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Community and Q&A

DIY Dense-Packing of Cellulose

SierraWayfarer | Posted in General Questions on

Hi All,

If I built a Mooney wall (2×6 with 2×2 furing) and used a typical low powered cellulose insulation blowing machine from a Home Depot or Lowes would it help if I installed not just the netting but one layer of sheetrock at a time to help with the dense packing? Dense pack the wall-space behind the first layer of sheetrock as tight as I can (without bowing the sheet rock, maybe set something in from of the sheetrock while I am blowing to keep it from bowing out?) then add the second layer of sheet rock (9′ ceilings) and blow the remainder of the wall? Has anybody done this, does it sound like it would help with the dense packing?

Would adding a horizontal layer of netting help with settling.

Would rolling it down from the top side as I go help with the dense packing.

I notice they have wet pack insulation to seemingly help glue it in place and stop it from settling. Is there anyway to apply the spray independently of the machine, say after each time I roll it down?

I know all of this is labor intensive but it would be mostly my labor and I am retired, bored, and not yet decrepit.

Thanks for any experience or advice you can offer.

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Replies

  1. Andy S | | #1

    A crotchety old carpenter liked to say "If that worked, everyone would already be doing it."

    1. Deleted | | #2

      Deleted

  2. SierraWayfarer | | #3

    I saw this: https://www.greenbuildingadvisor.com/article/how-to-install-cellulose-insulation
    ---So I don't guess you can pack to tight.
    In double walls he loose packs the whole cavity then changes to a smaller nozzel (higher pressure) and starts again at the bottom of the cavity feeling around for looser spots to pack tighter. I guess filling the cavity first creates a useful back pressure.

    1. DCContrarian | | #4

      Might also be faster than filling the whole thing with a small nozzle.

      1. SierraWayfarer | | #6

        Hi DCContrarian,
        Yeah, thats what I thought. Doing the whole job at loose pack density, 4/5ths of the cellulose, and then coming back around with the smaller nozzel and working in the last 1/5th probably saves time.
        Thanks for your view

  3. Eric Habegger | | #5

    "I know all of this is labor intensive but it would be mostly my labor and I am retired, bored, and not yet decrepit."

    I hate it when people talk about me. Actually it really is a perfect description of my own stage of life. I did exactly what you are thinking about and it was not a problem at all with a low power blower. You just have to use one with a slide gate and reduce the ratio of cellulose/air so it will compact at the density you need. Doing that in combination with Insulweb and the use of a roller can get you to densities you would not think possible with a machine that doesn't pressurize past 3lbs/cu ft. You can though by bellying it out sufficiently and then rolling it back flush with the studs.

    I did it with 2x6 walls. The people who say it can't be done are selling you short. I would try it first without the extra step of installing a separate layer of drywall and see how it goes. I think it will work if you take your time and do frequent pressure check just with fingers. Also, roll out at least one cavity space before you commit so that you insure you can compress it flush with the outer edge of the framing. It would do you no good to get what "appears" to be proper density without being able to get it flush. Also, definitely use an adapter with a smaller nozzle because you need to do that to get to high density with the typical low power blower. I don't think you would need to go smaller than a 1.5 inch diameter hose. Smaller than that is probably just too time consuming because you would need to close down the slide gate too much to keep it from clogging. It's a balancing act.

  4. SierraWayfarer | | #7

    Hi Eric,
    Thanks for sharing your experience, again...(I read your comment on Martin's blog about dense packing with a low power machine).

    I think you are right on the foregoing of 'putting up of the sheetrock layer by layer'. I have to get a machine that has a 'slide gate' and/or a reducer nozzle (like a smaller venturi on a rocket engine increases thrust) and/or possibly be prepared to 'rig' the machine's nozzle to a smaller size with: hacksaw, file, tubing, glue, duct tape, and no telling what... I won't know this until I look at the machine when I schedule it's use and I may end up having to pay the 'box store' for another nozzle... I will use your '1.5" nozzle diameter as my lower limit. That is an inside diameter, isn't it????

    First, I will pack all of the walls to a loose pack density which will increase back pressure and should allow me to more easily dense pack on the second pass after I shutter the "slide gate and reduce the ratio of cellulose/air" and reduce the nozzle size appropriately and thereby increase the pressure with which the added cellulose is pushed into the existing loose pack. It should be easy enough to check density by both pressing on the webbing and as I get further along by calculation of bags used against stud bays filled. Obviously I will need some sort of roller and I will have to be more cautious in the early going until I get a 'feel' for it.

    Doing without the layering up of the sheetrock will alleviate the problem and likely increased expense of keeping the machine longer than the 'box store' would like.

    The netting will require more effort because of using a Mooney Wall. I wonder about furring out the Mooney wall using an internal Larsen Truss (and only a 2x4 load bearing exterior wall) rather than 2x2's run horizontally on a 2x6 wall. Does that make any sense? I apologize for not having a detail but the idea just came to me.

    I don't like Larsen Trusses (to the exterior). I don't like cantilevers in general and I especially don't like cantilevers that create a shaded sheltered space close to the ground for little creepy crawly things in my dry/cool winter/hot summer/high diurnal climate. I could certainly be wrong. A Larsen Truss to the exterior would provide more livable space for the dollar but I wonder about it's long term viability. I still need to use BEopt or calculate an estimated optimal wall depth. I won't be using external foam board. I think it is an unnecessary expense and modest risk (tunneling bugs) in this dry moderate winter climate but again, I could be wrong.

    Between not liking Larsen Trusses and external foam I am beginning to wonder if I am a 'conservative designer' in spite of my forum moniker and social inclinations...

    I don't want to mention this but if tapping on the sheetrock at the top of the wall produces a hollower sound at some time in the future there is always a messy, modest expense, time consuming, DIY solution.

    Thanks for any and all help.

    1. Eric Habegger | | #8

      I think that idea of making two passes with separate size nozzles is a good one. I'd coordinate that with the adjustment in the ratio of air/cellulose on the slide gate. I wish I'd thought of that when doing mine as it would have gone much faster. You'll find that blowing with a larger nozzle requires less movement of the nozzle to get an even blow throughout the stud bay but the machine will stall quicker as the stud bay fills up. It is absolutely necessary to use a blower with a slide gate or the machine will stall out before getting even close to the needed density. That's just the nature of a low power blower.

      Good Luck!

      1. SierraWayfarer | | #11

        Hi Eric,

        That interior Larsen Truss thick wall to the interior instead of a double stud wall coming off a [email protected]"o.c. exterior bearing wall would help with the blowing too, the narrower 16" stud bays. I guess the sheetrock would hold up to that. Would it need to be furred do you think?

    2. Expert Member
      Josh Salinger | | #9

      Hi SierraWayfarer,

      We have a general rule at our firm that when we are insulating walls thicker than a 2x6 we switch from cellulose to fiberglass as we have heard anecdotes about and seen some settling in walls thicker than 5.5" over the years. We haven't experienced this with fiberglass, however.

      If you do go with cellulose (which I do prefer for a number of reasons) I would be sure to measure the density you are actually achieving vs. just 'using ones hands'. We have a tool designed specifically for this from Green Fiber (pic below), but all it is, essentially, is a tube with a 2 1/4" I.D. that attaches to a drill. One could easily create something similar with a ABS pipe in order to extract a core of cellulose. Once you have this core sample it can be weighed-- we just use a simple digital scale we keep for this purpose. I've attached a simple spreadsheet/calculator that calculates the weight of the core to pounds per square foot. This way you can check various areas of the wall to make sure you are being consistent and achieving the target density of 3.5 pounds per sqft. We tend to find that corners, tops of walls and hard to reach areas don't quite meet this and we will mark it as such with a sharpie on the netting. We then go back and add additional insulation to these areas in one swoop.

      Another thing we do is use two layers of insulweb netting and we will attach it to the face of the studs with a simple stapler. We then go back and staple with a upholstery stapler about a 1/4" in from the face of the stud on each side. This really tightens up the netting and allows for the bowing of the insulation to do its thing and give it a fighting chance to be rolled back into submission. It also helps achieve a good density. We typically use a 5/8" gypsum on the exterior walls as we don't want to see this bulging get reflected through to the finished walls. This thicker gypsum is also more robust and can also help squeeze the bulges into the cavities. We have had some grumbling from the hangers, but they always manage to get it installed. The thicker gypsum also adds thermal mass to the building, so it has additional benefits.

      Hope this is helpful!

      1. Eric Habegger | | #10

        This is good advice.

      2. SierraWayfarer | | #13

        Hi Josh,

        Thank you so much! Lots of good information there!

        I think I might go with the dense fiberglass. The link I posted near the top of this page is to a GBA interview by Martin with a cellulose insulation professional who said they go to higher densities when they do double stud walls (to avoid the settling I guess, just like you said). Higher densities cost/require more fiber and fiberglass has a higher R-value than cellulose though less wind or sound resistance.

        Cellulose collects moisture, something I wish to avoid for several reasons. My location is blessed with low humidity and I would like to keep the stud moisture content at a low level to lower the chance of mold and termites.

        That is a wonderful idea about how to check the density. So simple!!!

        That cellulose professional in the link above also said that corners were more difficult to pack heavily and said he tended to do them first.

        I like your idea for using two layers of insul-web too. Makes sense...and the 5/8" sheetrock too. I will be using it on the ceiling to support all of the insulation up there so I might as well make it simple and use it everywhere... I like things sturdy and yes sir on the thermal mass! There is a reason that Adobe was prominent in early New Mexico (and not uncommon now). Our dinural (daily) temperature swings in Southern NM and West Tx are greater than almost everywhere in the country or the world for that matter so thermal mass is important here.

        Thanks again for all of your valuable help!

  5. Expert Member
    Dana Dorsett | | #12

    Instead of a 2x6 + 2x2 Mooney wall, 2x6 + 2x4 Mooney wall with Intello Plus (detailed as an air barrier) on the edges of the 2x6s allows enough space to dense pack the 2x6 and 2x3 cavities independently, and 2x3s are deep enough to run all the electrical boxes & wiring exterior to the Intello layer, which lowers the amount of air sealing detailing required.

    Even in large rolls Intello is still slightly more expensive per square foot than a layer of gypsum board, but it's cheaper than gypsum board + 2-mil nylon (or vapor barrier paint), and easier (Iess DIY labor) than dense packing gypsum board. (slit & tape vs. drill & patch)

    See:

    https://foursevenfive.com/blog/how-to-install-dense-pack-cellulose-with-intello-plus-airtight-membrane/

    With ~R10 cellulose to the interior side of air-tight Intello and R20 on the exterior in most of the US the air tightness or vapor retardency of the interior finish wall would hardly matters. Unless the indoors were kept at TROPICAL relative humidity the vapor retardency of the Intello would stay in the Class-II zone all winter even in zone 6 and the warm half of zone 7.

    For the record, in what IECC climate zone is this house being built?

    If for some reason 2x2s are still preferable to 2x3s, splitting compressing R13 cellulose batts for the 2x2 layer hits a reasonable density:

    https://www.cmsgreen.com/sites/default/files/ecocell_35_batt_sheet.pdf

    1. Eric Habegger | | #14

      Dana, I'm not convinced intello-plus is right for this install. Although I've had success with low power cellulose blowers with gate valve to do it there are some modifications required. Here's what it said in that 475 memo:

      "Because the membrane is flexible and translucent, inspection to ensure a complete fill is facilitated. Once the dust is settled (there won't be much as INTELLO is airtight), make the membrane dust-free and apply 6" patches of TESCON VANA 150 over the blow-in holes to reconstitute your airtight layer."

      Notice it says intello is airtight. That seems like a problem. You need displacement of air to get the full required density. Sure, you can cut slits to allow air to escape but it's not ideal. With a low power blower you can't use it behind drywall with 2 holes drilled in it because the intrinsic power of those blowers is below the 3.5 lbs/cf needed to eliminate settlling. This same idea seems to eliminate blowing cellulose behind intello. The blower just won't have the power to belly the cellulose out if air doesn't escape. It's the same reason you can't use a low power blower to blow cellulose behind drywall with 2 holes cut in it. You need that elasticity and expansion room so you can compress it with a roller to get the 3.5 lbs/cf required for it to not settle later.

      1. Expert Member
        Dana Dorsett | | #16

        >Notice it says intello is airtight. That seems like a problem. You need displacement of air to get the full required density.

        What are the odds that the rest of the assembly will be truly air tight? It's somewhere between slim & none, sez me, unless the sheathing is glued to the framing and SierraWayfarer spent gobs of time detailing it as an air barrier, which is possible of course.

        In the case of an air sealing obsessive SierraWayfare, installing the Insulweb in place of the Intello would be better (and more inspectable), which is fine, as long as the interior side air sealing is attended to.

        >"With a low power blower you can't use it behind drywall with 2 holes drilled in it because the intrinsic power of those blowers is below the 3.5 lbs/cf needed to eliminate settlling."

        The density may be what's required to eliminate settling is 3.5lbs in zone 6A, but that isn't universal. Even 2.5lbs should be plenty in the zone 2B/3B parts of the desert southwest, 2.8-3lbs in the Gulf coast or zone 6B (with a class II vapor retarder). Settling is a function of creepage from seasonal moisture cycling in & out of the cellulose. The greater the moisture cycling, the higher the density required to keep it in place. This was studied extensively by people at Aalborg University in Denmark in the '90s and early 'oos. Don't assume 3.5lbs to be gospel even in zone 6A, and with a single stage rental blower you'd be lucky to break 3lbs.

        I've personally seen sub-3lbs density cellulose that was installed using 2-hole method in a 2 story balloon framed antique in zone 5A that had not settled after more than 25 years of service. This was in an 1890s house being gutted as part of a deep energy retrofit. The crappy 1lbs(?) density early 1980s open blown cellulose in the vented attic (with huge seasonal moisture cycling) had settled, but not the stuff in the walls.

        It's easy enough to count the bags and calculate the volume to come up with an estimate of what the average density is working out to be. If the cheap single stage box store rental blower just isn't cutting it, 2-stage blowers can be rented (from sources other than box stores.)

        The other option is going to fiberglass, of course. Fiberglass only needs to be 1.8lbs density to be reasonably air-retardent, but even 1lbs density fiberglass will never settle.

        1. SierraWayfarer | | #17

          Hi Dana,

          Air sealing, caulking, taping, ... is going to be one of my main jobs on the build. Free up the young strong guys for the heavy work. I think a tight structure is very important, especially with blown dense pack fiberglass. Did you see my questions below?

  6. SierraWayfarer | | #15

    Hi Dana!

    Thanks for your input. This is zone 3-b, hot/dry and windy.

    Did you see my idea for changing my exterior load bearing wall to 2x4 @ 16" o.c. and then extending to the inside with an 'interior Larsen Wall' (ripping an appropriately sized i-joist in half) which would increase my thermal bridging but shrink my stud bays and help with the insulation blowing. Is this reasonable? Would I have problems with cracks in 5/8" sheetrock? Would it be more expensive than what you are suggesting?

    Also, I now think I want to use dense pack blown fiberglass to help keep the stud-frame's moisture levels lower and hopefully reduce long term chances of mold (pretty sure of that) and termites. I have read that even drywood termites need at least 10% wood moisture levels and with fiberglass insulation I think I have a pretty good chance of keeping the studs below that for long periods of the year, especially winter.

    There aren't great problems with termites around here. The old stucco houses, 70-100+ years, commonly have stucco touching the ground. But, I don't like to take anymore chances than I have too. Am I being too cautious with mold and termites?

    Thanks for any input you can give.

    1. Expert Member
      Dana Dorsett | | #18

      In zone 3B the target density is less of an issue, and with cellulose fill the vapor retardency of the interior is MUCH less important (even in zone 3A). And the borate content of cellulose fre retardents is known to be a mitigating factor against termites. You're on the right track!

      In your zone 3B case using Insulweb instead of Intello would be both cheaper and achieve a somewhat higher density, all of which is good. With a 2x6 + 2x2 Mooney wall dense packing the 2x6 using blowing mesh and splitting R13 cellulose batts to fill in the 2x2s after running the electrical is as good as trying to dense packing it. If you install the 2x2s before dense packing you won't even need to glue the blowing mesh to the glue- a bit of pillowing between the mesh and 2x6 edge won't hurt anything, and the 2x2s won't be blown out by the pressure if using ring shank nails or screws.

      But if going with a Larsen Truss, why not do it on the exterior? On the interior a double-studwall or 2x4 + 2x3 (staggered stud or straight) studwall approach would have less thermal bridging than I-joists.

      Since it's not structural, going with the cheapest grade of FINGER JOINTED 2x3s on the interior wall can result in an even flatter interior wall than with milled x 6s. Finger jointed lumber isn't usually stocked at the local lumber yard, but it can be ordered. (I just looked- they are in stock at the big orange box store in Vancouver B.C.- up for a road trip? :-) Oh yeah, the border is still closed... :-( ) Finger jointed lumber is not a straight as LVLs but it's a heluva lot cheaper (and cheaper than I-joists!) but straighter than standard milled framing lumber, and WAY straighter than milled 2x3s.

      With a 2x3 + 2x4 double studwall approach at the same total 7" framing depth as the proposed Mooney wall you'd need to install a layer of Insulweb on the 2x4s and dense pack it first. With in-line (non-staggered) configuration, install some 1.5" wide 1" foam board to the 2x4 studs with foam board construction adhesive (Bonfiglioli strip style) before dense packing and before installing the 2x3s. With staggered studs, first roll the dense packed 2x4 wall flat, and apply the foam board strips to the exterior side of the 2x3 wall before tipping it up into place, which will compression-fit nicely against the dense packed blowing mesh on the 2x4 wall, providing enough of an air barrier between 2x3 stud bays to dense pact them separately.

      When using blowing mesh on the interior finish wall side, after stapling the mesh in place, lay a bead of wood glue on each stud and work it in with a brayer, to keep the mesh from separating from the stud and filling with insulation. That keeps the interior wall surface nice and flat.

  7. SierraWayfarer | | #19

    Thanks! That was a lot of alternatives. A lot to think about!

    1) You didn't care for the interior Larson Truss idea because of expense and thermal bridging. I get that. I haven't crunched the numbers but I am sure you have crunched similar numbers and so have an idea off the top of your head what you are talking about. I don't.

    You asked about why I don't like external Larsen Trusses. I think they might be ok in a cold climate up higher on a wall but I don't much care for cantilevers anywhere I especially don't like them near the ground. Stuff collects under that 6-8" gap to the ground.

    Further, Larsen Trusses in the desert and especially Larsen Trusses close to the ground would provide a nice environment for insects up underneath them. I have a very healthy respect for the creatures (and plants) of the desert. It is a tough environment and it produces tough creatures that I don't want them around my home. I don't want to be fight spiders up under the Larsen Trusses for the rest of my life.

    I am leaning toward a metal rainscreen that help will cook them out in the summer and freeze them out in the winter. Hopefully the creatures will go next door and get in the neighbors stucco which is everywhere in Southern New Mexico cracked, moister, and more comfortable than most of the surround.

    Those 'FINGER JOINTED 2x3s' are a way for a mill to utilize shorter pieces of lumber? Not sure about utilizing those, an interesting efficiency though. But, I might be willing to pay a little more for a little more than those.

    I am building a small house (for my old age) that I hope wont give me many problems. I am keeping it small so can afford durability, reliability, comfort, and pretty good efficiency. Like many here, I want to build a pretty good house and not let the perfect get in the way of the good.

    I think I want to fill the walls with dense pack fiberglass so I can keep the wall interiors at a low moisture content.

    I don't think foam is necessary here. I want this house's walls to be pretty vapor open. Judging by the old houses here, vapor and air open works here. I will forego the air open.

    As far as the insulation density goes: I will reach toward the optimum r-value density in order to narrow walls and preserve interior space. But, I want this thing well insulated and air tight. In fact, I may even insulate under the slab, haven't decided. Certainly I will insulate the edge and the perimeter of the slab. I spent last winter in a home here with concrete/tile floors and they were cold.

    We get big daily temperature swings here. Regionally, Southern NM and West TX have bigger swings than anywhere in the country. So, not only slab and 5/8" sheet rock for thermal mass but also maybe insulate the slab to help moderate indoor temperature swings.

    I have got to get around to crunching numbers so0n. All my design talk doesn't mean much until I start attaching numbers to it.

    Thank you for all of the alternatives. It is always healthy to get outside your own box and comfort zone.

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