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Vault ceiling after switching attic from vented to unvented?

AnonymousUser | Posted in Green Building Techniques on

In the upstairs portion of our 1949 Massachusetts Cape, the “walls” are roof with 45 degree pitch. There are gables at each end, with a vent near the peak of each gable. There is an unfinished attic, accessible only if I move the 18×22 inch lid in the ceiling of my bedroom closet and climb up there with my step ladder.

From the flat ceiling of each bedroom to the roof’s ridge, there are only four feet (no floor coverings in attic; just bare joists). The portion of roof which envelopes each upstairs bedroom, and also each kneewall, is insulated with 5 inches rockwool batt.

Inside the attic, there is no insulation in rafter bays nor gable walls. We used to have 14 inches of cellulose on attic floor, but recently I discarded nearly all of it (still have a bit more to discard in the next week or so). I am going to have an insulation contractor spray densepack fiberglass (John Manville Spider) into the rafter bays and gable walls of attic. Manufacturer says this product needs no strapping or netting and “breathes” to the inside.

I am going to either switch my gable vents for fixed windows in the same size, or close up those openings entirely. I am also going to undergo reroof. Between sheathing and new roof covering, there will be polylefin barrier plus several layers of polyiso, staggered and taped at seams, for a total of 6 inches.

I want the bedrooms to have cathedral style ceiling instead of flat, so I am thinking to remove the ceiling that separates each bedroom from the attic above. I will consult a structural engineer to ask if it is safe to remove attic floor joists. I think the only structural function of these is to support the attic floor (which I am dying to get rid of!), but I could be wrong. I cannot see how they could be supporting the roof, since the eaves are at the floor of each bedroom, not the floor of attic…?

has anyone on this forum ever switched attic from vented to unvented, insulated roof outside sheating, and then removed attic floor to make cathedral ceilings?

I may keep part of my attic and will make sure that whatever portion I keep has access through the hatch in the closet. Or I may give the entire upstairs cathedral ceilings.

Who has done this, and what advice have you, if any? Thanks

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Replies

  1. Expert Member
    Akos | | #1

    JM Spyder is fiber insulation, as such it can't be used in an unvented roof unless there is enough exterior rigid foam above the roof deck. The "breathe to the inside" only works in warm climates, in colder mass climate, that can result in condensation and mold/rot.

    Search this site for articles on how to build a proper vented or unvented cathedral ceiling.

    Typically the floor of the attic are the collar ties for the roof. In some cases they can be removed but in most cases these need to stay. You can still remove the attic floor and continue the ceiling up and have the ties exposed. With this type of roof, your best bet is spray foam as it is very hard to air seal around all the ties.

    1. AnonymousUser | | #4

      Thank you, Akos. My rafters are 5 inches. Filling them with John Manville spider, this would give R value inside rafters of 5 inches x 4 per inch = 20.

      Outside the sheathing, we are having 6 inches polyiso applied. A building scientist advised me to use a conservative R value of 5 per inch (not the 6 or 7 some manufacturers claim) because in Massachusetts cold, this is polyiso's "true" R value. Therefore, outside the sheathing I will have R value of 6 inches x 5 per inch = 30.

      I understood the building scientist to say that, to avoid condensation in my Zone, R value inside sheathing should be 60% or less of total R value. In my case, R value inside sheathing would be 40% of total and outside would be 60%.

      Regarding joists at the attic floor, I came across this:

      https://www.finehomebuilding.com/2001/03/01/removing-collar-ties

      but will consult structural engineer before removing any joists. Even if she/he says I need to keep the 8 inch joists at the floor of the attic, I may still remove the attic floor and expose those joists.

      1. Expert Member
        Akos | | #7

        With 6" polyiso above the deck, you can pretty much skip fiberglass insulation on the inside in your climate.

        In zone 5, an R30 assembly gives you most of your energy savings, so unless you are targeting something like passive house, there is very little money saved on operating cost by going much above code.

        With your 5" rafters, R13 foam (~3" polyiso) is more than enough for condensation control and a pretty solid roof.

  2. Expert Member
    Zephyr7 | | #2

    Since you’re going to be consulting with a structural engineer anyway, if the floor joists are collar ties as I expect that they are, ask what you can do about them. Sometimes you can remove two out of every three, sometimes you can modify the structure a little so that you can remove all of them. The engineer will be able to tell you what your options are. If you do leave some (or all) in place, you can use them as “rustic beams” and add valence lighting on top. Make them into an architectural feature in the space.

    Bill

    1. AnonymousUser | | #5

      Thank you, Bill. I will see what he/she says. The rustic look may work if I have to leave some or all joists in the attic floor.

  3. Tyler Keniston | | #3

    >"Between sheathing and new roof covering, there will be polylefin barrier plus several layers of polyiso, staggered and taped at seams, for a total of 6 inches."

    As Akos mentioned, this must all be implemented correctly. How deep are your rafters? I suggest reading this article: https://www.greenbuildingadvisor.com/article/how-to-install-rigid-foam-on-top-of-roof-sheathing

    Your proposal could work as long you you get the R-value ratios right, and make it air-tight.

    Since you're re-roofing, you can make your roof sheathing an air-tight layer. Possibly a peel-and-stick membrane, or taped seams of sheathing if things are in-tact.

    Spend the extra time thinking about how to lap that air-tight layer onto the tops of your walls from the roof.

  4. AnonymousUser | | #6

    Thank you, Tyler. I am familiar with the article you linked and will add 6 inches polyiso outside sheathing (R value 30, assuming "conservative" R val of 5 per inch in cold Massachusetts), plus 5 inches denspack fiberglass inside my rafters (R value 20).

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