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Community and Q&A

Insulating a cathedral ceiling

Katie Grams | Posted in Energy Efficiency and Durability on

Hello!  Looking for some advice on current predicament.  We have a new 2nd story addition consisting of vaulted 7/12 pitch (14′ max. ceilings), 2×12″ hand framed.  Addition was framed with idea to vent vaulted ceiling from eave to ridge. Builder is proposing installing baffle at eave end ONLY in each bay and installing 10″ batting to ridge, which leaves a 2″ air space above insulation.  We have installed Metal Sales Image II standing seam metal roofing with ridge vent on roof.   I am now, in the 24th hour, second guessing this design and wondering if a hot roof would be more efficient and allow us to get highest R-value.  We are in Lansing, Michigan, Zone 5.  Contemplation points are as follows:

1) if batting is used, it seems wrong to me that builder isn’t extending baffles continuously from eave to ridge.  Seems like moisture will condense over time in batting as well as dirt, dust, and air contaminants severely degrading the R-value over time.  He says this is how it’s “always” done.  Is it?  I don’t want to be tearing down my drywall ceiling 10 years from now due to moisture damage and inability to keep room comfortable temp.

2) I had a spray foam company come and give an estimate on filling the cavity 5.5″ with closed cell, below a continuous baffle allowing 1″ air space.  (They thought I was odd wanting to do it this way).  My concern is finding a baffle that can hold up to the pressure of the closed cell without cracking (thus losing our VENTED chase).  The company couldn’t offer a great resolution to this other that build your own baffle from furring strips and osb (time consuming) OR let us make it a HOT roof.  So now I’m thinking maybe it’s better to make it a HOT roof?  We are set up for a COLD roof. I have an open ridge with baffles already in place under the metal ridge cap.  Sealing the eaves is easy, but if we pump foam in the ridge crack that’s been cut in the OSB, could I have potential moisture issues under the baffled metal ridge vent?

Our goal here is energy efficiency, maximizing R-value and  saving on cooling/heating costs while maintaining the integrity of the sheeting/framing and new standing seam roof.  I do realize that closed cell is not a “green” product.

Any input would be greatly appreciated!!  Thank you.

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Replies

  1. User avatar
    Dana Dorsett | | #1

    >"I do realize that open cell is not a “green” product. "

    Really? Open cell uses about half the polymer per R as closed cell, and is blown with water- it's about as green as it gets for foam insulation. The only thing that beats it is reclaimed foam from demolition/re-roofing projects.

    Closed cell foam blown with HFCs is at the other end of the spectrum.

    A 2x12 rafter bay is 11.25" deep, a 2x8 is 7.25" deep. At 4" of closed cell applied directly to the underside of the roof deck you would still have sufficient space and ample dew point margin for R30 rock wool.

    The greener, and higher performance solution (that may even be cheaper than 4" of closed cell foam) :

    With 5" of reclaimed polyiso ABOVE the roof deck you would have sufficient dew point control at the roof deck for a full cavity fill of cellulose or low density R38 batts.

    A lesser performance but still code-min+ solution would be 4" of reclaimed polyiso + R30 rock wool batts snugged up to the bottom side of the roof deck.

    1. Deleted | | #6

      Deleted

  2. Katie Grams | | #2

    Thank you for your reply, Dana. I mis-spoke. I meant to say "closed cell" is not "green".

    I have also considered spray and batt type installation, similar to what you are proposing with 4" closed cell+ rock wool, but I'm reluctant due to all of the negative articles I've read about getting it "just right" so not to trap vapor between the two layers.

    Unfortunately, the standing seam roofing is already installed so modifications to the roof deck are impossible. In retrospect, insulating the deck would have given us a better starting point than where we stand now in terms of max. R-value.

    My main question here is what is the better solution, cold or hot roof? You seem to be advocating for the hot roof. Aside from being able to get more R-value and simplicity of installation (no baffles, etc) are there other reasons you prefer this? Thank you for your time!

  3. User avatar
    Dana Dorsett | | #3

    The risks are lower with the unvented solution when the roof can't be properly vented soffit to ridge.

    At 4" most closed cell foam is still over 0.25 perms, many are over 0.35 perms. That's still only a class-II vapor retarder, not a total moisture trap the way 0.05 perm polyethylene would be. In zone 5A Lansing the drying season is at least twice as long as the moisture accumulating season when the roof deck's temperature average is colder than the indoor dew point average. The roof deck CAN dry toward the interior through 0.25 perms, eliminating it's wintertime moisture burden well before mid-summer. If the roof deck is dry when the foam is installed, it stays dry.

    Also, at 4" the foam itself is structural (a big selling point in hurricane territory), and it's waterproof to liquid water. Even if the roof decking rots in places over then next 75 years from a chronic roof leak or something the roof isn't going to fall apart.

    If you still want the vented deck, baffles made of 1/2" MDF (R1.3) with 2.5" of HFO blown foam (R17.5) is enough dew point control for R30 rock wool in most of zone 5A. If the inspector won't buy it, half inch MDF baffles (R1.3) + 3" of HFO blown closed cell foam (R21-ish) and compressing R30 rock wool into the to 6.75" (R28+) would get you the code min R with more than the IRC prescriptive dew point margin at the foam/fiber boundary.

  4. User avatar GBA Editor
    Martin Holladay | | #4

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

    Here is a link to an article about ventilation baffles. Both you and your builder probably want to read it: "Site-Built Ventilation Baffles for Roofs."

    1. Katie Grams | | #5

      Hello Martin,

      Thank you for the link, I've read it several times. :) At this point, I'm leaning towards installing Smart Baffles, 5.5" of closed cell and filling the remaining cavity with fiberglass or rock wool, as Dana had suggested. I'd prefer to fill the rafter completely with closed cell but understand the build-up limitations of the foam products on the market as well as dew point considerations Dana mentioned. I'm still extremely leery of the prospect of making this a "hot" roof even though it is, by far, the easiest option.

      If we were to go with our builder's proposition of installing ventilation baffles at the eave end and ridge vent of each rafter bay ONLY (not continuous to ridge) along with fiberglass R-38C insulation in the 2x12 bays-- are we looking at big issues with degradation of the R-value of the fiberglass due to wind-washing? Vented soffits are on N and S facing, pitch is 7/12. Is the Is the only way to correct this to install a continuous baffle from eave to ridge? The insulation manufacturer even shows their cathedral install without a continuous baffle so perhaps I've read too much and am overthinking a really simple install?

      Your site is top notch and my first go to for all of my building questions! We previously used your advice in creating a conditioned crawl in the original 1912 portion of this same project.

      Thank you!
      Katie

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