Building a high-r roof without foam
Hello everyone!
I am looking for very high-r value cathedral (zone 6) ceiling options for a conditioned attic. I’ve read Martin’s article several times, but we would like to avoid foam if and where possible. The thermal bridging, limited r-values and interior air barrier make a traditional vented roof unappealing. I recently came across the idea to attach TJIs to a sheathed roof truss to create a deep insulation cavity. The roof sheathing is taped to the wall sheathing for a continuous air barrier. The TJIs are wrapped in a membrane and the rafter bays are filled with dense pack cellulose. https://www.youtube.com/watch?v=MeoOeFAr1og
I really like the concept. The center of cavity r-value would be well above 60. The simplicity of gable room-in-attic trusses also would provide the attic floor at the full 24′ span for storage and include the structure for the roof in one straightforward application. This is all without the need to design for tributary load paths and interior beams. Additionally, a ridge beam assembly doesn’t inherently include a floor. In a way, the trusses seem more straightforward, but maybe I’m way off or forgetting something.
From a building science standpoint, I think everything makes sense: the roof sheathing will stay above the dew point in the same way that foam would keep the roof above the dew point, but I do have a lingering concern about inward vapor drive in the summer. Is the plywood roof sheathing enough of a vapor barrier to mitigate this risk? Would 1/2″ Zip be better or the same?
It’s obvious to me that this is a vented assembly beneath the roofing material (standing seam metal roof) with the battens and cross battens, but I’m a little wary of the code official rejecting the design since it seems like a hot roof design (based on where the roof sheathing is) that doesn’t use the code prescribed thickness of foam. Are my concerns about code approval warranted, is there another obvious solution that meets my roof r-value goals that I haven’t considered? Any other thoughts?
Thanks again,
Derek
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Replies
Derek,
I'll give your post a bump.
Derek, your design sounds a lot like Ecocor's Passiv Roof: https://ecocor.us/roof-systems/#passiv-roof, third option. It's an excellent system and meets IRC requirements for a vented roof.
Last night on The BS + Beer Show we talked about vented roofs with Martin Holladay, Ben Bogie, Kohta Ueno and Aron Jones about vented roofs and we touched on this type of detail, but did not dwell on it. The episode should be posted here on GBA on Sunday.
Michael,
You seem pretty familiar with this sort of stuff. Do you think piling lots of rigid wood fiber panels on the exterior (akin to exterior rigid foam) is a silly idea? I'm getting some pushback from folks suggesting it's perhaps not a practical assembly, but I can't place my finger on what specifically their reservations are. Would something like the Ecocor roof you linked above make more sense?
(And as a side note, I wonder what purpose that extra layer of 2x4's infilled with Gutex is serving in the 'Optima Plus Roof' on that same page you linked?)
Tyler figuring out these details is fun for me. I also worked at Ecocor when we developed the roof I shared so I'm pretty familiar with it. I don't think exterior wood fiber is silly at all; in fact I think it's a great product and would like to use more of it. It is currently not the most economical choice, compared to rigid foam, but it's not outrageously expensive either. Like many others, I am looking forward to GO Lab's wood fiber insulation plant to start up; it's less than an hour from me and I know they plan to be cost-competitive with foam. The main downside to installing thick exterior rigid insulation that I've heard and experienced is trying to hit the framing with long screws. If your air control and/or water control layers are under the rigid insulation you don't want any shiners!
I'm not sure how costs for exterior wood fiber on a roof would compare to a system like Ecocor's Passiv Roof. They developed the Optima Plus roof more recently so I don't know the details but I imagine the idea is that instead of just installing 2x4s on the flat to create an over-vented roof, you might as well turn those 2x4s on edge, put 2 3/8" Gutex between them and still have 1"+ for a vent space, gaining about R-10 more than the cavity insulation alone.
Thanks Michael, I'll check it out! I think Ben Bogie is one of the carpenters who exhibited the system in the Youtube video I posted, and you were mentioned in a wall assembly on the same channel. It seems like the green building world is quite small.
Derek, that's funny, I hadn't noticed that Ben was in that video--I only watched the very beginning. The green building world is relatively small, especially those of us who enjoy sharing what we are doing and learning, but it's growing all the time!
Is this new construction or retrofit. For new the vaulted parallel chord truss is outstanding in my opinion.
It's new construction. I considered the parallel chord truss. I like that I could loose-fill them instead of dense-pack, and the ventilation would basically be on par with an attic. However, I would still have to build the attic floor. I understand this is not a difficult undertaking, but it is one more thing. With a room in attic truss I get the roof structure, attic floor, and a profile that is very easy to air seal. The attic trusses are available at a cost that is only about a third more than a TJI floor assembly or floor truss assembly would be by itself.
This is an interesting question.
Another method might involve exterior rigid wood-fiber insulation, but unfortunately at this time it's not a readily available product. They seem to have the systems for this pretty dialed in Europe where it's far more common. But it's worth noting that a Maine company is planning to release these products in the next few years, and for now one can import from the likes of Gutex or Steico if desired.
475 is a good resource for these sorts of foam-free assemblies, including roofs. I recommend downloading their Smart Enclosure guide.
You don't need to build the structure on top of your roof. Instead just put the sheating on the inside of the roof and rigid wood fiber insulation on the outside then pack the roof with dense cellulose. It doesn't matter if your roof is made of trusses or tji. With trusses, you'll need to install some netting to split the cavities though, otherwise it's hard to dense pack the cellulose. Another option if you are not in a high wind zone would be to replace the interior sheathing by intello and install some metal bracing to resist shear loads.
Note that you'll need to tape both side of the roof and a water barrier on top of the wood fibre board. Some europeans manufacturer sell a waxed water resistant board but those boards are really expensive here in NA.