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Hygroscopic characteristics of mineral wool

DFVellone | Posted in General Questions on

Can anybody offer me a comparison of rock wool’s ability to redistribute wintertime moisture accumulation, and to what extent it can (or can’t) do this without losing function. I’d be most interested in a comparison to cellulose in this regard. Thank you, Daniel

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  1. Expert Member


    Mineral wool does not take on water, so it doesn't lose function, but neither does it re-distribute moisture.

    1. DFVellone | | #2

      So, any moisture that may accumulate in a wall cavity insulated with rock wool will migrate to the studs?
      If so, then rock wool doesn't protect the structural wood at all where cellulose actually does protect it?

      1. Expert Member
        Dana Dorsett | | #13

        >"So, any moisture that may accumulate in a wall cavity insulated with rock wool will migrate to the studs?"


        In winter the bulk of the moisture will accumulate in the (colder than studs) exterior sheathing first, in summer it'll accumulate in the wallboard first (assuming air conditioned interior).

        Where the studs meet the sheathing/wallboard the seasonal moisture content peaks will be higher than in the middle of the stud.

        1. DFVellone | | #16

          Dana, you've given me a lot of advice on my wall stack-up and I've finally gotten to the point where I'm deciding between dense-pack cellulose and rockwool having finally found a densepack installer with good recommendations willing to travel to my location. But I can save quite a bit detailing the rockwool install myself. The cellulose quote is a bit higher than I anticipated. **Moisture accumulation is my concern at this point as well - if the rockwool won't cut it I'll have to bite the bullet and hire out the cellulose install. My stackup, if you don't recall, form the outside is 1" hemlock siding, 1" airspace, tyvek, 2 1/2" reclaimed xps, 5" stud cavity, intello plus, blueboard + plaster. *There is no structural sheathing: house is timberframed and the insulation framed has diagonal let-in bracing throughout.

          1. Expert Member
            MALCOLM TAYLOR | | #17


            With your stack-up you have no condensing surfaces on the exterior to worry about. You can use anything you want to fill the walls.

          2. Expert Member
            BILL WICHERS | | #18

            I agree with Malcolm. The thing you’re trying to protect from condensing moisture is usually the inside surface of the exterior structural sheathing, which is usually plywood or OSB. If that sheathing gets wet, it can rot or grow mold. You don’t have any wood sheathing in that location, and with 2-1/2” of reclaimed XPS on the exterior, you’re in pretty good shape in terms of thermal/moisture performance anyway since that surface won’t be getting very cold. You don’t need to worry about your cavity insulation choice here.


  2. Expert Member
    Rick Evans | | #3

    Daniel, you are correct.

    Cellulose is hydroscopic- so it can, as you described, absorb and re-distribute water vapor within a stud cavity. Mineral wool is hydrophobic. It will not absorb and release moisture. Moisture will essentially pass through it as though it were not even there. Same with fiberglass.

    1. this_page_left_blank | | #9

      Fiberglass is definitely hygroscopic. It saturates with water readily. But whereas cellulose expands when it gets wet, fiberglass does not, so it turns into a clumpy, useless mess. It does not regain its original shape after drying out either.

      1. Expert Member
        Rick Evans | | #10

        Trevor I agree that fiberglass turns into a sloppy mess when exposed to bulk water. But in terms of water vapor I still think glass fiber is defined as having hydrophobic properties.

        1. Expert Member
          Dana Dorsett | | #14

          Fiberglass isn't quite as hydrophobic as rock wool, but neither is going to redistribute moisture very well, and none of the above will protect from bulk water incursions.

          With the diffusion moisture and minor air leak moisture loads both fiberglass and rock wool will take on a molecule-thick layer of adsorb over the fibers, and as the adsorb saturates there is some amount of wicking. The hollow tube characteristic of cellulose fibers gives it many times more capacity than either fiberglass or rock wool for storing and redistributing moisture in the form of adsorb, before it becomes more than a molecule thick.

  3. Expert Member

    One thing I wonder about is whether moisture-buffering is useful in climates where the high moisture conditions are not seasonal, but are pretty consistent throughout the year?

    The usefulness of it seems to pre-suppose the moisture accumulates during the winter, and will dry out in the summer, before the excessive wetting can do any damage. If your climate doesn't have the swings where adequate drying occurs, I wonder if using the wall insulation as essentially storage for excessive moisture is a good idea? Maybe in those climates it would be a better strategy to try and move all the moisture out through the wall as quickly as possible year round? Walls built that way would also perhaps be less susceptible to damage from bulk water intrusions?

    1. Expert Member
      Rick Evans | | #7

      Malcolm- good point.

      If the winters are short enough, or the interior heat isn't sufficient enough, I can imagine a scenario where all of the inward-driven moisture into the cavity is not entirely reversed back out of the wall. At some point the cellulose might start to compress from all of the moisture storage?

      I suspect there are two things that keeps the cellulose from becoming too saturated:

      1. The inward driven moisture is sent indoors through interior, vapor open drywall and removed via AC.

      2. Higher outdoor temperatures allow the air to hold more moisture making any outward drying that does take place to be more efficient.

      I suspect vinyl wall paper or an interior vapor retarder would cause serious issues. Obviously, exterior rigid foam makes everything better.

      Anyway, I'm thinking out loud here. Worthwhile though experiment.

  4. DFVellone | | #5

    I'm in zone 6, so the presumption I understand is that moisture may accumulate during winter and dry out during the warm months.

    1. Expert Member
      MALCOLM TAYLOR | | #6


      Yes. I was more musing about the situations in hot, humid, or temperate climates without large swings in humidity.

  5. Expert Member
    BILL WICHERS | | #8

    Malcolm, in an area with fairly consistent moisture levels year round I doubt cellulose’s buffering ability would be of limited, if any, benefit. The reason is that the constant moisture level would build up in the cellulose until a point of equilibrium was reached, at which point no further buffering would be possible. With seasonal changes, the moisture in the cellulose would drop when the surrounding environment dries out. With constant moisture levels, this drying will never happen, so the equalizer moisture in the cellulose just sits there.

    I don’t think cellulose is bad in a climate of constant higher humidity, but I don’t think it offers the same benefits there as it does in climates with large seasonal changes. In “always damp” climates it might be better to use something like mineral wool which won’t hold onto any moisture and will allow very rapid moisture migration whenever needed.


    1. DFVellone | | #11

      Does that benefit in climates with large seasonal changes outweigh any benefit of mineral wool? I'm referring mostly to mineral wool being a product that, with attention to detail, can be a grade A diy install. And in this climate is there a benefit to a hydrophobic quality?

      1. Expert Member
        MALCOLM TAYLOR | | #12


        The advice from builders like Dan Kolbert and others who do double-wall construction in New England, is that the moisture buffering is necessary to keep the sheathing safe from excessive moisture during the late Winter and Spring. Whether the presence of a rain-screen gap on it's own would be en0ugh to mitigate the problem I don't know.

        In one sense the question I asked about cellulose in climates without big swings in humidity is probably moot, as those climates tend to be mild enough you don't have walls thick enough, or that stay cold enough for it to matter.

      2. Expert Member
        BILL WICHERS | | #15

        I don’t think it’s a negative for mineral wool in seasonal climates. It MIGHT be a BENEFIT in consistently moist climates like the PNW where Malcolm is. In seasonal climates, I don’t think it’s as much of a benefit, but it doesn’t have the same tendency to get water logger that Fiberglas has which is a good thing.

        I use mineral wool in my own home and I’m happy with it. I also use exterior rigid foam, a lot more than code minimum, which makes seasonal moisture issues in the wall much less of a concern.


  6. DFVellone | | #19

    Can't begin to express how appreciative I am for all the advice I've been able to use to make decisions that could have gone the wrong way. I've been working on our new home here for several years myself, plodding along at a snail's pace, but am now closing in on it. After I did the architectural we hired an engineer that advertised a specialty in passive solar, energy efficient design and we learned a little too slowly along the way as mistakes in dimensions in the plan and a general lack of ability to comprehend or communicate in the language and terminology of residential construction revealed the engineer's lack of experience and knowledge. She was in way over her head. We paid the bill, licked our wounds, and turned to you folks and this site's extensive library for advice and guidance that has been invaluable. I appreciate it, Daniel

    1. Expert Member
      MALCOLM TAYLOR | | #20


      Good luck with your build going forward!

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