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Dense-packed dry cellulose: any downsides?

css1813 | Posted in Green Products and Materials on

I’ve been going over insulation techniques to use on a new home. Every insulation contractor that we’ve talked to has a different system. One in particular that is being pushed is dense pack dry cellulose. I have personally witnessed wet cellulose settling in our own home, however based on everything I have read about and heard about dense pack is that it will not settle and that our current home’s insulation was not installed properly. I know that cellulose is somewhat springy so I understand the theory is it will hold true in the wall for hopefully many many years. The contractor that is pushing it uses a netting type of material to hold in the dry cellulose until the drywall is up.

My question is then, what are the downsides to dense pack dry cellulose? The only one I’ve seen so far, is that any exposure to moisture will cause it to settle or condense. But, my take is that if you have that much moisture in your walls then you probably have a bigger issue at hand. I’m curious what some unbiased experts have to say on it.

I live in Michigan, zone 5. The house will have 1/2″ sheathing on the outside with Tyvek regular housewrap, and vinyl siding over that. The inside is standard 1/2″ drywall.

Thank you so much for the valuable information on this website.


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  1. Expert Member
    Dana Dorsett | | #1

    With dense pack the settling is determined by the normal seasonal moisture cycling and the density. Moisture cycling creates creepage issues causing it to settle, but above some density it won't happen. The density required varies with both climate and the particulars of the wall assembly, the indoor & outdoor seasonal relative humidity, etc. In a zone 5 climate it'll usually be fine at 3.2lbs per cubic foot or higher, but 3.5 lbs or higher might be called for at the cold edge of zone 5 under some conditions, so shoot for 3.5lbs. While some installers & blowers can hit 4lbs & up that isn't likely to be necessary in your location.

    With vinyl siding and 1/2" OSB or plywood code doesn't require in interior side vapor barrier to protect the sheathing in zone 5, but a "smart" vapor retarder such as 2-mil nylon (eg Certainteed MemBrain), or Intello Plus under the wallboard will most likely result in lower peak and average moisture content in both the cellulose and sheathing.

  2. GBA Editor
    Martin Holladay | | #2

    Dense-packed cellulose is an excellent insulation choice for closed cavities (for example, stud bays). It has low embodied energy; a high recycled content; reduces infiltration and exfiltration; and fills odd-shaped spaces perfectly.

    The only downside I can thick of is that in some regions of the country, it can be hard to find an insulation contractor with dense-packing experience. In these areas, the cost of dense-packed cellulose, if it is even available, may be higher than other options.

  3. lance_p | | #3

    Dana, is there a chart of recommended cellulose densities for different climate zones and/or anticipated low temperatures? I'm on the cold side of CZ6A and planning a dense pack for my wall assemblies. Based on what I'd read so far, I was planning to hit a minimum of 3.7 lbs/cuft, but that was a very general recommendation if I remember correctly, not specifically tied to a climate zone.

    I thought, and once again I may not be remembering incorrectly, that the 3.7 lb/cuft recommendation had come after shaker/vibratory and climatic testing of wall assemblies in Europe? I don't remember where I read that. Either way, I was planning to go a little above just to be safe.

  4. css1813 | | #4

    Thank you both for this, as always we appreciate all of your help!

  5. Expert Member
    Dana Dorsett | | #5

    Lance- I don't know if anybody has assembled a chart on this- there are lots of factors, and most of the research has been done in Europe. Torben Valdbjørn Rasmussen of Aalborg University in Denmark has published several scientific papers on the topic going back 20 years or so eg:

  6. Expert Member
    Michael Maines | | #6

    What I learned from cellulose guru Bill Hulstrunk is that the required density has a lot to do with the size of the cavity. The larger the cavity, the denser the cellulose needs to be blown, up to 4.0 pcf or more in 12" x 24" cavities. Cyclic wetting will allow the cellulose to compress a bit; at the recent NESEA conference, Kohta Euno of Building Science Corp showed images of a test they are running where cellulose, well-installed in an unvented roof, settled away from the roof sheathing after becoming saturated. So while cellulose shouldn't settle, there are situations where it does. Installing in a 2x6 cavity with decent moisture control should be safe, though, and a minimum of 3.5 pcf is usually recommended. At densities over 3.8 pcf the R-value per inch starts to drop a bit, though not enough to matter in most cases.

  7. lance_p | | #7

    Interesting. Their research suggests 50-80% RH swings at 23C require about 4.5 lbs/ft3 (73 kg.m3) for a 12" (0.3m) wall cavity, and about 3.9 lbs/ft3 (63 kg/m3) for a 6" (0.15m) wall cavity.

    I'm doing 12" deep cavities, so it looks like aiming for upwards of 4 lbs/ft3 is prudent. Thanks for the link!!!

  8. lance_p | | #8

    Michael, this publication by Joe L. suggests dense packing a flat roof is seldom a good idea without some sort of rigid insulation attached firmly below the roof, or spray foam:

    Sounds like Bill H. is right based on the paper referenced by Dana above. I think I'll shoot for well over 4 lbs/ft3 to be safe.

  9. GBA Editor
    Martin Holladay | | #9

    It's not just Building Science Corporation that advises readers that dense-packing rafter bays only makes sense if either (a) there is a ventilation channel between the top of the cellulose and the underside of the roof sheathing, or (b) there is an adequately thick layer of closed-cell spray foam between the top of the cellulose and the underside of the roof sheathing, or (c) there is an adequately thick layer of rigid foam above the roof sheathing.

    That advice has also long been the standard advice here at GBA. For more information, see these two articles:

    How to Build an Insulated Cathedral Ceiling

    Insulating Low-Slope Residential Roofs

  10. Expert Member
    MALCOLM TAYLOR | | #10


    That réponse answers a lot of questions that came up in the discussion here:

    But if you have the opportunity, I'd appreciate your views on the efficiency and mechanisms at work when the venting is above the sheathing.

  11. GBA Editor
    Martin Holladay | | #11

    There have been a few discussions of vent channels above roof sheathing on GBA. The best designs are created with 2x4s, installed "flatways" to create channels that are 1.5 inch high, with the 2x4s installed from soffit to ridge (in other words, perpendicular to the ridge).

    You need soffit vents and a ridge vent. You also need (ideally) roof sheathing that is relatively vapor-permeable -- meaning that board sheathing or plywood is preferable to OSB -- and roofing underlament that is relatively vapor-permeable (for example, #15 asphalt felt or one of the vapor-permeable synthetic underlayments).

  12. lance_p | | #12

    Good to know, thanks Martin. I was only familiar with that particular article as I'd just read it recently.

    Regarding the settling in the wall issue in general. I've been thinking about this for the last day or so and wondering what the severity is when compared to the alternative options, namely batt insulation. If we consider that batt insulation never really fills a cavity as well as blown in cellulose to begin with, I wonder how much settling (gap at the top of a wall cavity) is necessary with cellulose before it degrades to the point where it's equal to batts?

  13. GBA Editor
    Martin Holladay | | #13

    If your walls are dense-packed by an experienced installer, there is no reason to worry about settling.

    Settling isn't a widespread problem, and hasn't been for decades. The "cellulose can settle" issue is mostly a red herring promoted by manufacturers of rival insulation materials.

  14. lance_p | | #14

    Thanks Martin. After reading more than I can remember about cellulose I've come to that same basic conclusion. I'm still planning to use cellulose, the benefits sure seem to outweigh any potential drawbacks.

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