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Community and Q&A

Drying out a wet dense-packed cellulose wall

severaltypesofnerd | Posted in Energy Efficiency and Durability on

What rules of thumb can I use for when a dense pack wall will dry out itself, and when insulation must be mechanically removed? Here in a 1938 stucco home.

The underlying cause was a pair of downspout nails installed in 1938, but angled somewhat downward. Chance are it leaked from the start, but maybe the rate went up as the nail rusted. In 2012 the wall was dense packed from the inside. Nothing remarkable appeared on an IR camera: that wall was looked at a month ago. Then last week, 77 years of paint peeled itself off the interior wall leaving baby smooth bare and slightly clammy plaster.

The house is painted stucco exterior, diagonal old growth sheathing, painted interior lathe and plaster. The diagonals mean that 5-6 stud bays could be involved, two have been exposed so far. The exposed two were full of inconsistently wet cellulose and completely wet sheathing. There seems to be no visible remaining paper on that exterior wall, but wood rot is fairly minor. The exterior stucco appears to be saturated (too wet to caulk to).

Presumably the relatively permeable wall dried itself for the first 75 years, but the addition of dense pack held water until the point of failure. The dense pack smells moldy (the stink is horrible).

Hopefully a few people in addition to Martin can weigh in?

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  1. GBA Editor
    Martin Holladay | | #1

    My advice is to remove and dispose of all the damp cellulose.

    Without more information, it's hard to know what exterior measures, if any, are needed to avoid the problem in the future. Factors include certain exterior details (like the roof overhang width) and the local climate.

    As I have written many times, stucco is a relatively risky cladding, especially over an insulated wood-framed wall. Ideally, stucco should have an air space between the stucco and the wall sheathing, along with two layers of WRB to protect the sheathing.

    Here is a link to an article with more information on the topic: To Install Stucco Right, Include an Air Gap.

  2. severaltypesofnerd | | #2

    Martin: In a 1938 home, there's no chance to install a drainable WRB. And at that point it's very difficult to remove the dense pack from the walls, though clearly insulating this home was a mistake. The 5 or 6 stud bays of wet newspaper are one thing, but removing it from the entire house is rather an undertaking.

    In this case it's a 1938 Spanish mission style home with no overhang whatsoever (the walls form a parapet for the flat roof). Climate is non-freezing central California.

    Was this this house ruined for the long term by the insulation effort?

  3. GBA Editor
    Martin Holladay | | #3

    Here in Vermont, people cover problematic siding all the time. Of course, the usual remedy is vinyl siding, which some homeowners don't like. But a dry house with vinyl siding sure beats a rotting house with stucco.

  4. severaltypesofnerd | | #4

    Vinyl siding on a stucco house is not a viable solution, even assuming one could somehow make it waterproof at the roof line. And question was about how wet dense pack has to be before it needs to be mechanically removed, not about recladding the house.

  5. GBA Editor
    Martin Holladay | | #5

    You wrote, "The dense pack smells moldy (the stink is horrible)."

    I advised, "Remove and dispose of all the damp cellulose."

    Assuming that you follow that advice, you still have to decide (a) whether to re-insulate the stud bays or leave them empty, and (b) whether you need to worry about future water entry. You wrote that the two stud bays you examined had "completely wet sheathing."

    Obviously, you are on site and know more about this house than I do. But if the house has "no roof overhangs whatsoever," as you wrote, then I think that it's worth talking about what you plan to do to protect these walls on the exterior.

  6. Terryssr | | #6

    Re removal & disposal of the moldy cellulose insulation. (1) Martin's absolutely correct--does NOT solve problem unless you determine how moisture is getting in, otherwise, it will just get wet & moldy again or other areas will do so. Mold will continue to be a problem until you solve the moisture problem. (2) You would be well advised to take/use full professional mold remediation procedures in removing & disposing of contaminated insulation. That includes being sure--until it can be remediated-- you seal over with heavy gauge plastic sheeting the open wall area so that the mold spores & fragments are not pulled into the house indoor air. ALL molds contain toxic materials in their cell structures. Recent research proves that (1) toxins are contained in even non-viable spores & in mold fragments, (2) when inhaled, these mold toxins are capable of causing serious illness in many people, (3) mold spores & fragments can become widely disseminated in the indoor air via HVAC systems. So it is wise to seal-off/encapsulate any contaminated areas to prevent the house indoor air from spreading mold spores & fragments, and wise to use full professional remediation procedures including full body protection of people remediating an area, sealing off the HVAC vents/ducts from contamination & the contaminated area from rest of house, doing remediation with negative air pressure HEPA filtration unit, etc. etc.

  7. GBA Editor
    Martin Holladay | | #7

    I disagree with your statement that "ALL molds contain toxic materials in their cell structures."

    Many delicious foods contain mold, including blue cheese. I like to smell this mold, and I eat this mold with relish.

    Most people aren't harmed by molds, but some people can become sensitized to some species of mold and have adverse health reactions.

    Luckily, I'm not one of those people, so I'll keep eating moldy cheese.

  8. iLikeDirt | | #8

    You're not in a great situation, that's for sure. I understand that long-term, architecturally your options are limited and redoing less rather than more is better for the budget, obviously. And you don't want to spend a lot of money to undo the insulation work that you already spent a lot of money to have done in the first place.

    The best solution would be to remove all the stucco to expose the sheathing, let it dry and repair any damage, take the opportunity to add rigid foam insulation, and then apply a new WRB in a way that incorporates air gaps, or else use multiple layers of WRB material; 3 or more is probably not overkill for heavily-exposed stucco. Then apply new stucco. Needless to say, this is very expensive. But you will solve the issue once and for all and be able to keep your insulation work.

    If you're not willing to spend all that money, a next-best long-term solution will involve keeping water out of the original stucco. Your painted stucco isn't doing you any favors because it's vapor-impermeable, significantly reducing drying to the exterior. In theory it's waterproof but inevitably there are gaps and holes and water will get in anyway (as you've discovered). You should remove all moldy insulation and dry out any wet wood. Kill all mold you find with a serious mold killer product (not bleach). After that, let me suggest a somewhat unorthodox long-term approach: re-paint the exterior painted stucco with a vapor-impermeable waterproofer, then install lath over it, and apply a new two-coat stucco right over that. If you can find someone willing to do it, you could save money by having them install only a color coat over the surface using a chemical bonding agent for adhesion instead of lath, which would eliminate the expense of lath and all the fastener penetrations through the paint. This makes the painted surface of the original stucco into a new "poor man's WRB" that will keep water from migrating deeper into the original stucco, and each half of the wall on either side of the vapor-impermeable waterproof paint can dry to its own side, so there is no vapor trap. Then, you can paint the new stucco with a waterglass-based vapor-permeable paint or sealer. In fact, you can also add waterglass to the cement mix for the color coat to make the entire color coat more waterproof. Waterglass is amazing stuff whose benefits are practically unknown in the USA but it's heavily used in Europe. This approach will keep water out of the new stucco, and any water that does make it in can evaporate through the exterior surface, and any that's driven deeper by solar vapor drive will hit the WRB-like paint and not go any deeper. The extremely small fraction that makes it into the original stucco layer will be able to dry to the inside and be wicked away from the sheathing by the cellulose, which can handle small amounts of moisture (but not bulk water, as you've discovered).

    I had this done to my own house a year ago: we treated the existing painted stucco surface as a new WRB and applied foam, lath, and new stucco right over it. No moisture problems at all so far.

  9. Tim C | | #9

    Martin, as a professional pedant, I feel the need to point out that those two statements aren't contradictory. Many compounds are both toxic and safe (or even necessary) to consume. Copper is an excellent example - it is sufficiently toxic to be an effective biocide but at the same time it is an essential nutrient that you cannot live without.

    Of course, I do agree that the average mold will do little harm to the average person. Although I'd also caution that one should be careful about exposing their lungs to large volumes of airborne particulate matter, mold spores included - toxic or not, it is a good way to join those who are sensitized and suffer adverse health reactions.

  10. severaltypesofnerd | | #10

    OP here.

    Yes, all the dense pack is out of the open wall cavities, and it was done in a reasonably mold-safe manner.

    The real question here is how many of the closed-but-possibly-wet cavities do I need to excavate? Is there a guideline for a moisture meter reading that might help decide which ones? Remember it's diagonal sheathing, so watter trickles sideways down the gaps between boards.

    For Nate G: a major complication with any new cladding that's wider, meaning either replacing all the windows or extending all the original old-growth redwood frames. That's hardly trivial.

    For all: I can't find the stupid leak. It makes me feel that other "looks fine" walls are placed at risk by the cellulose.

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