Bathroom Remodel Conundrum - Insulation for unconditioned space behind shower

[sirguynate]

I just took apart my bathroom and am in the middle of a bathroom remodel. I removed my fiberglass shower and behind the shower was insulation behind a vapor barrier. My understanding that a vapor barrier behind a fiberglass shower is going to create a mold issue (vapor barrier behind vapor barrier.) However, I have an issue:

WA State Code:
502.1.6 Moisture Control:

502.1.6.1 Vapor Retarders: Vapor retarders shall be installed on the warm side (in winter) of insulation as specified in the following cases.

EXCEPTION: Vapor retarder installed with not more than 1/3 of the nominal R-value between it and the conditioned space.

502.1.6.6 Walls: Walls separating conditioned space from unconditioned space shall have a vapor retarder installed. Faced batt insulation shall be face stapled.

The wall behind my shower is an unconditioned space - attic. The insulation there along with the plastic vapor barrier had some mold on it. I do not have an option but to install faced insulation. I just have no idea what R rating i need.

I am in zone 5, but if someone could give me a hint on the appropriate R rating, or perhaps some other solution that would be appreciated.

I included a very rudimentary "drawing" of what I am dealing with. (there is plastic sheeting over the insulation)

https://ibb.co/zf2x4Kc

 

[Jadnashua]

Tile and grout are not waterproof. Fiberglass is, or should be. There should not be any moisture sandwich installed as it was. Did you find evidence of any moisture trapped?

Depending on what you use for the walls of a new shower, there are considerations you should address. If you use a conventional shower, you'd want a moisture barrier behind the wallboard and probably not the facing of common fiberglass insulation. In that case, you could just use a razor knife and make some slits in it. Should you choose a surface applied waterproofing, you could do the same thing. There's vaporproof and waterproof, two different things.

 

[Sponsor]

 

[Dana]

In zone 5 a partition wall to an unheated uninsulated attic is treated the same as an an exterior wall, from an insulation requirements point of view. In US climate zone 5 the IRC calls out R13 + R5 continuous insulation as code minimum for walls, or R20 in 2x6 framing.

Furring out the studs to 5.0" with 2x2s and compressing R23 rock wool (designed for 5.5" deep 2x6 bays) into that space would meet the IRC code minimum requirements. At 5.0" the batts would still be performing at ~R21, and despite the 9% shorter thermal bridge through the framing than a full 2x6 depth the "whole wall R" performance would be the same as a 2x6/R20 code minimum wall.

A kraft facer is not a true vapor barrier-it's a "smart" vapor retarder, that becomes vapor open when the humidity of the proximate air is high, and less than 1 perm when dry. This is far superior to polyethylene sheeting, since it becomes a release valve of sorts. But it's impossible to make perfectly air tight without another layer such as gypsum board plywood or OSB.

Cutting slits in polyethylene sheeting doesn't change it's vapor retardency- it's still a true vapor barrier (blocks moisture transport via vapor diffusion), even if it's no longer air tight. Vapor retardency is about vapor pressure x surface area (not air pressure) , and a slit has no surface area to speak of. Without some convective drive or wind to move air through the assembly (not a good idea on many levels) it isn't going to dry via air movement either. If the existing polyethylene is serving as the only air barrier on the interior side (no wallboard?) the last thing you would want to do is to compromise it's air tightness.

Installing a true vapor barrier next to another true vapor barrier isn't a problem. As adjacent very low-R layers there is no water vapor drive, and even if it leaks a bit of air between the two they are the same temperature- there is no condensation. But designing it so that the wall assembly to dry via vapor diffusion toward the exterior into the unconditioned attic would be good if there is any substantial air between the fiberglass surround and the insulation layer, since humid air leaking around the fiberglass could conceivably be taken up as adsorb into the susceptible subfloor.

There is no science behind the "1/3 - 2/3 rule" that came from Canadian lore in the 1970s, and it's odd to find it in WA building codes. The IRC prescriptives in chapter 7 would allow R5 rigid foam on the exterior of a 2x4/R13 wall as long as there is at least a Class-III vapor retarder (= between 1-10 perms) on the interior side. But with vapor-impermeable fiberglass shower or polyethylene sheeting on the interior side you don't really want to do that.

Since I can't see the images, is the polyethylene sheeting on the attic side or bathroom side of the assembly? (I'm guessing bathroom side.)

Is the MOLD on the attic side or bathroom side of the polyethylene? (I'm guessing bathroom side here as well.)

Fiber insulation needs air barriers on all sides to hit it's labeled performance levels, and the air barrier on the attic side of the assembly needs to be at least semi vapor-permeable for the wall assembly to dry toward the attic. High density "cathedral ceiling" or rock wool batts can almost hit their numbers even without an air barrier, but installing 2-mil nylon (aka Certainteed MemBrain" or unpainted wallboard on the attic side of the assembly will work. Wallboard would be more robust AND more vapor open (=better) than thin nylon sheeting, but may be more awkward to install depending on how cramped the attic space is. With enough staples and tape, lapping the seams between sheets only where supported by framing it's possible to make it air tight. It's 2-3x more expensive than polyethylene, but it doesn't create a moisture trap the way polyethylene on both sides of the framing would.

 

[sirguynate]

If the existing polyethylene is serving as the only air barrier on the interior side (no wallboard?) the last thing you would want to do is to compromise it's air tightness.​

The framing behind the fiberglass shower has, from the floor up, 4 feet of sheet rock on the attic side. Above that there is insulation covered by poly sheeting with no wallboard on the attic side or interior side (8 foot wall).

Since I can't see the images, is the polyethylene sheeting on the attic side or bathroom side of the assembly? (I'm guessing bathroom side.)​

Correct, Bathroom side.

Is the MOLD on the attic side or bathroom side of the polyethylene? (I'm guessing bathroom side here as well.)​

Correct, Bathroom side.

It is a very small shower, 30x30, in an alcove - however there is 8 inches of wall in front of the shower between both walls so what I want to do is make a custom shower pan to get a 30X38 shower so my wife can have a little wiggle room. Code requires cement board, I was going to put that up as my wallboard and to use redguard/aquadefence as my barrier. I have checked all measurements and I am at the required code spacing for toilet/shower/vanity/ ect.

I don't want to put myself in a situation where I am setting the bathroom up to get mold.

Thank you for your help and advice!

 

[Dana]

If you can remove the polyethylene before installing the tile backer / cement board and replace it with MemBrain (box stores have started carrying it in my area in smaller rolls) the assembly will dry into the attic space whenever the humidity levels on the bathroom side reach mold potential.

RedGuard runs only 0.35 perms, which is VERY vapor tight (though not as tight as 6- mil polyethylene), and would allow seasonal drying, but at a glacial pace. That is sometimes a necessary compromise on an exterior wall that has to dry through plywood or OSB sheathing that needs to be protected, but when you have a vented attic space to dry into a 0.35 perm layer increases the mold potential compared to a somewhat more vapor-open vapor retarder would.

MemBrain has more than 10x the drying capacity when it's needed, but is still less than 1 perm when the adjacent layers are dry. At 50% relative humidity (= low mold hazard territory) it runs about 3 perms, and when it's drier drops quickly to 1 perm or less, so while it impedes moisture most of the time it won't trap moisture at mold-hazard levels: