing. The vapor permeance without tile is not specfied, but it's probably under 2 perms.

The Duma-Pan is an extreme vapor barrier, more vapor tight than 6-mil poly, so adding a poly layer would create a moisture trap- don't do it.

The Sheetrock Mold Tough is highly vapor permeable, but with two layers of standard latex becomes a class-II vapor retarder, which is good enough for your climate as long as you have a back-ventilated siding such as vinyl. Fort Erie is the equivalent of a US climate zone 5 climate. The precriptive IRC stackups that allow you to use standard paint as the interior side vapor retarder are:

------------

Vented cladding over wood structural panels. (vinyl siding over plywood qualifies)
Vented cladding over fiberboard.
Vented cladding over gypsum.
Insulated sheathing with R-value ≥ 5 over 2 × 4 wall.
Insulated sheathing with R-value ≥ 7.5 over 2 × 6 wall.

------------

Where you CAN get away with only a class-III vapor retarder on the interior, you SHOULD, since that gives the assembly a much higher drying rate, making it more resilient to moisture drives from either side. It's fine to have the extremely low-permeance section at the Duma-Pan, as long as you don't trap moisture with yet another vapor barrier. If your code inspectors give you a hard time, direct them to the IRC documentation, or use "vapor barrier" latex primer, which would reduce the permeance to about 0.5 perms (the rough equivalent of a kraft facer), which would allow at least SOME drying toware the interior.

And as long as both the interior sheet-goods and exterior sheathing are tight to the fiber with no gaps and air tight you can get the thermal performance without the air-transported moisture issues.