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Community and Q&A

Best Practice for Installing Dense Pack Cellulose in Rafter Bays

sierra801 | Posted in Green Building Techniques on

I’m planning an R-60 unvented cathedral roof assembly with 3” of rigid foam above the roof deck, dense pack cellulose in the rafter bays, and 5/8” drywall on the ceiling as the primary air barrier. What is the best installation practice for blowing dense pack cellulose in the rafter bays? The rafters will be 2×12 on 24” centers and will span 16 feet, ridge to eave. Would you advocate dense-packing through InsulWeb netting? Would 1×3 or 1×4 cross strapping be required or recommended with the netting for my assembly? Or would you blow the cellulose behind the installed drywall? I know that I should rely on a professional installer for this advice, but all of the contractors I have talked to in my local area have tried to steer me towards open cell spray foam (at very minimal depths!) and seem reluctant to even consider DP cellulose. I am requesting your advice for two reasons. Firstly to use as a standard for evaluating any future potential insulation contractors, and secondly in case I end up having to rent the equipment and do the install myself. I have already read the excellent interview with Bill Hulstrunk and other related articles on this site, but am looking for a little more detail on the installation.

Climate Zone 5 Central AZ

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  1. Billy | | #1

    I would use cross-strapping on 16 inch centers and InsulWeb netting so you can blow the insulation before the drywall.

    The cross-strapping does a few things. Placing it at 16 inch centers helps keep your drywall from bowing. It will reduce thermal bridging through the rafters (not a huge deal because you have exterior foam on the roof deck. It also will help eliminate nail pops from the 2x12 lumber twisting with seasonal changes.

    Make sure they staple the Insulmesh to the strapping, not the rafters.

    Can you go to 4" of foam on the roof deck? Make it airtight, including at the ridge.

    It would be good to spray foam the ends of your rafter bays at the exterior walls before you blow the cellulose.

    Another option for dense-pack is Spider fiberglass insulation. The fibers are very fine and it dense-packs very well. And it won't get moldy if you have a roof leak...

  2. GBA Editor
    Martin Holladay | | #2

    It's important for the insulation contractor to be comfortable with the chosen installation method. Here in New England, most installations include ceiling strapping. This strapping is often installed over (actually, under) a layer of rigid polyisocyanurate, installed before the cellulose is blown in the bays.

    For more information, see How to Install Cellulose Insulation.

  3. user-788447 | | #3

    I think what requires the most attention with cathedral ceilings is how the air tightness plane will be achieved. Its easy enough to say the drywall is the air barrier but detailing it at all the different transition conditions and executing it in the field is another matter. What's your plan for lighting? for example.

    Concerning DP cellulose install I'm pretty confident from what I have heard that you don't want to install this after the drywall is up.

    Because the rafters bays are considerably larger than a 2x6 stud wall cavities, DPcell. install requires greater densities and will bulge drywall especially at 2' on center.

    Some local guys developed a technique to help keep the InsulWeb from bulging during install. After stapling they pinch it down with a metal channel (see photo). Even with this extra measure I would recommend 1x strapping and locating the InsulWeb between the strapping and the rafters. Your drywallers will thank you for it.

    That leaves you yet with the challenge of executing a continuous air barrier (including the transitions to the air barrier of the walls) while attaching the drywall to strapping. Detail it on paper, not in process in the field.

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