Mold inside exterior walls, WTF!!!

[Baldrick]

This is quite a shocker to me and my wife. We have been working on our house for a while and finally got to doing some interior finish. Opted out of drywall and went with 3/4" TG wide pine flooring. We found mold growing on the inside of exterior sheating, as we pulled fiberglass insulation to run some CAT5 before installing wood finish. We now checked several areas and mold is in almost every stud bay.

Profile of the wall looks like this (all new materials, foundation up):
- no siding yet
- windows not taped yet
- 1" polyiso foam, foil side to the house, not taped yet (I am debating this as the cause of a problem here)
- housewrap
- 1/2" OSB
- 2x6 studs
- R-19 unfaced batts between studs
- at some areas we had 4mil plastic, to avoid contact with fiberglass (but mold is also in spaces without plastic)

Mold is growing from the sill plate up, about 4ft high. It's also present under insulation between floor trusses (above the space we are talking about) and in the walkout basement wall, where we actually used 1" pink foam (taped at the seams) - which has damaged my theory that it happens because we haven't taped polyiso.

I am literally speechless and devastated... what do I do next?
Pull insulation, spray with borax/peroxide mix?
Tape seams outside, tape windows?

Help!

[johnfrwhipple]

Does the sheathing on the outside of the house have holes for natural convection between all the stud bays?

Here in Vancouver you are required to have a hole or slot in two locations to help with ventilation between the wall studs. Often this is achieve by not placing the sheathing tight to each other. Or a few holes drilled in in areas like under the window framing.

Can you post a picture?

JW

[Baldrick]

Nope, no holes. It's first time I head about it. No mention of it by any code here in WI.
Sheeting is tight, all vertical joints at the studs. Typar on the top and then foil faced foam.

Got air cleaner with ozone maker running at the furnace, just in case.

[Dana]

With foil faced iso on the exterior of the stackup and poly on the interior side, the assembly has effectively ZERO drying capacity (none!). Any moisture that gets in, by whatever path (bulk-moisture wetting behind the iso at window flashing, or capillary wicking up from the concrete foundation, etc) has no means of escape.

But without an interior side vapor retarder, with only R6 rigid foam the sheathing's average temp during mid-winter will be well below that of the conditioned space air, and without a vapor retarder to slow the rate of vapor diffusion through the wall it will load up with moisture, creating mold conditions. With ~R11 or more you would have been fine in any WI climate. With 2" polyiso on the exterior and R19 in the stud bays the sheathing mid-winter average stays above the ~40F dew point of interior air, limiting it's moisture uptake. Even in the warmest parts of WI a mere inch of iso is not enough for a 2x6 wall, but it would be enough to protect 2x4/R13 walls.

Bottom line- with your stackup and climate mold problems are all but GUARANTEED to occur, but there are a few ways out of this mess.

Most of Wisconisin is in US climate zone 6, but northern WI is in zone 7:



The IRC-code prescribed minimums for exterior R that would protect framed walls using only the interior latex paint as a vapor retarder can be found here.

The pink foam is XPS, and is semi-vapor permeable at about 1 US perm,or just a bit above, which is pretty vapor-tight, but not a true vapor-barrier. Below grade you have issues with both groundwater moisture year round AND interior moisture drives in winter, but if it's foam/caulk/tape sealed at all seams and edges 1-perm would usually be sufficient to protect a band joist or sill plate from interior moisture drives. But if it's not air sealed to the interior and convection currents can take place, the amount air-transported moisture will far exceed the amount of moisture that can get through via vapor diffusion alone.

Air tightness to the interior is critical to any of your wall assemblies, but the foil facers on the iso are true vapor barriers- you MUST have at least ~0.5 perms of permeance for the assembly to dry at any reasonable rate, but it also has to be 1 perm or less to avoid moisture loading of the sheathing via vapor diffusion. RIP OUT the 4-mil poly- it's too vapor tight to be part of any solution. Ripping it out means completely removing it, not just ripping some holes- vapor diffusion rates are a function of exposed area, not it's air tightness. A slashed up sheet of poly is useless as an air barrier, but will still block vapor diffusion, and thus blocks drying, trapping moisture in the wall about as effectively as an un-slashed sheet of poly. Once you've cleaned up any mold issues in those cavites you have a couple of options:

A: seal the sheathing on the interior with a flash-inch of closed cell spray polyurethane foam (ccSPF) then compress an unfaced R19 batt into the ~4./5" nominal space or fill it with cellulose (better), and paint the interior of the gypsum with standard latex paint. The ccSPF forms a non-wicking condensing surface from interior moisture drives, yet is still has 0.8-1.2 perms, which both limits the uptake of moisture, yet gives the sheathing a reasonable drying rate when the temps warm up.

B: Fill the cavity with high density R21 fiberglass, cellulose, or R23 Roxul batts, or open cell spray polyurethane foam, and use "smart" vapor retarder such as Certainteed MemBrain between the gypsum and insulation. (There are others, if you can't find a source, but MemBrain seems to be the most widely availble) When the air in the house & wall cavity is dry (as in winter) smart vapor retarders run <1 perm, but when the sheathing warms up in spring and starts releasing it's moisture into the cavity, it become vapor open, and the drying rate is limited by the permeance of your interior wall finish. Latex paint runs 3-5 perms, and is preferred. Vinyl or foil wallpapers run 0.03 perms, and are as big a mold-disaster as 4-mil plastic.

On the parts that have no 4-mil poly, first make it as air tight as possible- taking care to caulk all electrical boxes (including the wiring holes) with fire-rated caulk, caulking behind all kick-boards and trim molding, spackling any nail holes, etc., sealing any gaps to the gypsum behind window & door casings with low-expansion can foam. Then re-paint, starting with a layer "vapor barrier latex" primer (there are several vendors, and it tends to run ~0.5 perms give or take 0.1 perm), which is on the vapor-tight side, but still allows the assembly to dry toward the interior, albeit at a much slower rate that with solutions A or B.

[Baldrick]

We hang 4" poly because first, I had no idea that polyiso would act as a vapor barrier. We needed it though, to protect us from the batt fibers. I am ripping it down as I go with the wall finish, which is 3/4"x7" TG pine flooring. No drywall planned anywhere in the house.

Finished wall profile at this time is:
- 1" polyiso foam, foil side to the house, not taped
- typar housewrap
- 1/2" OSB
- 2x6 studs
- R-19 unfaced batts between studs
- 3/4" TG pine flooring

As we are inspecting our walls, mold is present at most of places, behind the R-19 batts. Doesn't matter if there is or was plastic there or not.

My plan of action for now is to wait until weather outside hits 50s during the day and:
- tape all polyiso seams, tape windows (all outside)
- remove R-19 batts
- wash the OSB with boric acid and hydrogen peroxide mix, maybe follow up with Sporicidin, allow to dry
- spray 2 coats of PermaGuard
- reinstall insulation and carry on with wall finishing

With seams taped, sheeting will be only able to dry from the inside of the house. PermaGuard is not supposed to create a vapor barrier film, but 1" of closed cell sprayed on probably would. What do you think?

[Dana]

Air leakage to the exterior does fairly little for drying capacity, but it can draw HUGE amounts of air-transported moisture into the stud cavity from a less-than-air-tight interior side. On the 1" iso it's 1000x better if it's foil side out, seams taped to make it as air-tight as possible. If you can mastic-seal the OSB and caulk/foam very edge and hole, as well as caulking the studs & plates to the inside of the OSB with acoustic sealant or 1-part foam, making it your primary air barrier that's good too. All sides of each stud bay need to be air-sealed, and using the foil facer of the iso as a secondary air barrier is worth it/ since the foil facer is still a vapor barrier you would have to establish a cavity vented to the outdoors between the iso and OSB to get appreciable drying around it, but a vented cavity would defeat it's insulating function.

If you don't put a better air-barrier than t & g planking on the interior side of the wall you're screwed, since there will be very substantial convection occurring between the interior air and the air in the cavity. A smart vapor-retarder such as MemBrain really works, but it's not super-cheap. Another approach would be to use a semi-permeable (not perforated) 1/4"-1/2" XPS fan-fold siding underlayment, taping all seams with housewrap tape and sealing all edges and electrical boxes (including the wiring side with can-foam. But read the specs- you need a version that is under 1.5 perms, preferably 0.8-1 perm. (The Pactive 3/8" goods would work, but not the 1/4", the half inch fan-fold Owens Corning stuff would work too.

To reduce convective transport of moisture into the cavities you need to go with something more air-retardent than low-density fiberglass- R19 batts are the 3-legged mangy dog of the wall-insulation world- it's a fluffed-out R13- exactly the same weight per square foot as an R13 batt, which makes it 5x as air permeable, it's as crummy as it gets for 2x6 framing insulation by almost any measure. The better cavity-fill alternatives (in descending order of max-air-retardency to least ) are:

Open cell spray foam (which would run ~$2.00-$2.25 per square foot, installed)

Dense-packed cellulose (similar, cost o.c.foam, if blown in mesh)

Dense packed fiberglass (1.8lbs density Optima, L77, or Spider- more expensive than o.c. foam, but higher-R)

Damp sprayed cellulose (usually cheaper than o.c. foam)

R23 rock wool (Roxul is available at Lowes and Home Depot these days, other vendors are usually from commercial distributors

R21 "cathedral ceiling" fiberglass batts (any vendor, but definitely NOT low-density R22 or R23s).

The air retardency factor slows both exfiltration transport of interior moisture into the cavity, as well as convection. With open cell foam it's essentially blocked. Closed cell foam works for the exfiltration part too, but going with 5" would be ungodly expensive at $5/foot, whereas a flash-inch (at a buck a square foot) + kraft-faced R19s compressed into the remaining 4.5" would be sufficiently protective, since an inch of foam puts a ~1 perm vapor retarder between the interior and OSB, and puts more R outside the fiber layer, dramatically reducing the number of hours that the exterior of the fiberglass is below the dew point of the conditioned space air. Moisture will still get in past the kraft-facer on the batt, but the kraft facer behaves somewhat like a smart vapor retarder- it's ~0.4 perms when dry, but can be as high as 2-3 perms when it's 70% RH on the cavity side (a moisture point when mold takes off like crazy), which allows it to dry more quickly.

At only 1" closed cell foam still has 20x the drying capacity of poly sheeting. At 2" it still has 10x, so unlike polyethene and foil, it's not a true vapor BARRIER, but a vapor retarder. The same is true of the XPS fan-fold stuff I recommended above. Latex paint on gypsum has about 5x the vapor permeance of 1" closed cell foam, but that's too vapor-open for your stackup and climate (but latex paint would work just fine with that stackup in the more temperate US climate zone 4.)

The reason there is mold either with or without the plastic I tried to explain in the previous post but I'll boil it down to two sentences:

1> In the no-plastic case, in YOUR climate, with only 1" of exterior iso the OSB sheathing stays below the dew point of the conditioned space air for way too many hours over the course of a winter, loading up with moisture to levels that can't dry quickly enough in spring to avoid mold growth.

2> Where there is plastic on the interior, the wintertime absorption rates are much slower, but the drying rates are now YEARS long, rather than months, so it gets moldy too- it's a moisture-trap.

Dealing with moisture issues in order of importance:

1> Getting the flashing on windows/doors lapped correctly to the housewrap is of absolute primary importance, 10x more important than any of the rest.

2> An order of magnitude below that, but still critical, air sealing all layers of the wall, all 6 sides of the stud bay, including the interior side, and making the insulation layer air-retardent, to limit air transported moisture from accumulating in cold OSB.

3>Also important, guaranteeing that the OSB has a path to drying via vapor diffusion, but not so vapor open as to load up excessively over the winter months. How vapor open you can keep the interior side without loading up the OSB in winter is a function of the local climate, and the ratio of exterior foam-R and cavity-fill R.

A lot of mis-placed focus in the 1980s was on limiting vapor diffusion of moisture rather than air-sealing, with rampant mis-use of polyethylene vapor barriers that made the very mold issues they were intended to prevent even worse. Air sealing the cavities from the interior side is FAR more important, but requires a lot of detail work to get right, but it's do-able. A square-nch of air leak into cheapy-R19-insulated cavities moves more moisture to the sheathing than a whole wall's worth of latex-painted wallboard. Wallboard is much easier to air-seal than t & g planking, which is why you really need a smart-membrane type vapor retarder that is semi-permeable to make your wall work. (That's true even if you bite the bullet and go with open cell foam, since 5.5" of foam is still over 10 perms which would load the OSB in your climate, whereas MemBrain is ~0.8-1 perm in winter, and only becomes vapor open when the OSB releases it's moisture during warmer weather. It loads up slow, dries fast- fast enough that mold can't get going very easily.)

[johnfrwhipple]

Nope, no holes. It's first time I head about it. No mention of it by any code here in WI.
Sheeting is tight, all vertical joints at the studs. Typar on the top and then foil faced foam.

Got air cleaner with ozone maker running at the furnace, just in case.

Not sure about your local code but here in Vancouver there is a massive crack down on exterior wall construction.

Rain Screens

Window Flashings

Air flow.

It is required to mock up these aspects and call an inspection before proceeding.

Sounds like maybe your home is not "Breathing" from the outside???

JW

[Dana]

Wisconsin is a heluva lot colder than Vancouver, and has different wall stackups that work (or don't.) Even if he had rainscreens and 1" of semi-permeable foam on the exterior, it could still have problems in a WI climate, but would be fine in Vancouver.

In WI it takes more than a rainscreen to keep OSB sheathing sufficiently dry. A rainscreen would be a good start, and would be just fine with if it were unfaced EPS, but no foam with a foil or vinyl facer would provide sufficient capacity to dry toward the exterior, taped or untaped. With NO exterior foam, a rainscreen, and an interior poly vapor barrier it works, but only if the vapor barrier is also made extremely air tight.

The most resilient stack up would be 2" of exterior foam, no interior vapor barrier (but air-tight gypsum.)