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Loose-Fill Cellulose for Scissor Truss Attic

DavidDrake | Posted in Green Building Techniques on

The consensus here seems to be that loose fill cellulose insulation will slip or slump if installed on a slope great than 4:12 or 5:12. However, I didn’t run across an article or post describing exactly what the results of slipping or slumping were, so I thought I’d see for myself. And see if there was a way I could make loose fill work with my overly-steep trusses.

I assume what happens is, if installed on a steeper slope, the cellulose will eventually slip or slump until it reaches its angle of repose (equal to the angle of a 4:12 or 5:12 pitch).

To account for this, I installed 48″ long baffles between each truss. The trusses are 12:12 top chord/6:12 bottom chord energy heel scissor trusses, spaced 24″ OC. See attached sketch for details.

The sketch show three dashed lines: Condition A—the blue line, corresponding to 18″ of unrestrained loose fill, which has slumped to the angle of a 4:12 pitch; Condition B—the green line, corresponding to 18″ of loose fill restrained by two Insulweb ‘dams’ stretched perpendicular to the trusses, such that even if the cellulose does slump to 4:12, the results will still be similar to loose fill without slip or slump, as in Condition C (the orange line). Note that in all cases, the baffle extends above the top of the loose fill.

I expect installing Insulweb dams will be more labor but less materials than simply blowing in loose fill until 18″ is achieved at the peak, and that’s the option I’m going to try. I’m hoping the dams will result in a more or less uniform layer of cellulose, a bit thicker in some spots, but all areas at least 18″ when installed (16″ after settling). I should be blowing in the cellulose this week or early next.

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  1. Robert Opaluch | | #1

    I had similar concerns, and this is a great solution IMHO!

  2. Expert Member
    Akos | | #2

    Not sure it will work. The cellulose not only squishes down but slides down along the ceiling. This means you might get a gap with no insulation behind your insulweb baffles.

    I think a better solution here is to install longer vent baffles, frame in a very small flate ceiling at the ridge and overfill the whole thing with cellulose. This way any settling will be take up by the extra insulation above the mini attic at the ridge.

  3. DennisWood | | #3

    If you have had to disturb cellulose after a few years, you will observe that it "sets" to some extent almost like dry cake and has surprising cohesion. I suspect this is a function of both moisture and mass over time. If you use flexible web, there is a good chance the cellulose will crack more like dry earth nearer the peak as the cellulose below it settles. The web won't provide any support to the cellulose above it once settling occurs downward.

    I suspect you'll have better luck with 2x4 or 2x6 baffles installed across the bottom chords, every 3-4 ft or so. If you keep the base stable, the cellulose above it should stabilise once it sets in place.

  4. Expert Member


    Both Dennis and Ako's solutions sound like they would be effective.

    Since this bears a lot of similarities with the difficulties of slope stabilization, I wonder if there isn't a solution from there that might work? Maybe a cheap geo-grid mesh course enough to allow the cellulose to be blown through it?

    1. Expert Member
      BILL WICHERS | | #5

      I had thought of using a thin material (like the 1/4" waferboard I often recommend for air baffles) to make vertical barriers every few feet, to essentially "compartmentalize" the loose fill insulation into smaller areas. This would mean less force exerted on the lower levels of insulation, so probably less sliding/slipping over time. I haven't tried it though.


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