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Detailing a Foam-Free Vented Roof Assembly

B C | Posted in Energy Efficiency and Durability on

Howdy everyone,

I’m designing my future house (TX Panhandle, Zone 4B) and I’m intrigued by the roof design from the following article posted here/fhb:

Building a Vaulted High-Performance and Foam-Free Roof Assembly

I think this design looks promising for my plans for several reasons:
* Tradespeople in my area generally frame residential roofs with cut rafters, and this design won’t require a different skillset from them.
* Transitioning the air barrier to the bottom side of the rafters saves the hassle of sealing around the rafter tails or having to “monopoly frame” (to borrow a term from Matt Risinger) and attach the eaves separately.
* The vented approach is a big safety valve allowing the roof to recover should water actually get in it or should the house pump too much warm and humid air into the attic.

So I pose a few questions for the great minds here:

1) What framing members would you use to increase the depth of the cavity for insulation?  Our municipality’s current design standards (amended 2015 IRC) only calls for R-40 roofs, which can be met by this design with 2×12’s, but we are going through the adoption process for the 2021 IRC which calls for R-60.

I’ve considered a) 14″+ deep I-joists, which would require me to support them with ridge beams everywhere (the roof will be hip with gable walls, but plenty of hips and valleys to handle)  b) deep LVLs which will allow ridge boards, but seem to be prohibitively expensive right now c) scissor or parallel chord trusses which would end up being way too deep to be practical.

2) For the engineers: Can this method be used with ridge boards instead of ridge beams?  The detail diagram in the fhb article shows the attic floor to be balloon framed to the studs below where the rafters sit.  This makes sense, as setting the joists on the plate with the rafters would force you to seal around them and part of this design is transitioning the air barrier to the inside.  But can these joists be used as the rafter ties?  Does nailing the joists to the studs below the plate provide enough resistance to the spreading of the rafters?

Thanks for you input guys and gals.  This is my first question here, and hopefully I will have many (but not too many) more.

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Replies

  1. plumb_bob | | #1

    1) I joists can be great for rafters as the top flange can be used to create a ventilation channel. The manufacturer of the I joist should have details for roof framing (for example look at Roseburg framing system installation guide online). Having the rafters land on top of the ridge, instead of butting to it, will make roof ventilation easier.

    2) As above, the manufacturer should have approved details. Anything outside of this you should get designed by a structural engineer.

    On a recent build I did the same sort of method as you are considering, joining the floor to the wall below the top plate. However, I used a ledger system with hangers as it was easy to pre-instal a strip of poly behind the ledger to keep my air barrier continuous. It is never possible to say that a certain system will provide enough resistance against X without having the full details.

    1. B C | | #3

      Thanks, Bob. I-joists are looking more and more like the best choice. I'm thinking I'll need to make sure the plans can accommodate some columns to support the ridge beams. With a ridge beam there won't be any spreading force to contend with.

      I like your idea of the ledger to carry the ceiling joists.

      As for engineering, I've got a P.E. in the family, although I won't use them for this project as they are not practiced in residential design. I'm just trying to limit my options and get close before I engage someone.

  2. Deleted | | #2

    Deleted

  3. plumb_bob | | #4

    Try to ensure the structural ridge beams do not run continuously through the exterior walls as this detail is very hard to seal and insulate correctly.

    Also, like the ledger, you can drape a piece of poly over the ridge prior to installing the rafters, this can tie into your continuous air barrier. Try to be conscientious about not damaging this poly strip during construction.

    True balloon framing had the joists nailed to the sides of the studs, and then a "ribbon" was let into the studs below the joists for bearing. I used structural screws to attach my ledger, and then used a rough-cut 4x6 as trim (false timber frame) under the ledger, also structurally screwed. So lots of support.

  4. Expert Member
    Malcolm Taylor | | #5

    BC,

    A general caution: Moving the venting above the structural framing to eliminate the dead-ends that occur on hipped roofs or at gables doesn't automatically yield a well ventilated cavity. You still need a fairly straightforward path and a mechanism to encourage the air to move from eaves to ridge. If it's at all circuitous, and the material containing the blown insulation is not rigid, you may not end up with deep en0ugh clear channels to adequately ventilate the roof - although in your climate that's probably not as much of a concern as other places.

    I'm not that familiar with your codes. Under ours the wall plate would have to be directly attached to the subfloor for it to act as a restraint on the horizontal forces exerted by the roof, though an engineer could probably come up with something the would work if the balloon framed knee-walls weren't too tall.

    Another option for deepening the insulation if you go with dimensional lumber for framing is to add either strips of foam or 2"x2"s to the underside of the 2"x12"s. The foam has the added advantage of reducing thermal bridging, but makes drywall and trim more difficult.

    I've never done this type of roof assembly, and several people who have (like Josh Salinger) say it isn't too onerous to build, but it's one of the few I'd pass on framing if the roof was steep or high. Getting the layers above the framing on, tight and sheathed, while having nothing to walk on makes me nervous - but maybe I'm just getting more cautious as I age.

  5. Expert Member
    Akos | | #6

    I would not want to be walking on anything that is covered in Tyvek. Slippery as is but even a hint of water on it and it is like ice. Maybe building a short span where you can reach the top without having to walk on the roof would work, but nothing any long spans.

    Keeping the air barrier bellow the rafter is the simplest. Well detailed 6 mil poly, fancy membranes or taped OSB all work. Easy to install and visually check. You do have to figure out your transitions at the wall top plates and at the ridge. Plumb Bob's suggestion of draping poly works, I usually go for a wide piece of peel and stick tape with only enough of the backing removed to stick it to the faming, the rest of the backing is removed when the air barrier is up to complete the transitions.

    With a hip roof the easiest way to get vent channel continuity is to cross strap with perpendicular 2x2 on the top of the I-joists. Along with the vent channel along the top flange, this extra space above allows for flow not only along the rafter but also across it so all rafter bays can be vented.

    Simpler is to skip the hip roof and go for a gable roof. This also means the posts for the ridge beam fall onto the exterior walls and it is easier to carry the loads down to the foundation.

  6. B C | | #7

    plumb_bob, Malcolm, and Akos, thanks for the replies; they take time and are greatly appreciated.

    plumb_bob, the poly over the ridge is a good idea, as is going full-gable from Akos. Aesthetically, I'm a fan of the gables, too. And Malcolm's idea of adding additional lumber to the rafters to increase the depth is a great idea, and I hadn't thought about the slickness of the Tyvek. It's possible I'd have problems getting the framers to want to do it for safety reasons. Something else to consider.

    Thanks again, y'all.

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