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Choosing Between Mineral Wool and Cellulose

woobagoobaa | Posted in Green Building Techniques on

Getting near the point that exterior wall cavity insulation will be going in.  Zone 5 in Eastern MA.  Wall assembly is (out to in) exterior paint, cedar claps, rain gap, Blueskin WRB,  old board sheathing, wall cavity, vapor retarder?, skim coated blueboard, latex paint.

Walls are 2×4 or built out to 2×6, being a reno the stud spacing is irregular.  Many dense pack cellulose providers in the area, far fewer dense pack mineral wool but I have quotes for both.

So, dense pack cellulose or dense pack mineral wool, how would you make this decision?

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Replies

  1. Expert Member
    Michael Maines | | #1

    Dense pack cellulose insulates well, helps moderate moisture by absorbing and releasing moisture safely while also protecting the framing around it, the borate additive is a pest deterrent and makes the cellulose essentially fireproof, and it sequesters carbon.

    Mineral wool insulates slightly better and is even more fire-resistant than cellulose. In large cavities it may install easier than cellulose. It is not affected by moisture but it does not protect the framing it touches, nor is it pest resistant. Because to make it they have to melt rock, it has a relatively large carbon footprint.

    I like and use both products, but I generally choose cellulose.

    1. BlueSolar | | #2

      Michael, what do you mean that mineral wool isn't pest resistant? I thought that it was, and even read that it cuts pests.

      1. Expert Member
        Michael Maines | | #4

        I have a stack of mineral wool in my barn--mice, skunks and cats think it's great for nesting in. Most insulation marketing touts "provides no food value" but that should not be confused with being pest-resistant.

        1. Expert Member
          MALCOLM TAYLOR | | #5

          Michael,

          I've had the same experience.

          1. charlie_sullivan | | #9

            My shed smells a lot better now that I've thrown out the bag of mineral wool that the mice were nesting in. But it appears that they have decided to move into my attached garage now, so I think I need to do some trapping.

  2. Expert Member
    Dana Dorsett | | #3

    >"Wall assembly is (out to in) exterior paint, cedar claps, rain gap, Blueskin WRB, old board sheathing, wall cavity, vapor retarder?..."

    You seem to indicate you're not sure of the stackup? Or just not sure of the vapor retarder?

    To which...

    >"..., skim coated blueboard, latex paint."

    Standard interior latex paint on plaster IS a vapor retarder sufficient to protect the rest of that stackup, given the "...rain gap...", which I take to mean 1/4" or more of air between the clapboard & blueskin. In a zone 5 climate that would work at all wall thicknesses, even if the sheathing layer were as vapor-tight and moisture susceptible as OSB. If it's ship-lap or other plank sheathing (and not OSB) under the BlueSkin it's even less risk.

    Cellulose is far greener (it's sequestered carbon, & renewable) and far more protective of the structural wood from interior moisture drives than rock wool. The microscopic hollow fiber characteristic of cellulose gives it an order of magnitude greater capacity for storing seasonal moisture (as adsorb on the surfaces of the fibers), and will redistribute any moisture accumulation, reducing the moisture content of the wood that would otherwise take up the moisture. And with a rainscreen gap as a capillary break against exterior moisture drives the assembly continues to dry toward the exterior via the gap.

    While dense packed rock wool would yield a slightly higher center-cavity R-value, the higher thermal mass of cellulose reduces the magnitude of the swings the interior surface temperatures experiences over the course of a day, and delays the peak gains/losses relative to the high & low outdoor temperatures experienced by the siding. In a 2x6/R20-ish wall that thermal diffusivity mass effect isn't dramatic, but measurable, but becomes more apparent in higher-R assemblies. The overall the mass effect reduces net differences on annual energy use between R23-ish rock wool relative to R20-ish cellulose to something pretty small- nothing like a simple ratio of R23/R20. Depending on the actual framing fraction it yields something like a 1-2% performance benefit to go with high density rock wool.

    When furring up to 2x6 depths it may be work considering a Bonfiglioli-strip approach to increasing the cavity depth, which yields a much bigger improvement in performance than the cavity fill insulation choice:

    https://www.finehomebuilding.com/membership/pdf/9750/021250059.pdf

    1. woobagoobaa | | #6

      Thank you Dana et al ... yes we had not settled re: interior vapor retarder being needed or not. And I will have to confirm with my town. We indeed have a 1/4" gap between the claps and the Blueskin which is applied to the board sheathing (shrinkage gaps of 1/4"-1/2" between boards).

      I'm likely tilting towards cellulose for the moisture buffering ability and anti critter/bug borates.

      1. Expert Member
        Dana Dorsett | | #7

        >"re: interior vapor retarder being needed or not. And I will have to confirm with my town. We indeed have a 1/4" gap between the claps and the Blueskin which is applied to the board sheathing (shrinkage gaps of 1/4"-1/2" between boards)."

        MA code follows the IRC 2015, with amendments. The only amendments related to walls are fire-related. Under IRC 2015 in TABLE R702.7.1 the air gap qualifies as the "vented cladding" exception for climate zone 5 (= all of MA), allowing the use of Class-III vapor retarders:

        https://up.codes/viewer/massachusetts/irc-2015/chapter/7/wall-covering#R702.7.1

        ANY air gap between the siding & WRB allows the use of Class-III vapor retarders (= latex paint on gypsum) for wood-sheathed houses. Plank sheathing is already less risky than OSB or plywood, and cellulose would lower that risk even further.

        1. woobagoobaa | | #16

          Dang ... local building inspector will require Membrain or similar.

          1. Expert Member
            Dana Dorsett | | #17

            MemBrain is pretty cheap stuff- a fraction of the cost of Intello et al, and only ~2-3x the (material) cost of 6-mil polyethylene. The installation labor for 2-mil nylon isn't be any more than that for installing polyethylene vapor barriers. If you're working from bare studs the cost adder should be on the order of 20-25 cents per square foot of wall area (installed)- less than a grand for a whole whole house unless the construction boom has inflated everything dramatically (which may be the case).

            With the rainscreen gap the inspector shouldn't really be requiring it, but getting into a pissing match with the local inspector over the minutae of the letter of code is never a good strategy when the upcharge is low. That's how I ended up with a brand new low-flush volume toilet a dozen years ago as part of a heating system installation. A drain pipe needed to be moved over a couple feet to accommodate the gas venting location that had a very limited range to work with while still meeting distances from windows and exterior walkways. The inspector insisted that the toilet in the bathroom using that drain be updated from the circa 1923 version if I was going to touch any part of that drain plumbing, even though the branch serving the toilet wasn't going to change. It seemed like an irrelevant silly demand by the inspector, but arguing about it would have risked screwing up the schedule on the heating system.

  3. Tedleschi | | #8

    Dana,

    Thanks for your invaluable comments and insight. Could you explain the concept of thermal mass and how “... the higher thermal mass of cellulose reduces the magnitude of the swings the interior surface temperatures experiences...”? I understand that mineral wool batts have a density between 2 and 4 PCF and mineral wool boards have densities as high as 11 PCF, whereas dense-pack cellulose if properly installed has a density of only 3.5 PCF. Isn’t thermal mass directly related to actual mass, i.e. weight and density? Isn’t the massive weight and density of a masonry wall the reason it also has a high thermal mass and works so well to buffer temperature swings?

    Fred Williams

    1. Expert Member
      NICK KEENAN | | #11

      While I respect Dana's expertise tremendously, this answer puzzles me. "Thermal mass" is not a term used in building science, or any science. "Mass" as scientists use it is a specific property of matter, it takes no adjectives.

      Putting linguistics aside, a finished house weighs on the order of 100,000 lbs. The capacity to hold heat of the fluffy stuff in the walls is going to be negligible compared to the rest of the structure.

      1. BlueSolar | | #12

        You're saying you're not familiar with thermal mass, or that you dispute the validity of the concept? It's real, another term for heat capacity.

        Your claims about mass being a specific property of matter and taking no adjectives are bizarre. Are they supposed to be logically connected? Mass takes no adjectives *because* it's a specific property of matter? That's a non sequitur. Nothing about science requires or implies that properties of matter take no adjectives, or that mass in particular takes no adjectives. Mass takes whatever adjectives that language users find useful, like reaction mass, rest mass, relativistic mass, effective mass, unsprung mass, and thermal mass.

        Your argument about a house's weight isn't valid. Both thermal resistance and thermal mass are local properties, specifically pertaining to cross-sectional linear/lateral heat transfer and storage across a barrier material. The only thing that matters to a wall's thermal mass are the properties of that wall, specifically the thermal masses of its constituent materials. Houses could weigh a hundred pounds or million tons – that doesn't tell us anything about the thermal mass of any material we'd use in a wall. The thermal mass of cellulose is indeed negligible, but Dana noted that.

        1. Expert Member
          NICK KEENAN | | #14

          "Thermal mass" is not used in science. It comes from architecture, where the word "mass" has a very different meaning, that of "enormity," which comes from the original meaning of mass before it was coopted by physics. When an architect talks about the "mass" of a building he's not talking about putting it on a scale to see how much it weighs, he's talking about how weighty it appears.

          The reason I object to the use of the term is not just semantics. Many people have an intuitive sense of heat dynamics that is at odds with the science. That sense leads them to do things like incorporating unnecessarily large amounts of concrete into their designs, in the mistaken belief that it improves energy efficiency to have large amounts of, you guessed it, "thermal mass." The reality is that concrete is a very non-green material and houses with large amounts of it tend to be harder to insulate.

        2. Expert Member
          NICK KEENAN | | #15

          >Both thermal resistance and thermal mass are local properties, specifically pertaining to cross-sectional linear/lateral heat transfer and storage across a barrier material. The only thing that matters to a wall's thermal mass are the properties of that wall, specifically the thermal masses of its constituent materials.

          So where on the Manual J do you enter the "thermal mass" of the walls?

  4. charlie_sullivan | | #10

    I think that the skill level needed to install cellulose well is higher than the skill level to install mineral wool well, but if you have plenty of cellulose installers to choose from you can get a good install and get the various advantages people have noted. But it's worth asking around and getting the cellulose installer with the best reputation, not just the cheapest bid. As with many trades, but the necessary skill is more obvious in some other trades.

    Deep cavities require more skill but even in your case I would want someone skilled to get uniform densely packed walls.

  5. CCM_CONSTRUCTION | | #13

    Price wise it's almost 2x more for mineral wool in my neck of wood (1$ vs 1.85$ for sq foot. Both product are good, I was in the same situation as you and I decided to go with cellulose, there were no real advantage to pay $ for mineral wool.

  6. pnwnerd | | #18

    Easily cellulose for walls. If unvented ceilings were in the mix, discussion might be more spirited than it already is.

  7. cococchio | | #19

    "Microscopic hollow fiber characteristic of cellulose gives it an order of magnitude greater capacity for storing seasonal moisture". Do we have any idea of what is the effective R-value of cellulose when it adsorb water? Also, I believe that cellulose can settle. I read about the necessity to use baffles to prevent settling but wouldn't they increase the final cost?

  8. woobagoobaa | | #20

    FYI the installed assembly (out to in) ... cedar claps, 1/4" air gap, Blueskin WRB, original board sheathing, dense pack mineral wool, Membraine, skim coated blueboard.

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