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Blown vs batt insulation for vaulted, non-cathedral ceiling

BJHuffine | Posted in General Questions on

In my climate (4a), code calls for R-49 in the ceiling.  We asked for the roof to be framed so that there’s plenty of room for the insulation.  There are two rooms with vaulted ceilings, non-cathedral.  The roof pitch is 8/12, but the ceiling joists in these rooms are 6/12.  I’ve looked everywhere on the best way to insulate this and batts are what mostly comes up.  I did see a similar issue for an attic reno on this site where a response was to put netting on the top and bottom of the ceiling joists to blow the insulation in.  However, I don’t know if that was done and how it went.  Nor have I seen that anywhere else, so I still feel like we’re blazing a trail here.  Our plans are for the attic floor to be covered with cellulose.  With its high R-value, fire, and sound rating, it was a no-brainer.  However, this also means that these rooms do not leave room for fiberglass in areas where the roof joists pinch down close to the ceiling joists. 

Our walls will be filled with mineral wool batts (R-23 in 2×6 studs), which also have a high R-value in a more compact space.  We actually pulled our building permit just before code change acceptance in our area so technically we only have to meet R-35 in the ceiling.  With that in mind, if I compare the netted cellulose with two layers of mineral wool batts in these two rooms, the batts cost $1,300 more.  

Unfortunately, the shape of the house causes some of the corners and sides to not be easily accessible without significantly squeezing yourself between the ceiling and roofing joists and around corners, so whichever direction I go, I want to make sure it’s  both effective and solid.  We do not want to revisit it later.  That alone has made me think the $1,300 may be worth it, but then again, the cellulose will fill all the spaces better and might be more solid than I would think. 

So my question is whether its worth the $1,300 more to just go with the batts?  Or is it worth blazing this trail with the cellulose?  Do those of you in the know see where there may be significant issues that would need to be overcome?  Maybe you’ve been here yourself?

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  1. nshirai | | #1

    Hi Jason,
    First up, full disclosure - this is Nathan with Insulation Unlimited located in Chattanooga. This is NOT a sales pitch, truly a neighborly effort to try to be helpful to you. Since we are not familiar with your project, I assume that whomever is your GC has an insulation sub already lined up and we do not want to interfere with any relationships there. When I saw you mention the climate zone, I checked out your profile and noticed that you're in our area, and so this is really just me geeking out on your project since performance-minded builds are somewhat of a rarity around here.

    With that out of the way, what is the minimum vertical distance between your exterior wall top plate and roof deck at the areas in question? That may help narrow down the most practical approach. Standard practice would be to just put batts between the joists, as perching oneself at the peak of the vault and shooting blown material down into a converging eave would be somewhat challenging. I can imagine an approach involving plenty of ventilation blocking detailed prior to drywall being in the way that might increase the chances of success with the blown approach, but it would need to be planned for of course. If the "pinch" above the top plate is narrow enough, best practice might be to get your ventilation baffle in and then get as much closed cell foam in that area before transitioning to fiber insulation at whatever point the joists and rafters diverge sufficiently. Another approach might be to batt the joists with a single layer of mineral fiber and then "top off" with blown to supplement R value and combat thermal bridging by covering the joist top edges. This could reduce that $1300 cost overrun to switch to all batts. In our experience a 6/12 pitch is gentle enough that you shouldn't get any issues with the blown material sliding around. A possible safeguard against this (especially if you're on the brow) would be to make sure your ventilation baffles terminate at a high enough elevation that the blown material on the roof side of the vault is not subjected to any weird direct wind pressures from the soffit.

    To speak to a couple of your crawlspace questions, for what it's worth:
    Your daylight walls would be best approached as Dana recommended - just like any other above grade framed wall. Air seal them, then insulate the cavities. One detail to make sure not to skip is to put some sort of air barrier on the studs after you insulate, so that you don't have bare fiber insulation exposed to the crawlspace area. Depending on how your inspectors feel, this may need to be an ignition barrier a la drywall.
    Also, regarding the dehumidifier question. Even though your crawlspace is conditioned, in my opinion a dehumidifier is still worth having. We get long enough shoulder seasons here (mild weather when neither the heat nor AC cycles very long) that you'll want a way to deal with the excessive humidity during periods when the AC just doesn't run very much.

    Happy building to you!

    1. GBA Editor
      Brian Pontolilo | | #3

      Hi Nathan.

      Thanks for the thoughtful reply to Jason's question. It's great that we have a lot of GBA members who care enough to understand the best way to... air seal, insulate, provide vapor control, etc. ... but an installers point of view sometimes puts a different and real world spin on things that can't be ignored.

  2. BJHuffine | | #2

    Thanks Nathan! And for those reading this, the "pinched" area should be (if memory serves) 16-18 inches (might be 20, just can't remember). We asked for enough to allow for air movement with planned blown cellulose. And also in full disclosure, this is a fantastic forum with experts from many areas. But with this, I was curious as to how local contractors also handle this situation. So literally, Googling nearest insulation contractors, I happened to select Insulation Unlimited as my first and only place to call. Nathan answered and laughed because he was in the middle of typing this response! How funny and what were the chances! It was a pleasure talking with you and nice to see someone in the construction business with a building science mindset. That's definitely a rarity in our area.

    1. GBA Editor
      Brian Pontolilo | | #4

      Hi Jason,

      The idea of blocking the eaves and spraying closed-cell foam over the top plate seems like a pretty smart way to go for me. Not only will it give you the R-value you need in the pinched area, but it will give you an air seal at a tough area to detail. Keep in mind that air sealing your ceiling is just as important as getting these insulation details right.

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