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Community and Q&A

Dense pack cellulose: facts or fictions?

Donald Lintner | Posted in Energy Efficiency and Durability on

For dense packing cellulose insulation, does it matter if it is blown dry behind netting or done as a damp spray? Any difference between the two techniques in the R-value or convection properties in the resulting wall assembly?

On another aspect of cellulose, is there a limit to how deep a roof assembly can be dense packed? The example I was given is that typical 11 7/8 inch I-joists would pack OK but that 16 inch deep parallel cord trusses are too deep to truly dense pack.

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  1. John Klingel | | #1

    I know that 18" walls are dense packed. It has also been mentioned on here, I believe, that the netting (in a wall) is not stiff enough to dense pack properly. To get the right density, which is about 3.25 lb/cf, the netting would bulge out between the studs, making sheet rocking a nightmare or impossible. You may want to check me on that. What would be confining the cellulose in your roof so that it could be dense packed?

  2. GBA Editor
    Martin Holladay | | #2

    According to Bill Hullstrunk at National Fiber, a cellulose manufacturer, it is possible to dense-pack cellulose to 3.5 pounds per cubic foot behind Insulweb netting. Here is a technical document listing the steps to achieve that density behind netting:
    Directions for Installing Dense Pack Cellulose behind Insulweb Netting.

  3. Jesse Smith | | #3

    My recollection is that damp-sprayed cellulose reaches a density of roughly 2 lbs/cf, and therefore shouldn't be considered dense-packed. Depending on the assembly you're designing, this may or may not matter. If you're looking for cellulose to help achieve significant reductions in air leakage, damp-sprayed cellulose probably won't cut it. Building Performance Institute standards call for 3.5 lbs/cf to be considered dense-pack, and in my experience this density will achieve significant leakage reductions (@ 50Pa). I believe National Fiber's site (which is fantastic btw) has information on R values at specific densities.

    In principle, any depth of cavity can be dense-packed by using multiple insertion points. A 16" deep cavity probably wouldn't require more than one insertion point provided the single insertion point was central, i.e. 8" above the ceiling. Depending on the equipment and machinery settings, the material seems to disperse pretty evenly for 1'-2'.

  4. Donald Lintner | | #4

    I'm working with an insulation contractor with a good reputation who connected me with an energy auditor to blower door test my house. The auditor told me one can't dense pack to 3.5 psf a 16" deep roof and that damp spray cellulose doesn't have enough density to qualify as being dense packed either. I have panels between the trusses creating a vented roof and 2" rigid foam (air sealed) on the bottoms of the trusses for the ceiling dense pack and the walls are 12" double stud. The insulation contractor blows in dry cellulose in ceilings but uses a damp spray for walls. I thought I'd be getting dense pack cellulose for both walls and ceiling but according to the energy auditor, I won't be getting dense packed cellulose anywhere.

  5. Donald Lintner | | #5

    Your answer posted just as I did mine. Thanks for the details. It is a cathedral ceiling and we are leaving out the top foot or so of foam at the ceiling peak and blowing each truss bay and then netting the the last bit and finishing that bay off so I should be getting close to 3.5pcf for the roof. I couldn't find the r-value chart on the site but National Fiber's Nu-Wool product description appears to be a damp spray similar to what my contractor would use and they indicate it achieves 3 to 3.5 pcf. This contradicts your memory and my auditor's information.

  6. Jesse Smith | | #6

    That's why you really shouldn't take advice from guys with the last name 'Smith' on the internet.

    You're right, it looks like there's a system for achieving high densities with damp-spray cellulose. It's probably important to distinguish between the system described for use with Nu-Wool and the practice used by many insulation contractors of adding water to cellulose to make the material more workable and reduce dust.

    If you're looking to achieve a certain density, then specify as much with the contractor. Count bags - ideally for specific areas - to verify the material was installed properly.

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