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Cathedral Ceiling and Conditioned Attic – Same Thing?

swooley | Posted in General Questions on

In the research for my new house, which has both a conditioned (unvented) attic and a large stretch of cathedral ceiling, I’ve been struck by the reality that these two things aren’t different – are they?

Both are directly below the trusses/rafters. Both are conditioned spaces inside the envelope. Both run into, therefore, the same reality of rising moisture.

Is there any point in making a distinction between them?

I ask because both Allison (Energy Vanguard) and Joe (Building Science) advocate for supply/return in conditioned attics to alleviate any moisture concerns with open cell spray foam against the roof deck (with added vapor barrier in 5+).

If this does indeed work just fine, would open cell foam not also be acceptable in cathedral ceilings, given that they are similarly conditioned?

I constantly see “closed cell in cathedral ceilings” alongside “open cell in conditioned attics as long as they’re actually conditioned” and it feels very much like there’s a mistake in understanding here.

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Replies

  1. Expert Member
    MALCOLM TAYLOR | | #1

    swooley,

    I can't think of much practically different between how you would build the two. The distinction really comes from what is below them - and that can have some implications - in things like the protection against fire spread which may be required for the underside.

    I agree. If open cell-foam can be used safely when the space below is conditioned to address humidity, it shouldn't make much difference whether it is an attic or room under the sloped roof.

    1. swooley | | #2

      Thanks so much Malcolm. I always appreciate your input. I keep going back and forth between my desire to justify an open cell-only approach to my cathedral ceiling/conditioned attic in CZ3A for environmental reasons (the closed cell here is still HFC), and just throwing in the towel and following the blanket recommendation to do 2" of closed cell first.

      Since everyone is fine with open cell in conditioned attics, I feel more comfortable doing the same on my cathedral ceiling.

      1. Expert Member
        MALCOLM TAYLOR | | #3

        swooley,

        Being in a climate where vapour-diffusion ports aren't allowed I haven't paid that much attention to them, but wouldn't they add a lot of resiliency to an open-cell roof in CZ3A?

        1. swooley | | #4

          Malcolm,

          That may well be the case! We're already decked in with Ice and Water over everything, but I suppose it's possible to add that before the standing seam goes on.

          It's a single pitch, 2:12 roof, so it doesn't have a proper "ridge". I've looked at maybe doing under-deck Accuvents out to the high side of the roof, I'm just not familiar enough with, nor can I find it spelled out, how you detail these features in a single slope/shed style roof.

          Joe's recommendation is also to only use vapor diffusion at 3:12 or higher pitch: "We need a sloped roof. Buoyancy, baby — thermal or hygric or whatever. At least 3:12." and that also seems to be the thinking for attic ventilation writ large.

          So it's a low slope roof (weird) with a single pitch (weird), 24” deep heel trusses, and 70% cathedral ceiling (weird) that will be unvented and (likely) insulated with 12" of open cell spray foam, with supply/return spaces under every part of the roof, attic included.

          I'm hoping that'll work. Everytime I look at other solutions I seem to run into limitations or even more question marks. Any advice is always welcome.

          1. Expert Member
            MALCOLM TAYLOR | | #5

            swooley,

            If you have that deep trusses, and a 2/12 pitch, why aren't you venting the roof and using either cellulose or batts?

            Leave a deep vent channel, and add vented soffits on the rakes if you are worried about adequate air-flow.

            You can vent the ridge either at the top much as you do a typical metal gable roof, or through the soffits at the peak.

        2. swooley | | #6

          Malcolm,

          I'm completely into the idea of putting vent baffles in and having a vented space below the roof deck and above the insulation. I wasn't sure that 2:12 was sufficient slope to get that kind of air flow. I actually saw a comment of yours from a couple of years ago early on in my research that helped drive me away from venting:

          "- Low sloped roofs are typically defined as those under 3/12. That's the point below ventilation becomes difficult, and codes begin to call for different detailing of the roofing material." on the thread "Baffles and Spray Foam for Low-Slope Vented Roof"

          The problem is I'm just not entirely sure how to achieve this kind of vent baffle assembly, as I know the details *really* matter. The contractors around here either spray foam everything or put fiberglass on the underside of attics, so I'm trying to carve out my little unique design more or less on my own. I'd be thrilled to use venting and a non-foam solution.

          I've attached a picture of what I believe this assembly would look like, borrowing from the "5 Cathedral Ceilings That Work" blog.

          I've only got a bit of attic, and it has all of our HVAC and electric water heater in it. Where my ignorance is most expressed is in not understanding how an air permeable insulation layer would be sufficient to prevent both my heat leaving the house in winter and the humidity of CZ3A from causing problems with all that moisture floating around freely.

          Assuming this works, I'd just look to get some Smartbaffles and define what, 6" of ventilation space? Do I need my insulation fully up against the baffles, taking up the entire cavity between baffles and ceiling?

          Forgive the UV plant lighting making my sketch look insane, haha. Also ignore the other roof drawing below it!

          1. Expert Member
            MALCOLM TAYLOR | | #7

            swooley,

            Going to 2/12 definitely calls for a deeper vent space and more diligent air sealing. Going below that becomes very problematic because each reduction in pitch - say for 2/12 to 1.5/12 - represents proportionally a big drop.

            Your situation gives you a large advantage. The depth of the trusses means you have room for as much cellulose or batts as you would like and still allows plenty of space for a deep vent channel. The simple roof shape also allows you to vent the soffits on all four sides.

            I would not install baffles. You can make up for the slight diminution in R-value due to wind-washing by adding a bit more insulation, and it will be well below the air inlets If you leave a deep cavity. With R45 you are looking at that cavity still being at least 10". That's the direction I would explore.

            This house, just down our road, has almost exactly the roof assembly I'm suggesting. The same pitch, metal roof, 2 ft trusses and permeable insulation.

        3. swooley | | #8

          Malcolm,

          So wait -- I can just have the following stack?

          Standing Seam
          Ice and Water
          OSB Roof Deck
          --AIR SPACE--
          Batts/Cellulose
          Drywall

          If that's the case my options for insulation absolutely skyrocket. I was looking at $2,000 in SmartBaffles which really made the cost and labor difference non-ideal.

          That would be incredible news. And then I just make sure to keep the side soffits and main soffits unblocked by insulation and I'm good?

          1. Expert Member
            MALCOLM TAYLOR | | #9

            swooley,

            That's exactly how the house I posted is built, although I don't think they bothered to vent the soffits on the sides.

            The baffles are there for two reasons:
            - To reduce wind-washing. But the reduction in R-value can be compensated for by using slightly deeper insulation, as we do on the floor of vented attics. And if the inlet is well above the top of the insulation, as yours would be, then scouring should not occur. If you wanted to be extra safe you could tack house-wrap as baffles over the first few feet at the perimeter.
            - As a secondary air-barrier. I've never been convinced this is particularly important.

        4. swooley | | #10

          Ok Malcolm - you may well have prevented a house from getting spray foamed here. I just want to make sure I'm tracking the proper science behind a vented roof, which as far as I can tell is just making a tiny vented attic.

          Images attached of looking down and up my trusses. I'm up about 18ft on scaffolding. The black box is the wood stove pipe and the soffit is just beyond it. I've also attached a picture of the sidewall, where the ICF meets up with the roof. I'll clearly need to airseal this (going to do EPS on the exterior and probably just run a line of foam on the interior).

          There's not exactly a super clear channel for air/ventilation given the 2x4s run perpendicular to the trusses around where the soffits are.

          My plan would be to get a set of R30 Knauf Ecobatts, as well as a set of R19 Ecobatts. Start with an R30 batt, push it up in the ceiling a little bit, then slide the R19 under it until it's flush with the bottom of the truss. That's 16.25" of batt, which leaves about 8" of empty space between the roof deck and the insulation.

          Then I'll install drywall and focus heavily on air sealing the drywall. I only have 4 roof penetrations (4 ceiling fans) so that shouldn't be too difficult.

          You'd say this is the best, or at least a recommended, assembly for all the variables involved? Can this really vent soffit to soffit when the holes are on the underside of the soffit on both the high side and low side?

          For posterity, my options ended up being:

          $4984 — R49, I can pickup at Home Depot w/ truck + trailer (not ideal for t&g)

          $7722 — R48, HempWool, requires semi transport (not ideal for t&g)

          $7020 — R42, Open Cell 12" (easier for T&G)

          1. Expert Member
            MALCOLM TAYLOR | | #11

            swooley,

            That would be what I'd do. Leave the first batts slightly low so when the second ones go in you know you have at least 8" clear vent space.

            I don't see any problem with the vents being on the soffits. If it worries you, it's easy to vent the metal at the peak, much as you would a gable roof.

            I have no reservations about the sasembly, but so far this discussion has just been between us. I hope other posters will weigh in to provide you with reassurance or second opinions.

        5. swooley | | #12

          Malcolm,

          Agreed. I’ll kick this out to a relevant thread for that reason, and so others can find it easier later on. Deeply appreciate the the help.

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