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Dense pack cathedral roof assembly redux

Ethan ; Climate Zone 5A ; ~6000HDD | Posted in Green Building Techniques on

Some of you may remember me from such posts as “Roof / wall connection showing vent above structural sheathing.”

Well, after talking to standing seam metal roof suppliers, it seems that the only viable underlayments for standing seam metal are a variety of (practically) zero perm HT Water shields (Weathermaster, Grace, Lastobond, etc).

So I’m back to the attached detail, which shows the free-floating Solitex Mento (I think should be Mento Plus, not Mento 1000 as called out) underneith a vent cavity created by battens at 2″ )C on 16″ rafters which are also 2″ OC.

A cellulose installer has expressed concern about he Mento bowing upwards during cellulose installation, thus cutting off my vent cavity. I can’t figure out how I could mitigate this, except by leaving the sheathing off until after the ceiling is dense packed (this makes me nervous!) Another idea I have is some sort of space block running at 12″ OC between the rafter bays… not sure how this would be installed…

In addition, is it your experience that cellulose is generally blown right up against these fancy membranes (Intelly, Mento, etc)? Again, a cellulose installer has expressed concern about dense packing against an air-tight membrane, and ahs recommended installing against an Insulweb fabric and then installing the Intello later.

This seems like extra material and work if not necessary.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Ethan,
    Most people create a ventilation channel under the roof sheathing by installing site-built ventilation channels made out of rigid foam, plywood, or fiberboard. Do you care to remind us why you don't want to follow this conventional approach?

    More information here: "Site-Built Ventilation Baffles for Roofs."

  2. Ethan ; Climate Zone 5A ; ~6000HDD | | #2

    Martin, maybe I'm misunderstanding your question, but after extensive conversations about this detail, I was under the impression that I am essentially creating a site-built baffle with the Mento Plus. My reasoning for avoiding another course of plywood is reduction in material, labor, and roof weight. Maybe I am wrong about this. My reason for trying to figure this out is it seems that sealing individual baffles in each rafter bay would be more difficult than stretching the Mento over the whole roof. Also after extensive reading here at GBA, I was under the impression that materials suggested for creating the baffles in the article that you linked to, such as foam, plywood, and potentially fiberboard, are not vapor open enough to effectively vent insulation cavity. In other words, if you look at previous conversations about my roof assembly it has been recommended that the event Channel be below the sheathing. It seems that these site-built baffles acting away Lake sheathing and reduce the efficacy of the vent channels I could be wrong, I could be overthinking it, but that's why I'm here. Maybe I'm wrong about the fiberboard in that would be the appropriate solution in this case?

  3. GBA Editor
    Martin Holladay | | #3

    Ethan,
    Site-built vent channels work, without causing any moisture problems. As Dana Dorsett explained on a different thread, it's preferable for the vent channels to be located below the sheathing rather than above the sheathing, since the source of the moisture is interior air, and since it's easier to carry away the excess moisture if the vent channel is located on the side of the sheathing where moisture accumulates.

    While I understand your desire to choose a material for your site-built vent channels that is highly vapor-permeable, you are overthinking this. There is no evidence that using plywood, OSB, or rigid foam as vent channel material causes problems, for the reasons stated in my article, "Site-Built Ventilation Baffles for Roofs." (I noticed that you quoted these reasons in a different thread, so I assume you are aware of them.)

    You may think that using Solitex Mento will be easier than the conventional approach, but there are two hurdles to your plan. (1) The first hurdle is buildability. Where will you place your feet as you stretch and install this membrane above your I-joist rafters? (2) The second hurdle is the refusal of your insulation contractor to go along with your plan.

    I'm not going to contradict your insulation contractor, since he or she has more experience blowing cellulose than I do.

    I advise you to follow the conventional approach described in my article, "Site-Built Ventilation Baffles for Roofs."

  4. Expert Member
    Michael Maines | | #4

    Ethan, there is no reason your detail wouldn't work just fine, assuming you change to Mento Plus (made for dense-packing against), stretch it very tightly and fasten it liberally. It will bow as much as 1" at the center of the bay but the remaining vent cavity is large enough to function well. Depending on your location you may be required to attach the structural sheathing directly to the rafters, but if it's not a high-wind or seismic zone, or you have another plan for lateral bracing, you should be fine. Installation is trickier than for standard roof sheathing, but with judicious use of toe boards and membrane staples it's really not that bad. In all a bit more work/risk than fully ventilating the roof from the inside.

    Your insulator's request to blow through Insulweb and apply Intello afterwards is common, and ensures a better installation than blowing directly behind Intello. A hybrid approach is to install Intello at the top and bottom of the rafters, and use Insulweb just at the center, to be patched in with Intello after the insulation is blown. When the cellulose is being installed, a large volume of air needs to evacuate the cavities, and although there are twin-walled blowing tubes that allow this, they are clunky and slow.

    On the other hand, I agree with Martin that there is nothing wrong with a more typical approach. If you're using I-joists, you don't need to use plastic baffles, you can just rip sheets of 1/4" plywood and attach them to the underside of the rafter top flanges.

  5. Ethan ; Climate Zone 5A ; ~6000HDD | | #5

    Thank you both for your input. My current thinking is to eliminate the vent Channel underneath the roof sheathing and allow drying upwards via Solatex um, as discussed in this blog post:
    https://foursevenfive.com/blog/vented-high-performance-standing-seam-metal-roof/

  6. Expert Member
    Michael Maines | | #6

    Ethan, that approach will probably work; just be aware that it does not meet the building code definition of a vented roof, which requires a minimum 1" clear space (and Lstiburek says that 1 1/2" or 2" is better). Also make sure your roofer will install over the product--it's a brand new system.

    475 has a lot of details that don't meet code but probably work well as long as every detail is done perfectly.

  7. Expert Member
    Malcolm Taylor | | #7

    And here we go again sliding into potentially risky assemblies when there is no pressure from existing conditions or the architectural design to do so. There are several boringly conventional approaches to designing a high performance cathedral ceiling that we know work fairly well. I don't understand the impetus behind deliberately introducing a potentially problematic assembly.

  8. Ethan ; Climate Zone 5A ; ~6000HDD | | #8

    Malcolm, My experience in working with old buildings has given me a (perhaps irrational) fear of vermin in the structure. Where some see water risk mitigation via vent cavities, I see holes in the roof assembly at the eaves through inevitable failure of the vent channel at some point along ~100' of eave... I see squirrels up there and bees and wasps and what have you. I see also a lack of logical consistency in the various justifications given for roof venting. Why require a zero-perm membrane over the roof sheathing when this will categorically inhibit drying to that direction? I will think about the 1/4" plywood baffle solution that Michael Maines mentions above...

  9. Expert Member
    Malcolm Taylor | | #9

    Ethan,

    I agree about vermin, although I don't share your worries about vents. The perforated flashing I get bent for soffit venting is not the weakest point of entry. Why not just concentrate on making sure they can't get in?

    i'm missing the logical inconsistency you bring up. If the roofing is essentially impermeable, why does the permeability of the underlayment under it matter, unless the ventilation channel is above the sheathing? Cathedral roofs are inherently more risky that attics, but are much less risky when they have sufficient ventilation space under the sheathing, and a sufficient slope to move the air.

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