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Community and Q&A

Cellulose insulation hides leaks and moisture problems

bluesolar | Posted in Green Building Techniques on

Hi all – I thought this article by Michael Chandler at Fine Homebuilding was interesting:

He explains why he doesn’t use cellulose or blue jean insulation:

On two occasions, homes I have built with cellulose insulation had plumbing leaks. The moisture was absorbed and spread by the cellulose and it took days for the homeowner to notice. By the time we located and repaired the leaks, the cellulose had wicked up the water and become saturated. While the cellulose itself hadn’t mildewed, the studs, exterior sheathing and drywall were covered with mold. We removed the sheetrock and cleaned the wood and set the cellulose aside to dry. It was remarkable how long it stayed wet.”

So it sounds like cellulose is acting like an anti-canary – it muffles a house’s critical signals re: leaks in particular. I’ve seen a lot of promotion of cellulose here on the idea that it fruitfully manages moisture in wall cavities. I’ve never seen any discussion of what happens when cellulose is exposed to serious moisture, like a pipe leak.

I think it’s also important to note that the argument for the purported subtle moisture management benefits of cellulose are premised on wood-framed houses, but there’s not much reason to use wood in a new build, which makes that argument moot. Assuming a robust house with concrete or CEB walls or steel framing, cellulose wouldn’t compromise the structural materials as it does with wood, but it would still be concealing leaks and sitting there soaking wet for long periods. You could overcome its signal loss effects with in-wall sensors, but most people won’t want to do that.

I’ve been thinking about cellulose inside and something punchier outside, like solid polyurethane, polyiso, or a phenolic foam like Kooltherm. But if cellulose is going to hide major leaks, I’m now leaning towards mineral wool. (I don’t trust foams’ fire resistance, and I feel like there’s no reason to build a house that can burn down if it’s trivial to build one that can’t.)

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  1. Expert Member
    BILL WICHERS | | #1

    This is similar to the argument against spray foam in roofs because it “hides leaks”. Should we also not use ice and water shield? That hides leaks too, in a way. It’s better to build a structure with less chance of leaking to begin with. In the case of plumbing leaks (which really constitute “bulk water” and not “moisture”, they’re two different things in the building science world), it’s best practice to not put any plumbing in exterior walls, and that would include attic spaces in this case. That means the places you’d use cellulose should be free of any plumbing and thus not at any risk of getting soaked by plumbing leaks.

    Cellulose has many good properties, but it has tradeoffs like anything else. I’ve never thought the “it will hide leaks” argument is a good one though, but that’s my opinion.


  2. Expert Member
    Michael Maines | | #2

    I agree with Bill--this has always seemed like an odd argument to me. Spray foam hides leaks much more than cellulose, in my experience--by the time you know there are leaks the wood can rot, vs. just a few days for leaks to show up with cellulose. Meanwhile, with good-quality cellulose, the borate content protects anything it touches from mold growth. Interior vapor retarders also hide leaks. If you go on vacation for a week you might miss a leak.

    As we have discussed here many times, there are a lot of good reasons to build with wood and it's not all that hard to do it. If you can't figure out how to build without a lot of foam and concrete, I'm not sure what to say to the vast majority of builders who have figured out how to do so.

  3. Expert Member

    Where would the cellulose come in contact with pipes? Exterior walls should not have plumbing in them. In the rare cases where it is unavoidable, the plumbing should be in a service cavity, or isolated from the rest of the wall by a layer of foam in that one stud bay.

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