Mold in wall - advice on cleaning

[WolfB]

Howdy All,

Remodeling the master bath and found some mold in a couple of the exterior wall cavities.
Being that we live in northern WI, wondering what the best course of action is to clean it and prevent it from coming back? Old man winter doesn't want to give up and we are currently in the 20s here. Don't know if they helps or hinders the cleaning/prevention process.
Currently I have the exterior wall open with just the insulation in the bays.

Items of note:
PREVIOUS wall construction, from outside to in: vinyl siding, house wrap(Typar I believe), OSB, studs and R19 fiberglass batts, poly, and then drywall.

NEW wall construction, from the outside to in: all the same, except I will be I am replacing the poly with Membrain and want to put PVC shiplap up instead of drywall.

Thoughts and ideas welcome.
Thanks,
Brian

 

[Dana]

MemBrain is 2-mil nylon, which is not a very robust air barrier. Shiplap isn't even close to being an air barrier (though PVC is extremely vapor retardent.) A layer of thin wallboard detailed as an air barrier would still be a good idea.

If you're ripping open the walls, take the opportunity to pull the batt insulation and caulk the sheathing to the framing over the full perimeter of each stud bay with polyurethane caulk (not the cheap stuff.) Caulk the seams between any doubled up framing such as jack studs, window headers, top plates etc. Put a bead between the bottom plate and the subfloor too. Any seams between the sheathing should be taped with the appropriate tapes- if the tape adhesion to a moldy/dirty sheathing seems sketchy paint over it with 1/8" of fiber-reinforced duct mastic to keep the tape from separating over time. The more air-tight the sheathing, the less air gets pulled into the wall cavities from wind pressures, etc.

R19s are really only R18 when compressed to 5.5" in a stud bay- it's one of the crummiest excuses for insulation sold. Even the box stores are carrying R23 rock wool designed for 2x6 framing now, and it's worth it. It's far more air retardent, and completely fireproof (fiberglass melts). Even for a small project it's worth buying a purpose-made batt knife to trim the batts to a perfect fit. When installing the batts split them to accommodate any wiring or plumbing. Tuck them in at the corners and edges to ensure there are no long skinny gaps where the framing meets the sheathing, then tug them out gently until they're just proud of the stud edges for a compression fit when the next layer goes up.

Sculpting around electrical boxes and air-sealing the electrical boxes is also critical. It doesn't take much of an air leak to move a lot of moisture in, but the moisture gets out primarily via vapor diffusion. With PVC on the the interior the primary drying path will be vapor diffusion to the exterior through the OSB into the air space behind the vinyl siding, which makes air tightness to the interior critical.

 

[Sponsor]

 

[WolfB]

MemBrain is 2-mil nylon, which is not a very robust air barrier. Shiplap isn't even close to being an air barrier (though PVC is extremely vapor retardent.) A layer of thin wallboard detailed as an air barrier would still be a good idea.

If you're ripping open the walls, take the opportunity to pull the batt insulation and caulk the sheathing to the framing over the full perimeter of each stud bay with polyurethane caulk (not the cheap stuff.) Caulk the seams between any doubled up framing such as jack studs, window headers, top plates etc. Put a bead between the bottom plate and the subfloor too. Any seams between the sheathing should be taped with the appropriate tapes- if the tape adhesion to a moldy/dirty sheathing seems sketchy paint over it with 1/8" of fiber-reinforced duct mastic to keep the tape from separating over time. The more air-tight the sheathing, the less air gets pulled into the wall cavities from wind pressures, etc.

R19s are really only R18 when compressed to 5.5" in a stud bay- it's one of the crummiest excuses for insulation sold. Even the box stores are carrying R23 rock wool designed for 2x6 framing now, and it's worth it. It's far more air retardent, and completely fireproof (fiberglass melts). Even for a small project it's worth buying a purpose-made batt knife to trim the batts to a perfect fit. When installing the batts split them to accommodate any wiring or plumbing. Tuck them in at the corners and edges to ensure there are no long skinny gaps where the framing meets the sheathing, then tug them out gently until they're just proud of the stud edges for a compression fit when the next layer goes up.

Sculpting around electrical boxes and air-sealing the electrical boxes is also critical. It doesn't take much of an air leak to move a lot of moisture in, but the moisture gets out primarily via vapor diffusion. With PVC on the the interior the primary drying path will be vapor diffusion to the exterior through the OSB into the air space behind the vinyl siding, which makes air tightness to the interior critical.

Thanks for the info, Dana!

Don't know if this makes a difference, but the PVC shiplap in in 6" board form, not solid sheets.
Also, I forgot to ask for any advice on actually cleaning/preventing the mold. I had read somewhere that borax does a good job of killing/preventing mold.

Thanks again. Brian

 

[Dana]

I assumed the ship lap was plank not sheets which is why it can't be detailed as an air barrier. But just because would (on it's own) leak copious air doesn't mean it's permeable to water vapor. Vapor diffusion is surface area x permeability function- there isn't much surface area in the crackage, and nothing to drive convection around the planks, so the extremely low permeability of the vinyl means water vapor can't easily get in or out by diffusion. But water CAN get in via air leaks, then take forever to get back out.

Borates are indeed anti-fungal, but most important thing is to just keep the wood dry enough to not support mold. Keeping it under 20% moisture content is critical, under 15% is better, and realistic if it's air-tight to the interior, and has back-vented siding such as vinyl.

 

[WolfB]

Dana,
Thanks again for your insight.

So do you think my best plan would be to caulk all the junctions on the studs and OSB with something like Loctite PL S40.
Then fill the bays with well trimmed Roxul.
Then use the Membrain on the interior and then put up the PVC shiplap?

 

[Dana]

Loctite PL S40 is a good wood-to-wood polyurethane sealant.

MemBrain is a lousy air barrier- it's only 2-mils thick making it easy to puncture, and with the PVC blocking drying toward the interior the variable permeance of MemBrain buys you nothing.

Use something more rigid and durable than sheet nylon as the interior air barrier, like a layer of wallboard, or 1/4"-1/2" plywood/OSB, or MDF. If MDF or wallboard paint it with vapor barrier latex to lower it's vapor permeance (not necessary for plywood or OSB, which are inherently variable permeance, low perm when dry.) Caulk all of the edges of the air barrier sheets to the framing, tape any seams that aren't supported by framing with the appropriate tape.

 

[WolfB]

Thanks again, Dana.
You time is greatly appreciated.

Just for clarity (i'm easily confused), I can just use something like wallboard/OSB/plywood and NOT use the Membrain?
Or do I put up the Membrain and then the wallboard/OSB/plywood?

Brian

 

[Dana]

Like MemBrain, the permeance of OSB & plywood varies with the RH of the proximate air. If it's damp enough inside the wall cavity to support mold the permeance of OSB & plywood is higher higher than standard latex paint. When dry OSB & plywood are less than 1 perm, which is good enough even if there is convection around the vinyl ship lap. The permeance of unpainted wallboard is high, on the order of 30 perms, but if painted with "vapor barrier latex" it drops to about 0.5 perms, and while it's probably not necessary to paint with v.b. latex (due to the extremely low permeance of the PVC) it's cheap insurance against potential air convection around the PVC.

The vapor permence of 6-mil polyethylene is 0.05 perms, an order of magnitude lower than v.b. latex. At even 1/8" thick PVC is on the order of 0.00001 perms (I'm not sure that's even enough zeros), several orders of magnitude below that of a standard vapor barrier, but it leaks air. A square inch of air leakage moves as much moisture or more as 250 square feet of wallboard painted with standard latex paint (3-5 perms), which is why you need a reliable air barrier behind the PVC. But the vapor permeance of that air barrier hardly matters unless there is a lot of air moving between the PVC layer and the air barrier layer. It's not worth spending the money on MemBrain in your stackup- there is NO drying toward the interior via vapor diffusion- the wall has to dry toward the exterior, through the structural sheathing (which is low permeance when dry and only medium permeance when wet.

At 60% RH MemBrain is about 5 perms, as vapor open as standard latex.

At 60% RH plywood & OSB are slightly more vapor open than MemBrain, but about 1 perm at 35% RH, which is good enough. The cavity air will be drier than that during winter, and the average indoor RH will be under 35% when it's cold outside too, unless you are actively humdifying the place (always a bad idea.)

But note that 6 mil polyethylene is always low permeance (the green line), and half-inch XPS (the brown line) is always ~2 perms- higher than you want in winter, a bit tighter than you'd want in summer, if you didn't already have the ultra-low perm PVC. Vapor barrier latex on wallboard would be tighter than the half-inch XPS, cheap insurance, but probably not really necessary.

If it's only a small section of wall and you're doing the air barrier with wallboard it's not worth buying a 5 gallon bucket of vapor barrier latex. A couple coats of standard latex primer would bring it down to ~3-5 perms, which should be good enough when combined with the PVC layer. But if the PVC is ever removed it'll need vapor barrier latex.

 

[WolfB]

Thanks again, Dana.

I'm learning ALOT! More than my brain can handle, that last post took me a while to process.
I was hoping to put the PVC directly over the Membrain mainly because we have a window in this wall that is 1/2" proud of the studs and the PVC would flush up to it nicely.
Since being educated by Dana, I've come to understand (in a rudimentary capacity anyway) the importance, and difference between, vapor and air barriers.

My plan now for this exterior wall stack is from exterior in:
Vinyl Siding
Typar
1/2" OSB
Rockwool R23 in the stud spaces
Membrain (because I've already purchased it and it will be necessary if the PVC shiplap every comes down but won't hurt in the meantime?)
1/2" moisture resistant drywall with 2 coats of latex primer
1/2" PVC Shiplap

Good?
Brian

 

[Dana]

That will work!

With the MemBrain in place it doesn't matter whether you paint the drywall or not (it's not the vapor retarder if you have MemBrain, and it's on the warm side of the insulation), but it has to be detailed as an air barrier.

It's important to detail the MemBrain as an air barrier too, making the seams & overlaps only where it's supported by the framing and won't flex, breaking the tape seal over time, etc.

 

[WolfB]

Thanks again, Dana, for be so patient with me.
With your extensive help, I have a plan.

One final question, I have a gap between two 2x6s that runs from the top plate to the bottom plate that I am thinking of filling completely with Great Stuff Big Gap Filler. It varies from about 3/4" at the bottom to 1 1/4" at the top, so filling with Rock Wool be problematic.
Is there any problem filling that whole cavity with that product?

 

[Dana]

Filling those kinds of gaps are exactly what can-foam is made for- go for it! It's impossible to properly caulk-seal a sub-1" stud cavity to the sheathing, but can foam does the job. (It insulates as well, but the real value is the air-seal.)

 

[WolfB]

Again, thanks a million, Dana.
We followed the plan and completed the following:

1) Caulked all stud bays with Loctite PL S40
2) Filled a few voids with Great Stuff Big Gap and Window and Door expanding foam.
3) Filled wall with Rockwool R23 batts, (Great stuff to work with!)
4) Hung the Membrain.
5) Hung the Mold Resistant Drywall.

You mentioned to 'detail' the drywall as an air barrier. Is there anything specific beside taping, mudding and painting that is included in 'detailing'?
As for the gaps between the drywall and window, light fixture mount and outlet, do these need special attention other than the Membrain?

Brian

 

[Dana]

Here's a short primer on air-tight drywall. (You may have to sign up for a trial subscription to read it.) Here is another.

 

[WolfB]

Thanks again, Dana.
Great info.

Is DAP Extreme Stretch Crackproof Elastomeric Sealant a good choice for around boxes and drywall edges?

 

[Dana]

I've not used it or seen long term data on it, but it looks like a reasonable product for the application.