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Vaulted ceiling Cellulose questions

blamus20 | Posted in Energy Efficiency and Durability on

I have a valted, trussed ceiling that i want to insulate with blown in cellulose. I have blocking where the trusses meet the wall, i installed foam boards 2″ from decking as baffles connected to soffit vents. But the foam boards (baffles) are only 4′ long. 

1. Do i need to continue installing baffles all the way to the ridge/top so theres a continuous 2″ channel all the way? If not how would i (or cellulose contractor) know the insulation wont block the vent channel? Theres only 2′ of depth (top coard to bottom cord) so i have to blow in from below and wont see how high it will fill?

2. Should i install insuweb to hold up the insulation or drywall and then cutting holes in the drywall to blow and then repair? I feel it might be less work to use the netting. Drywall patching a ceiling sucks. But im also worried that the netting will bulge down from the weight of the insulation and then its hell to install the drywall? 

How do people ensure they blow just the right amount into their ceiling bays? Should i install insuweb 2″ from the decking everywhere?

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  1. Expert Member
    Michael Maines | | #1

    The baffles need to be in place wherever there is dense-packed cellulose. Sometimes vaulted ceilings end in an open attic space; if that's your situation, you only need baffles to the top of the loose-blown cellulose at the attic. But if your sloped ceilings extend to the ridge, the entire framing cavity needs to have baffles.

    This is a good overview of how to install cellulose, by a friend and collaborator of mine:

  2. blamus20 | | #2

    I didn't realize that i should be doing dense packed for a ceiling. In which case i would have to partition the whole roof into small i know why everyone wants to do fiberglass batts.

    Its only 3-12 in slope. Dense packing the full 22 inches is alot more than the R60 i was going for?

    1. Expert Member
      Michael Maines | | #3

      I should have asked what depth you have to work with and what R-value you are aiming for, but the vast majority of similar questions here don't have enough space to work with, while you do.

      To get R-60 with loose-blown cellulose you need about 18" of settled depth. Where you have foam, the cellulose should be in direct contact, to meet code requirements and to reduce the risk of moisture accumulation. At the rest of the ceiling you could get away with loose-blown but in a cavity the size of yours it will be very hard to do so without a baffle, as installation is basically blind and based on feeling, experience and the number of bags used. I've been using R-60 in sloped ceilings without resorting to fiberglass once in the last ten years, so it's definitely possible. If you had 36" trusses or if your installer had access from above you might be able to do loose-blown but I don't see how it would be possible in this case.

      One potential work-around: use Insulweb as the baffle; it will contain the cellulose so it won't clog the foam baffle. It won't hold up to a dense-pack without closing up the vent space, but if blown in very lightly, it might work. I've always used dense-pack in this situation.

      1. blamus20 | | #4

        Ok. Sounds like i need to baffle 2000sqft of roof afterall......

        I actually have a giant pile of reclaimed EPS boards that i can use to baffle the whole ceiling. Of course i can save it for other things but maybe i should just use it up here. Honestly its not THAT much more effort to use foam boards than insulweb for baffles. Most of the work is moving scaffolding! And boards i can install myself one piece at a time while insulweb ill need more hands to pull it tight etc.

        Also, definately use insulweb to hold up the cellulose vs drywall?

        Would blown in fiberglass allow me to not baffle the whole roof?

        1. Expert Member
          Michael Maines | | #5

          I'm not an installer, just a designer and occasional builder who uses cellulose regularly. I think I've had blown fiberglass on one project and I wasn't directly involved, but I don't see how you could blow any material in blind and be sure that it's not filling the space or clogging the vent chute.

          I would talk with the installer you plan to hire. For dense-packed cellulose, you would likely want Insulweb on the interior before drywall, because the air needs to evacuate the space for the cellulose to get dense enough.

  3. blamus20 | | #6

    Out of curiosity I started pricing out material costs - blown vs rolls vs batts etc. And purely looking at Home Depot prices. And I'm surprised to find that the roll (fiberglass, unfaced) layered to R76 will cost more than double of the material cost for blown cellulose (including insulweb)

    The cheapest $/sqft/R in fiberglass is actually the unfaced R19 roll @ 0.037
    VS blown cellulose @ 0.016.
    @R76 thats 4 layers of batts, $2.8/sqft
    Compared to cellulose @ $1.18/sqft, plus insulweb @ $0.05/sqft, total $1.23/sqft.

    This is purely material cost, from home depot. I'm sure hiring out the install will make the material cost meaningless in comparison. The batts I can do myself and probably do a better job. The blown in I can also do but rather not due to the added complexity. Though at this rate, I probably will if I can find a 2 stage insulation blower to rent.

    If these numbers are right, then it makes even more sense for me to put in the work and install EPS baffles for 2000sqft so I can use cellulose...... I'll end up with R80 which, is a good place to be.

    Do my home depot material cost numbers sound inline with the rest of the country?

    1. Expert Member
      Michael Maines | | #7

      It's extremely difficult to insulate properly around truss webs. It's possible but it's far easier to get a good result with a blown-in product.

      I don't have cost comparisons for your situation but what you relay sounds reasonable. One rule of thumb is that you can hire a company to install fiberglass for about the same as you can buy the material yourself. Mineral wool and other materials don't have the same contractor discount pricing.

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