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Dense packing cellulose with the Krendl 425

JDuchek24 | Posted in Energy Efficiency and Durability on

Hi All,

I am going to attempt to dense pack several horizontal stud bays (2×10 floor joists, 12in o.c., between attached garage and bonus room) using the Krendl 425 (14amp 2-stage blower, pushing 1100 – 1400 lbs of cellulose per hour) over the upcoming weekend.

Each horizontal stud bay is about 8 feet in length and has about 5 cubic feet of total space to be filled (which is equivalent to about 1 bag of big box store cellulose at 3.5 lbs / cu. ft. of density.) I will be working in semi-cramped attic space, pushing the hose through insulation netting on one side of the kneewall (with the far end of the bay sealed with rigid foam)

Considering speed, quality and the “less likely to screw it up” factor, I am looking for guidance on whether I should:

a) “Loose fill” the bays with the 2.5in hose (damper fully open) and then go back in with a 1.5in flex PVC
b) Just start (and finish) each bay with the 1.5in flex PVC (reduced down from the 2.5in blower outlet)
c) Consider just using the 2.5in hose and a mostly closed damper

I assume that the bigger diameter hose / closed damper approach will be slower (much slower?) and perhaps slightly less effective? I am weighing this against the extra effort to get the hose reduction correctly down to 1.5in., etc.

My thinking is based on guidance provided in this thread:

And also Bill Hulstrunk’s comment (from 2011) when asked about the Krendl 425:

“With the Krendl 425, the air should be set at 100% and the feed gate closed to approximately 25% depending upon the size of the installation hose being used. The bag coverage chart can be used to confirm proper installation density of 3.5 lbs/cu. ft. If not enough bags were used, than close the feed gate more, reinsert the installation hose and reinsulate the cavity to increase the installed density to achieve a dense pack.

Any advice / opinions are certainly appreciated.



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  1. GBA Editor
    Martin Holladay | | #1

    Good luck! I don't have any advice -- except to inform you that those aren't stud bays. (The term "stud bays" is used for walls.) Those are joist bays.

  2. JDuchek24 | | #2

    Thanks Martin. I'll report back regarding results.

    Yes, I am definitely dense packing 2x10 joist bays. Thanks for highlighting.



  3. Jon_Harrod | | #3

    I did a lot of walls and floors with a 425 back in the day. I agree with Bill's recommendations for machine settings. In the interest of time, I'd pre-fill the cavities using the larger hose then go back with the small hose and dense pack. You won't be able to get a true dense pack with the bigger hose; you might get it dense enough that it won't settle, but it won't stop infiltration the way a true dense pack will.

  4. JDuchek24 | | #4

    Thanks Jon - I appreciate it.

  5. JDuchek24 | | #5

    Dense-packing the joist bays with the Krendl 425 was a success. Based on the bag count, I likely got to 3.5 or 3.6 lbs / cu. ft., with the ends of the bays definitely feeling like firm mattresses. I am appreciative of Martin, Dana and Jon’s helpful advice - I wouldn’t have been successful without it.

    Additional thoughts / comments for first-timers:

    - I started by filling the bays with the 2.5in flex tubing that came with the Krendl (using full flow and just pulling back when the flow stalled) - that alone got the density to somewhere between 2.2 - 2.5 lbs / cu. ft. (with the cellulose feeling like an "old lumpy pillow” at that point). I then followed up by reinserting a 1.5in Flex PVC (“schedule 40”) dense-pack tube, with the business end of the dense-pack tube cut at a 45 degree angle. I closed the feed-gate damper approximately 50% when using the 1.5in tube. The dense packing element took twice as long, even with all the bays already filled with the larger flex tube.

    - I used PVC pipe and PVC reducers to get from the 2.5in flex tube to the 1.5in dense pack pvc tube. I realized too late that Krendl steel hose reducers are available at reasonable prices online (but are seemingly not sold in stores). I would strongly suggest buying those reducers. I was able to make the PVC reducing system work, but I spent far too much time / energy trying to come up with the right “home center” solution, likely had some pressure leaks and almost couldn’t make the “amateur” version work (i.e. I had a few clogs and had to re-work things a bit).

    - It look me a couple of bays to realize that you need to work the dense pack tube around quite a bit to get the best results. Even just moving it forward and backward did the trick for me (as the cellulose then fills the recently vacated tube space).

    - Dana’s advice about marking the dense-pack tubing with tape about a foot from the business end and also using an old shirt / sweatshirt to cover the remaining hole in the netting were both very helpful tips.

    - I used Insuloc rather than Insulweb (both made by Hanes). Insuloc worked well for my application, was easy to staple using a light-duty staple gun and also held the cellulose well.

    - Having a remote-controlled, hard-wired start / stop function made things much more efficient.

    - The Krendl 425 had two plugs (for two separate 15 amp circuits) and I had no issues with over-loading / tripping circuits.



    1. Deleted | | #7


  6. GBA Editor
    Martin Holladay | | #6

    Thanks for the report! Good information.

  7. JMRtbay | | #8

    Thanks for the first hand account. My local HD has this blower available for rent. I’ll be starting my slightly better than PGH next year and I’m leaning towards double stud walls (similar to Lstiburek ideal wall without exterior sheathing) with dense packed cellulose. Im also considering purchasing an original Force 2 blower with an upgraded blower and agitator.

    Did you by chance take any video of the time it took to pack a joist bay and your process?

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