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Dense packed cellulose in deep roof joists

Anders Bostrom | Posted in Green Building Techniques on
I’m working on a building with a low pitch, 2:12, shed roof. 
The roof construction is OSB, ice and water shield, two layers of staggered 4″ foil coated EPS (taped and foamed), another layer of OSB topped off with ice and water shield, and a standing seam metal roof. That would be about R-32.
 
12″ roof I-joists span the width of the building and I plan on adding dense-packed cellulose below the roof decking. To be able to maintain 51%+ of the R-value on top of the roof decking I plan to limit the cellulose thickness to 7″ for a total of about R-60.
The issue I have is how to limit the dense packing to 7″ in a 12″ joist.
I can place the fabric at the 7″ level, fill the cavity and pack/roll the cellulose. With time and its own weight, the insulation will most likely be sagging and leave an air gap at the roof decking. 
 
One alternative could be to install some sheeting on the 7″ level leaving the remaining 5″ as a wiring chase on top of the final sheetrock. Install the extra layer of sheeting looks like a lot of extra work (plus parts of the ceiling is at 16′ level). 
 
Any suggestions on alternatives?
 
I’m in zone 6B   

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Replies

  1. Expert Member
    Akos | | #1

    If the rigid is not already installed, the simplest is to change the assembly to a vented roof.

    Staple baffles to the underside of the top flange and loose fill the cavity. That would get you close to R40. If you want more R value install a layer of rigid underneath which would can also serve as your air barrier. You can install drywall directly over rigid with longer drywall screws, commercial drywall places carry these in long length.

    You can also cross strap the ceiling to add extra depth to your joists or go for one size up the I-joists and skip the rigid completely.

    Either option is much cheaper than what you are proposing. I-joist have much lower thermal bridging since the web is a thin piece of OSB, the exterior rigid doesn't make as much difference as with dimensional lumber.

    Plus in heavy snow country, you would still need to vent above rigid to bring in outdoor air to keep the snowpack from melting. Might as well build a vented assembly in the first place.

  2. Anders Bostrom | | #2

    Akos;
    the exterior of the roof is already completed. With the rigid insulation and an airtight roof deck, I don't expect much snowmelt. 
    BTW, to my understanding venting a low pitch roof might not be the best solution.

  3. Expert Member
    Akos | | #3

    Probably the simplest is to go with faced batts and staple them to the side of the I joist at the right height. Make sure batts that are the correct width, not for dimensional lumber, typically meant for metal studs.

    You can also go with mineral wool batts as these are much stiffer and stay in place better. You would still need something to keep it tight against the roof deck. Standard insulation support wires won't work here as they won't reach between I-joist webs. You can probably get your local metal shop to cut you some to the correct size.

    Dense pack would also work provided you add in some extra support. For example, for 7" dense pack, cut some 2x4s and put them on edge above the bottom flange across the rafter cavity. The pillowing from the dense pack would now help push the insulation against the roof deck.

    P.S. Venting low pitch roof is possible, done around me (zone 5) all the time. You just need more vent area and a larger vent gap. Above all, the most important thing for these is air sealing the ceiling.

    Sample of one, but I just pulled the low slope roof off a 40 year old addition with a single 3.5" vent over 150sqft. Roof deck was in such a good shape that I ended up reusing it for strapping for the new siding.

  4. Anders Bostrom | | #4

    Akos;
    Looks like I found the way. A 2x4 as spacer on top of the I-joist flange will be just about the right height. Maybe rip 1/2" to make up for the sheeting I'm adding.
    Thanks for your input 👍

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