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Vented Cathedral Roof With Dense-Pack Cellulose and Smart Membrane

stephenr | Posted in GBA Pro Help on

Hello,

I have been flipping between roof designs and am hoping to come to a decision as my plans need to start taking shape.  Zone 6, 1000 square foot, cathedral roof, coastal Maine, owner/builder.

I switched from a vented truss design (because of cost) to an unvented outsulation design but am uncomfortable with the use of any rigid, including EPS and Polyiso, because of cost and GWP.  So, i am back to a vented design, as described below, and would appreciate feedback on it.

Its a 32 foot square shed cathedral roof.  I am going to bump up the slope to 2.5/12.  I will probably be framing with 2×10’s or 12’s and will hang gussets down to increase the depth to 16 inches or so to get my r-60 using dense pack cellulose.  I am interested in using the “vented, with vapor variable membrane” roof described on page 124 of PGH.  Basically, its tyvek over framing, strapping in line with the rafters on top, and then sheathing, creating a vented channel.  Strip vents at soffit.  Here are my questions.

1. Will the tyvek, supported by the strapping above, be strong enough to resist the pressure of the dense pack without tearing?
2. If I rip a 2×4 in half, I could create a vented space of  1 and 5/8″.  Would this be sufficient?
3. I would like to use zip all the way around as my air barrier.  Will the tyvek, then, also need to be taped and sealed, as a belt and suspenders approach?
4.  At my “gable’ ends, I will be cantilevering framing members over the gable end wall in a 1:1 ratio, with a 2 foot overhang.  Would I simply continue my above tyvek strapping at 16 oc to catch my sheathing, or is there a shear consideration here that needs to be made?
5. I will be blocking with wood between rafters.  is it advisable to cut rigid and install it with spray foam on the inside of the blocks (as one would insulate a rim) or will the dense pack be sufficient in this space?

Thanks!

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Replies

  1. Expert Member
    Michael Maines | | #1

    Stephen, here is a thread with more info on that type of assembly: https://www.greenbuildingadvisor.com/question/new-take-on-insulating-a-roof-in-northern-maine.

    Dense-packed cellulose is about R-3.6/in, and vent spaces should be at least 1" but 1.5" or more is better. I usually create 18" deep rafter spaces and use 1 1/2" vent spaces, for 16.5" of cellulose to reach R-60.

    To answer your questions:
    1. I would be wary about using Tyvek; while pretty tough, I have only used Pro Clima Mento for this application, which is even tougher than Tyvek.
    2. 1 5/8" is a good depth, but when you dense-pack the sarking membrane will bow outward, up to an inch or possibly more, so it's better to use deeper vent spaces with a sarking membrane system if possible.
    3. Your air control layer needs to be continuous, so if you are taping the roof deck for airtightness, you can't also have a vented space below the roof deck. A sarking membrane system uses the airtight membrane as part of the air control layer, and the roof sheathing is outside the air control layer.
    4. For a 2' overhang you need more structural support than the sheathing alone can provide. I'm not clear on your proposed detail; a sketch would be helpful.
    5. Do you mean bird blocking, above the exterior wall top plates? If you can get R-60 in the roof you don't need full R-value over the top plate, so solid blocking with cellulose meets code, and keeps your embodied carbon footprint small.

  2. Expert Member
    Akos | | #2

    For a vented roof, you generally detail the ceiling as the air barrier. This is easy to tie into the wall air barrier by putting a wide flashing tape over the top plates before any of the rafters are set.

    Trying to use the vent baffle (either membrane or other sheet good) as the air barrier is very difficult as you'll have rafters poking through it and no easy way to do this and support snow loads on overhangs.

    I don't know your local lumber costs but here I-joist are a bit more expensive than lumber but you can usually go for wider spacing, so overall not a big cost increase. You can also use the bottom of the top flange as a convenient built in vent spacer. For a vent baffle, you can staple a sheet good (house wrap/ insul mesh/thin OSB or fiberboard) to the bottom of the top flange and dense pack. A 14" or 16" I-joist would give you a pretty decent R value assembly and would be in the ballpark to clear span which simplifies your interior structure.

    A ceiling like this doesn't need to be packed to as a high density as a wall to avoid settling, this can help minimize the amount of bulging.

  3. stephenr | | #3

    Thanks Michael,

    Very very helpful. Here are my follow up questions to the numbered ones above.

    2. I guess 2x material with strapping would give me 2.25" and allow the venting channel to stay wide enough for good air movement after dense packing. The only drawback of going this wide, however, seems to be the extra long fasteners that would be required for applying the sheathing. With 5/8 plywood, we would be looking at 5" screws to tie the sheathing directly to the rafters. Is this about right?
    3. Interesting about the air control layer. Also, taping the seams of the pro clima in the field of the roof assembly might be tricky with no backer. Is this a concern?
    4. Please see attached screen shot pulled from GBA. This is not EXACTLY what I plan to do on the gable overhangs, but gives a visual image thats close. The roof framing members that are perpendicular to the rafters, on my roof, will be 4 feet in length, in order to create a 2 foot overhang. After I lay my mento over this, I imagine that I will just continue strapping as I have been; 16 inch oc running from the eave to the ridge. At the points over the perpendicular framing members, however, my roof sheathing would not be able to maintain the proper nailing schedule (6" on edge, 12" in the field if I remember correctly). Should I plan on spacing these perpendicular framing members on 12" centers in order to maintain the required nailing schedule with my sheathing and give added support?
    5. Yes, the eave blocking above the top plates and between the rafters. Glad to know I won't need rigid. This brings up a small concern. My air barrier on the walls in that area will be zip sheathing, cut to fit in between the rafters and tie in the eave blocks. I imagine that its important to tape and caulk my zip from the outside around these blocks so its nice and tight. Also, I am imagining some trickiness in connecting my air barriers in this area (zip and mento). It seems that I would be taping the very top of the zip to the underside of the mento, in between the rafters, from the outside. Is this right?

    1. Expert Member
      Michael Maines | | #4

      2. You can use a 2X rip to create a 1 1/2" vent channel, which will still allow air flow once dense-packed, and will probably settle slightly over time as well. With 5/8" sheathing and at least 1 1/4" rafter penetration, you would need 3 3/8" nails or screws. However, although for walls the building code requires the sheathing to be fastened directly to the studs, that is not the case for roofs; if the 2x rip is solidly fastened to the rafter, the sheathing is not required to be fastened directly to the rafter. Engineers and code officials may be wary, and rightfully so in high-wind zones.

      3. It is easier to do a good job of sealing the tape to the membrane when there is a backer, but it's possible without a backer. The membrane should be stretched drum-tight so it's not as hard to tape as you might think.

      4. Engineers would probably say yes.

      5. For both 4 and 5, you might find it easier to use ladder-type rafter extensions, screwed on after building a "monopoly house" without overhangs. That makes it easier to make all of the required connections. The sheathing should extend continuously over the extensions; it acts in tension while the framing is in compression, so it won't sag--as long as the framing is tight. If you really want to use more traditional framing with the sarking membrane as your air control layer, it will require a lot of careful cutting and tape origami. It's possible; I've done it, but it's time-consuming.

    2. Expert Member
      MALCOLM TAYLOR | | #6

      stephenr,

      Another option to avoid building and venting ladders is to support the overhangs with brackets, as you frequently see on craftsman style houses.

  4. StephenSheehy | | #5

    As an alternative, we used raised heel scissor trusses. Stapled a membrane to the bottom chord, then applied strapping, then drywall. The membrane was the air control layer and was easy to establish continuity with the wall membrane. The roof was vented. Plenty of room for R70 ish cellulose.
    Also in Maine.

  5. stephenr | | #7

    Thanks, a lot to consider here. I am almost leaning towards the origami approach since I am in my fifties and enjoy detailed work on ladders. I guess I would rather work with tape than prebuild ladder extensions and lift them into place. Plus, the building is on a tricky site, has grown taller, and will be challenging for pump jack and scaffolding placement. I will mostly be working solo on this, although I will hire guys for the heavy stuff. When I priced out the I-joists, they came in at about double that of lumber and although it would save a lot of work, I am trying to minimize my GWP and source local wood whenever I can. Also, the posts supporting the mid-span beam of the rafters fall in acceptable places in my floor plan. In considering the air barrier...I guess if I don't do the monopoly house approach, I would rely on the ceiling drywall and tape on the top of the plywood that connect the inner and outer stud walls as an air barrier. I would still oragami tape at the eave blocks, but it would be good to know that all that tricky work would be backed up by having the air control layer at the ceiling. I could then easily tie my variable vapor control membrane of my walls (interior shiplap) to my ceiling drywall for my vapor control layer. That's the direction I am leaning. Plus, high winds and exposure at my site have me leaning towards not doing ladders at the gable ends, but rather framing in the manner of the attached photo. It seems more solid.

    1. Expert Member
      Akos | | #8

      Stephen,

      When building a house, your shell is (or should be) one of the smaller big line items. If the shell is costing a lot or taking a lot of time, I would revisit and figure out other ways of doing it especially for a self build.

      I know when DIY it seems like your time is free, doesn't take a lot of that free work for the project to feel like it is just dragging on forever. The most important part of any build is finishing it.

      I would very carefully consider any "fussy work" and make sure it is worth it. For example, I did some hidden downspouts and gutters on a project. These small details probably used about 1/3 of the roof build time. At the end of the day it looks good, but if I could go back, I would design them out.

      P.S. Typical ceiling air barrier would be a membrane under the drywall. I've used 6mil poly for this but you can also use one of the fancier membranes. The drywall than acts as backup air barrier. Make sure to figure out air barrier continuity along internal walls as well.

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