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Vaulted ceiling – Considering two options

jschramkc | Posted in General Questions on

My wife and I recently moved and purchased a home that we are renovating for what we hope to be a long term residence.  It is in Kansas City, MO in Zone 4 (but mere miles from the southern Zone 5 boundary).  We are going to vault the ceilings in the kitchen/living areas.  I have engaged a structural engineer to evaluate sizing and configuration of framing.  It is a 1960s era ranch – currently 2×6 rafters and ceiling joists.  The roof was recently replaced and they installed OSB sheathing on top of the original 1×4 spaced sheathing.  The house has newer vinyl siding with what appears to be very good soffit ventilation.

I am considering a couple of options for the vaulted ceilings:

1.  2×12 rafters sistered to the 2×6 rafters.  2″ DCI smart baffle in each rafter bay from soffit to ridge + 2″ closed cell spray foam + 7 1/4″ mineral wool batts (R-30) + 1″ polyiso on the underside of the 2×12 + drywall.

2.  2×12 “ceiling joists” to essentially create a scissor truss-like configuration so that I would still have an effective attic space above, albeit quite small.   Obviously at the exterior walls it would taper to nothing, but otherwise there would be an attic space above.  I would install 1″ polyiso (maybe Thermax if inspector requires?) on top of the 2x12s (would have to cut and cobble around the interface of 2×6 and 2×12 at the exterior walls) then (2) 5 1/2″ R-23 mineral wool batts in the cavity, then drywall.

I imagine Option 1 is much better in terms of performance, but much more expensive and seems riskier the more I read the site.  I would think that Option 2 would perform just fine, but would appreciate any feedback.  I am certainly open to other options as well.

As an aside, I am tearing off nearly 100% of drywall at exterior walls both upstairs and in the basement to perform caulking at plates, insulate rim joists and headers, install an interior layer of 1″ polyiso, etc., so I’m not averse to spending some money to make this an efficient home for some time.

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, can you tell us your name? (I'm Martin.)

    Of the two options, the first is by far the best. The second option combines cut-and-cobble (usually a risky approach for roofs) with insufficient R-value in the foam layer of the stackup (usually not a good idea).

    For more information on the risks of the cut-and-cobble approach on roofs, see "Cut-and-Cobble Insulation."

    For more information on calculating the ratio of foam insulation to fluffy insulation, see this article: "Combining Exterior Rigid Foam With Fluffy Insulation."

  2. jschramkc | | #2

    Sorry about that Martin. My name is Jason.

  3. GBA Editor
    Martin Holladay | | #3

    No problem. Welcome to GBA.

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