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Dense-packed cellulose might go extinct

WildBunchFarm | Posted in Green Products and Materials on

I apologize for the clickbait title, but it seems all of the insulation contractors in my area are very wary of dense-packed cellulose in walls and often say, “we normally don’t do cellulose in walls anymore.” Part of their reluctance in my situation is attributed to my dpuble stud 10 1/4″ thick walls that are 24″ o.c. They think it will settle and bulge out. Still, I think even if I had standard 2×6 walls, 16″ o.c., they would all try to convince me to do spray foam and offer a lower price to do it.

I’ve spoken to 6 insulation contractors and only one agreed to do dense-packed cellulose and it was a pain to get them to give me an estimate, which is scary since it’s a sign they might not commit to doing it well. I even sent them this video that shows Natural Fiber representatives dense-packing a 12″ thick wall. Even after they claimed that they watched the video, they were still trying to sell me on spray foam.

In any case, we agreed to do a test of the dense-packed cellulose. If it doesn’t work out or they half-ass the job, I might have to end up going with spray foam after all.

My only other option is wet spray cellulose as much as they can and then fill in the rest of my walls with batt insulation.

What do you all think? I’m trying to avoid spray foam in the walls, but it’s looking like the best option in my area.

Here’s some pics of the house so far.

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  1. GBA Editor
    Martin Holladay | | #1

    In areas of the country where insulation contractors aren't familiar with cellulose insulation, the usual substitute is blown-in fiberglass, not spray polyurethane foam.

  2. charlie_sullivan | | #2

    Where are you located? Maybe someone will see this who knows someone who knows someone who can help.

    But I do agree with Martin that blown in fiberglass is the next thing to look at. I have no direct experience with it, but it should be easier than cellulose to do well in a 12" deep wall, and should be immune to settling if done well.

  3. WildBunchFarm | | #3

    Thanks Charlie and Martin. I can't find good information on dense-packed fiberglass. Is fiberglass less susceptible to settling than cellulose and why? Sounds like a good option over spray foam though.

  4. GBA Editor
    Martin Holladay | | #4

    You can start by reading the article in the GBA Encyclopedia: Blown-In or Loose-Fill Insulation.

  5. WildBunchFarm | | #5

    Well say this good thread. GBA really does have everything under the sun.

    Phil Lawson seems to really like fiberglass!

  6. GBA Editor
    Martin Holladay | | #6

    You may also want to read this blog by Alex Wilson: Getting to Know Spider Insulation.

  7. WildBunchFarm | | #7

    Thanks Martin. Great discussion on that article in the comments section. I'm not sure what you think of this idea for my 10 1/4" thick walls -

    3" of closed cell spray foam and then netted wet spray cellulose and cellulose for the rest of the 7 1/4".

    The closed cell might not let any potential wet sheathing from drying to the inside, but I already have a 1x4 rain screen on the outside that should allow for good drying to the outside.

  8. WildBunchFarm | | #8

    I meant to say "3" of closed cell spray foam and then netted wet spray cellulose or spider for the rest of the 7 1/4"

  9. GBA Editor
    Martin Holladay | | #9

    Like Charlie Sullivan (who asked you where you live in Comment #2), I'd like to know your geographical location or climate zone.

    You are planning to build a wall with a total R-value of about R-46, and about R-19 of that total R-value (41% of the total R-value of the wall) will be in the foam layer.

    That will work for Zone 6 and warmer zones, but it's not quite enough spray foam for Zones 7 and 8.

    For more information, see Combining Exterior Rigid Foam With Fluffy Insulation. While that article focuses on exterior rigid foam, not closed-cell spray foam between the studs, the ratios given in that article apply in your case.

  10. WildBunchFarm | | #10

    I'm in zone 4. I'm not sure why the ratios are important in my situation since the sheathing will always be cold. Since it's on the outside of the foam. The closed cell would also keep moisture from the interior facing side of the sheathing.

  11. brendanalbano | | #11

    The ratio still matters because you don't want the inside surface of the foam to be cold enough for condensation. You can think of the interface between the closed-cell foam and the fluffy insulation in your double stud wall just like the sheathing between the rigid insulation and the fluffy insulation in a single stud wall with exterior insulation.

  12. Expert Member
    Dana Dorsett | | #12

    "...I'm trying to avoid spray foam in the walls..."


    "...3" of closed cell spray foam and then netted wet spray cellulose and cellulose for the rest of the 7 1/4" ..."

    Plan-B- go all in, but with open cell foam:

    With 10.25" of half-pound density open cell foam you'd be at R38 center cavity, which is PLENTY of wall-R for a zone 4 climate. (more wall performance than it takes to get to Net Zero Energy). It would be far greener than 3" of closed cell + 7 .25" of dense packed cellulose, probably a lot cheaper too.

    The 10.25" of open cell foam would...

    ,,, use less total polymer than 3" of closed cell foam...

    ...use low-impact water rather than HFC (= extremely powerful greenhouse gas) for a blowing agent...

    ...would provide a drying path toward the interior for the sheathing...

    ... can be safely installed in two lifts of ~5-5.5" each...

    ... be more inherently air tight than 3" of closed cell foam.

    To manage wintertime moisture issues you could install a smart vapor retarder.

    Or, plan A.1:

    Install 1.8lb+ density dense-packed fiberglass would would deliver about R41. That would require a smart vapor retarder or half-perm "vapor barrier latex" primer to manage wintertime moisture drives.

    That would be far lower impact than 3" of closed cell foam or 10.25" of open cell foam and still a pretty good solution. At 18.lbs or higher density it's fairly air retardent, but still extremely vapor open, but a membrane type vapor retarder such as Intello Plus or Certainteed MemBrain detailed as an air barrier behind the wall board would give it excellent drying capacity, while limiting the rate of wintertime moisture accumulation, and very good resilience overall.

    Plan-B would be more inherently air tight but it's still possible to get there with air-tight drywall and/or membrane vapor retarders..

  13. WildBunchFarm | | #13

    Just the advice I was looking for Dana. You need a show on HGTV.

    The insulation guys are coming out in a few days and doing a test for dense-packed cellulose and dense-packed fiberglass. My only concern with using dense-packed cellulose was them not getting the density right as they don't have a density tester and it seems dense-packing is more of an art/feel than strict science.

    And it seems that at the lower density, the fiberglass install would have less room for error and future settling?

    We'll see how it goes. I'm guessing the only way to know if it's a good install is if there is some form of bulging.

  14. charlie_sullivan | | #14

    You can tell if it's a good install by feeling the mildly bulging netting--it should be like a firm mattress. You can look for spots they missed as well as feeling for the right density in general. But it's best to learn that in person from someone experienced.

    I think fiberglass settles less because it's lighter weight for the same pressure (firmness of packing), and because it tends to spring back better after getting compressed. So it's a little easier to get right. (Easy for me to say because I've never done it.)

    Looking forward to Dana's TV show.

  15. WildBunchFarm | | #15

    The TV show would be called "Dana Makes It Green" kind of like "Holmes Makes it Right." Why doesn't Green Building Advisor have a show?

    About vapor retardant primer, does it's application prevent drying to the interior? How much more vapor retardant is this primer over standard primer? I looked into the prices and it's almost double that of regular primer.

  16. GBA Editor
    Martin Holladay | | #16

    A vapor retarder works in both directions. It slows down (but does not stop) vapor flow from the interior of the house to the wall assembly. It also slows down (but does not stop) vapor flow from the stud cavities to the interior of the house.

    For more information on these issues, see these two articles:

    All About Vapor Diffusion

    Smart Vapor Retarders for Walls and Roofs

  17. Expert Member
    Dana Dorsett | | #17

    The new-school fiberglass is a lot "springier" than cellulose, and will not settle in walls even at 1lbs density (unlike cellulose which is hygroscopic and starts to settle due to normal seasonal humidity cycling below some density). Though 1.0 lb density IS a standard blowing density for a number of fiberglass products, it isn't very air-retardent at that density, and has a lower R/inch than the ~R4.1 - R4.2 / inch you get out of it at 1.8lbs density. At 1.8lbs density it's about as air-retardent as 3.2-3.5lb cellulose, which is pretty good.

    I don't know how I could cut it as a television personality, building on the portfolio of extreme (-ly silly) skiing video performances...

  18. WildBunchFarm | | #18

    Quick question - how important is it to do the vapor retardant primer if I go with dense-packed fiberglass. Would it be a problem if I just went with regular primer? I am in Zone 4 and have a 1x4 rain screen on the exterior. I've read vapor diffusion isn't as big of a problem as air leakage.

    Got some Icynene open cell spray foam applied to the house today. We needed to use the open cell spray foam to fill those traditional hard to reach spots (corners, small stud bays, stud bays behind chases, rim joists, etc.). Moreover, the spray foam helped fill those cavities behind headers and the gap between the gable end wall and attic. Hard to explain so I've attached the framing details and some pics. We made sure that our double stud wall covered the rim joists on all levels including the rim joists sitting on the sill plate. However, this leaves the aforementioned cavities behind headers and rim joists. Although, most of the cavities could be reached from the sides, we drilled 1" holes every 12" to make sure the spray foam insulation got in the middle.

    One of the pictures shows 8" flexible ducting in the attic - a big no-no for many. But I couldn't avoid it. The ducting was for my ERV that is carrying exhausted air from my bathroom. I decided to encase it in the spray foam. That also helped avoid any blown in cellulose insulation gaps that might have occurred behind the ducting.

    One of the pictures shows Roxul insulation with Tiger's Teeth to keep it between the trusses. We are going to blow in cellulose on top of this. Since the shed roof has a pitch of 3:12, we were afraid the cellulose would gradually settle towards the bottom. The roxul hopefully will provide some holding power and good minimum insulation just in case there is settling in some areas.

  19. Jon_R | | #19

    > That would require a smart vapor retarder or half-perm "vapor barrier latex" primer

    > Would it be a problem if I just went with regular primer?

    See here for discussion about actually getting 1/2 perm. I'd use Membrain for dryer walls in Winter and Summer. If you use fiberglass, consider a second, interior side air barrier (with compatible electrical boxes).

  20. WildBunchFarm | | #20

    Well, it looks like we're going with wet spray cellulose. The salesperson I was working with said that they couldn't do wet spray cellulose after about 6 inches, but when the actual applicators showed up they said we should test it. And it looks like it worked even in my 10 1/4" walls. I felt the wet spray was a little more firm than the dense packed cellulose, at least the way they applied it. This may mean less chance of settling over the years.

    Here's the test.

    Bay 1 - Lip stitched dense-packed cellulose. We lose about 1/2"-1" of insulation with this method, but it ensures no bulging. The worry here is that the netting will relax after a few years and encourage settling at the top. And I don't like the idea of having a 1" air gap in the walls.

    Bay 2 - Dense packed cellulose with netting stapled to the face of the stud. More chance of bulging, which to me made the applicator seem to not pack it dense enough.

    Bay 3 - Wet spray cellulose - Appeared the firmest out of the three. And less chance for user error.

    We didn't end up testing the fiberglass.

  21. Expert Member
    MALCOLM TAYLOR | | #21

    I'm no dense pack expert, but to achieve a firm enough install that future settling won't be an issue, there should be enough pressure to cause bulging of the netting. You get rid of it by rolling the finish bay. I'm worried that the walls are that flat.

  22. WildBunchFarm | | #22

    Yeah, that's why we're going with the wet spray. They did also roll the netted cellulose so that's why it's flat.

  23. Expert Member
    Dana Dorsett | | #23

    With a rainscreened exterior you don't need or want an interior side vapor retarder in a zone 4 location.

    Give it at least a day open air drying before putting up the wallboard. Don't use vapor barrier latex, or even standard paints on the wallboard for at least a week (two is better) after damp spraying a 10.5" deep cavity. If you want to be totally sure, buy a moisture meter and verify that the cellulose is below 25% moisture content before drywalling it, and re-test the drywall after a reasonable drying period to be sure the paper facer on the drywall is below 25% before painting.

    Damp sprayed cellulose doesn't settle in walls if they set up the equipment correctly. The density will usually end up between 2.5-3.0lbs after the moisture content stabilizes, but the moisture activated adhesives dramatically reduce the effects of mechanical creepage with normal seasonal moisture cycling.

    Without the adhesives the thicker the wall, the denser it has to be to avoid settling (as modeled and tested by researchers at Aalborg Univerity in Denmark.) National Fiber's dense pack coverage charts are based on that research, and for a 10.5" deep cavity you'd need minimum of 3.6lbs density to reliably get there without damp spray adhesives:

  24. WildBunchFarm | | #24

    Thanks Dana for the tips. Do they use the same bags of cellulose for damp spray and dense-packed? Or are the adhesives already in all cellulose? What is the adhesive?

    This is what the damp spray cellulose looks like so far.

  25. GBA Editor
    Martin Holladay | | #25

    Many -- probably most -- installers of damp-spray cellulose don't use adhesive. For more information, see my interview with Bill Hulstrunk: How to Install Cellulose Insulation.

  26. WildBunchFarm | | #26

    Thanks Martin. Great interview. I wish I could get Bill Hulstrunk to do my dense packed installation.

  27. Jon_R | | #27

    This company has a different cellulose product (with organic adhesive added) for damp spray. Other companies sell liquid adhesive to be added to the water. Seems like cheap insurance in case the density is lower than expected (and it's not clear how one verifies that it isn't).

  28. user-3428312 | | #28

    As I found out dense pack cellulose was not available in Northern Ontario, just foam and fiberglass. The cost of spray fiberglass was almost the same as spray foam so I went with foam. Reclaimed 2 x 4 foam, 4" exterior and 2" closed cell inside on 2x6 walls. Story I got was the mesh and cellulose was not approved, oh and the mesh was by the fiberglass mfg and I think they didn't want it used with another product.

  29. WildBunchFarm | | #29

    @John - sounds like you got a nice and insulated home. No cellulose up there? bummer. It took me probably 7 hours to blow in 41 bags of loose fill cellulose into my shed roof truss attic. Spray foam would have taken a contractor an hour to fill up those trusses. Moreover I had to work in concert with the drywall crew. They did the ceiling drywall in sections so I could actually reach with the blower hose and see what I was doing. I also had to make sure I didn't fill it up too high where the insulation blocked the ventilation ports.
    Spray foam saves time but you don't get to get into that meditative state watching the cellulose build up like snow four hours! But we did manage to get around R-60 up there. Do they make faster blowers that you can rent?

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