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Best alternative to dense-packed cellulose?

Doug_Epperly | Posted in Green Products and Materials on

First, thanks in advance to all that may provide a response. I enjoy this forum and all the good feedback.

We have been planning a home construction project, with the intent of using dense pack cellulose to achieve R-40. I have not been able to find a contractor in our area that works with cellulose, let alone having the equipment to do dense pack. Contractors in this area seem to principally deal in either glass or spray insulation. We liked dense pack cellulose because the material potentially took products out of the waste stream, contributed less to global warming than spray (at least as I understand it), potential to reduce thermal bridging with double stud walls, etc. I’d appreciate any thoughts on what the next best option might be. If it matters, we will be building in Zone 6.

Best wishes, Doug

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  1. charlie_sullivan | | #1

    If you like the blown-in format of cellulose, you could do blown in fiberglass. Or you could do batts, either mineral wool or fiberglass. Mineral wool is generally considered better, but dense fiberglass can work well too.

    Dense packed cellulose has some ability to mitigate poor air sealing or minor moisture problems, but it's not a real solution to either of those problems, so if you do things right as far as air sealing and moisture management, you won't lose performance relative to cellulose.

    The "embodied energy" of any of those will be higher than cellulose, but that issue is tiny compared to the global warming impact of most closed cell spray foam, because of the blowing agent used. So the main thing on climate impact is to avoid those blowing agents. Spray foam is also expensive, so you are better off buying more thickness of another insulation material to get R-40 for a reasonable cost.

  2. exeric | | #2

    I would say the technical question of when, or when not to use dense pack cellulose has to do with how thick the open cavity is that will be filled. Anything greater than about 2x6 stud filling requires advanced expertise. If you have thicker open cavities to fill then even tradespeople that say they do it are not a sure thing. It sounds like you have that situation if you are aiming for R40.

    I did it myself on my own home with 6" thick cavities. After a learning curve it worked well and came out to about 3.5lbs/cf. I wouldn't attempt anything greater than a 6" depth due to inevitable settling later. The only thing that "might" allow one to do it yourself is if one develops a scheme to divide the cavity into either smaller individual thicknesses, or alternatively to put in some kind of fire blocking that will resist the settling of the cellulose. It's basically not a diyer job unless you are super conscientious and actually enjoy that kind of thing. Alternative insulation types may be the best option for you in your location and climate zone.

    1. sbrooksvt13 | | #4

      Hi Eric,

      Did you achieve with a typical renter blower or a special machine? How did you measure this?


      1. exeric | | #5

        Hi Steven,

        Yes, I did it with a standard rental blower of the type the big box stores rent. They are able to accomplish it but there are two requirements to get to 3.5 lbs/cf:

        1. It must be the type of blower that has a slide gate so it can be adjusted to a high ratio of air to cellulose so it does not bog down due to their lower max pressure.

        2. It really requires the method of dense packing behind netting. This is true because you need to pack it so that the netting bellies way out. The reason you do this is because you then use a roller to pack it back in and you are then using your own labor to get it to that 3.5 lbs/cf.

        For instance, if you tried to drill holes in the wall and then pack it with a big box blower through the holes that blower just would not have enough power to pack it tight enough without the extra bit of human power with a roller.

        To get the weight of the cellulose used I just counted the empty bags used and multiplied it times the lbs per bag. Then to calculate the wall volume filled multiply the linear length of each wall times height and width of wall. For instance 5.5" by 96" by length for 2x6 standard construction. Then subtract at least 20% from that figure for the framing factor of the actual 2x6s. Finally, divide the total weight of cellulose used by that calculated volume of the walls. I hope this helps.

  3. GBA Editor
    Martin Holladay | | #3

    You didn't really describe your project. I'm assuming that you are talking about wall insulation or perhaps a cathedral ceiling. When cellulose insulation is installed on an attic floor, it cannot be dense-packed; it is simply blown in place and allowed to settle.

    R-40 would be considered a lot for a wall -- so I'm assuming that you are talking about a double-stud wall.

    R-40 would be considered insufficient for a cathedral ceiling in Zone 6 -- you should really be aiming for at least R-49 for a ceiling or roof.

    Assuming that you are talking about a double-stud wall, you can consider blown-in fiberglass -- but make sure that your contractor has experience insulating deep cavities before you go forward, because double-stud walls can be tricky to fill.

    Mineral wool batts can certainly work for a double stud wall. Even fiberglass batts can work, if they are installed conscientiously. (Conscientious installation of fiberglass batts is rare -- but that's another story.)

    No matter what type of insulation you specify, you need to have an air sealing plan in place before the insulation job begins.

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