Continuous Wall to Roof Assembly for Unvented Cathedral Ceiling
We are homeowners building our house in Northern Virginia (zone 4). We are faced with the challenge of locating high performance building professionals in our area. In order to make our house a “Pretty Good House”, we are left with the option of coming up with our own building enclosure design to meet proper water, air and thermal tightness. GBA has been our saving grace and an incredibly valuable resource. As we are complete novices, we would so heartily appreciate reviews and comments from the GBA community. Please see the attached drawings and assembly layout.
Some additional questions and explanations:
* This assembly is largely based on various articles in GBA and Joseph Lstiburek’s article: https://www.buildingscience.com/documents/insights/bsi-081-zeroing-in#F01
* There is an open carport and two veranda sidewalls attached to the house. We are not sure how to keep the water air and thermal continuity at these 3 junctures. (please see attached drawings)
* We’d appreciate confirmation on the code compliance of unvented roof. Our architect insists that the code requires vented roof, which contradicts with Martin Holladay’s “All About Roof Venting” https://www.greenbuildingadvisor.com/article/all-about-roof-venting#0
We checked Virginia’s construction guide and found no exception to the 2018 IRC.
* We have engaged Energy Vanguard for mechanical HVAC design. Would appreciate any recommendation for high performance building consultants and builders in the Northern Virginia area.
With much appreciation ~~ Lulu, Todd & Mr. Clyde (the Cat)
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Replies
Lulu,
The devil is in the details. You need to do a section through the wall/roof intersection to figure out how the Zip sheathing can be made continuous. A lot depends on what you are using as your roof framing - which in turn determines how the roof structure sits on the top plates.
There are a couple of options to attach floors and roofs to buildings with thick exterior insulation. You can omit it where the ledgers are and take an energy hit, or space the ledger off the wall with a proprietary bracket. Here is a link to one of them: https://deckbracket.com/
In the article you linked to Martin has provided code references for each of his assertions. The IRC code section R806.5 is titled "Unvented Attic and Unvented Rafter Assemblies.” Perhaps your architect was saying that the roof assembly you are proposing needs to be vented, as opposed to all roofs need venting?
Hi Malcom,
Thank you so much for your comment.
For wall/roof intersection, we are thinking of following Joseph's Darth Vader House concept, but using Zip sheathing to connect wall to roof like Matt Risinger's house. The rafters will be under the fully enclosed Zip sheathing. Then lookout overhang sits on the top plates with exterior insulation in the cavity and above it. Please see attached photos. As non-professional, I picture taping the wall/roof intersection with Zip tape all around. Is that a viable construction method?
Thank you for the suggestion of deck bracket solution. Can we use the brackets to connect walls of the carport and veranda to the house's thick exterior walls?
Regarding code compliance, the assembly we propose follows one of the unvented roof assemblies in Martin's article: air-permeable insulation in direct contact with the underside of roof sheathing, plus continuous insulation directly above the roof sheathing (2018 IRC, 806.5.1.2 and 806.5.1.3). Our architect believes that all roofs have to be vented according to code. We are 99% certain that is not the case, but would like confirmation from experts.
Thank you again!
Lulu,
That's a great detail, but both it and the attachment of the verandah and garage may require engineering. Not a big deal, just something to be aware of.
For the garage walls the simplest solution would be to just frame the walls conventionally and interrupt the exterior insulation. You can add it to the first stud bay of the garage wall instead. The verandah may take a bit more thought. It can probably just be lagged through the insulation with long structural screws (something like GRK RSS would work) with a couple of small blocks, as long as the verandah doesn't rely on the h0use for its lateral support.
- Your architect is wrong about roof venting. I guess he has just never encountered un-vented roofs in the projects he has done, but it it's still a bit surprising he hasn't read that part of the IRC.
Hi Malcolm,
Your comments are so appreciated! I have a much better idea now how to continue the conversation with our architect and structural engineer.
Lulu
You have simple shed roof, I'm not sure if there is a good argument for exterior rigid insulation.
You can build a much higher R value assembly for significantly less by going with a vented roof. Use 16" I-joists or trusses for your rafters and fill to R50 with cellulose. In case of I-joists you can staple house wrap/thin osb/fiberboard to the bottom of the top flange for a simple vent channel. Your roof slope is low enough that you don't need to dense pack, you can loose fill the whole space.
This avoid the cost of the exterior rigid plus the two separate roof decks plus makes the overhangs much simpler. Detailing the ceiling as the main air barrier is pretty simple and has been done in passive house builds for a long time.
The high R value PGH numbers assume double stud construction using cheap insulation in walls. Getting R40 in Zone 4 with exterior mineral wool on the walls has an ROI measured in centuries. I would look at building with either a single layer of rigid or going for a 2x8 24" OC wall with R30 HD batts without any exterior rigid.
Akos,
One of the things I appreciate about your responses is that you look that the whole thing afresh. I tend to just answer the questions being posed.
Hey Malcolm,
Not sure my wife appreciates it as much. She usually prefers that I answer her actual questions, not the question I think she is asking.
Let's not go there :)
Malcolm,
After reading numerous articles, Q&A's and taking pages of notes, we realized there is no straight answer to the "perfect" enclosure system. Your answer gave us tremendous relief that our understanding is at least on the right track. Now we can start taking other factors into consideration and focus on fine tuning.. Thank you so very much!
Thank you, Akos! I really like your cost analysis and the cost-effective suggestions.
As northern Virginia is at the top of zone 4, close to zone 5, we did take the overkill approach. Your ROI point is very valid especially considering the simple shed roof. I have done a quick sketch based on your suggestion except for still keeping the two separate roof decks. As we don't have a high performance builder on our team, we like the extra insurance of a second air barrier, plus the continuous Zip sheathing from wall to roof. Is that over thinking?
Please advise if my understanding of your suggestion is correct as presented in the sketch. Much appreciated!
The way things are headed I would not be too worried about being near Zone 5. Summertime cooling up here in the great white north used to be something that was needed for two weeks out of the year, slowly that has moved to two months.
Your drawing should work well. OSB is something that is often used as a ceiling air barrier, like you said, hard to screw up. Since it is not exposed to water, it doesn't need to be Zip, standard OSB will work just as well. You can still use Zip tape.
Thin foil faced rigid will also work as well, for the same cost as OSB you get some real R value bump.
You'll still have to figure out the exact details of your air barrier transitions. The important ones are foundation to wall, wall to ceiling.
The typical wall/ceiling transition is to install a 10"-12" strip of OSB on top of the wall overhanging to the inside. This strip gets taped to the sheathing on the outside. Install the roof framing over this followed by the OSB underneath. The OSB under the rafters can now be taped to this strip for air barrier continuity.
Because of the roof slope, you will most likely also need an extra tapered top plate above each wall to match the rafter slope.
You can squeeze a lot of efficiency out of pretty standard construction by getting the details right. Lot of these details are pretty simple but they do have to be figured out beforehand.
When you see complicated builds on the internet, lot of times it is for the entertainment value (or to promote a specific brand), not necessarily because it makes a better building.
Dear Akos,
First of all, I'm so grateful to you for your time and advice. As you can easily deduce, we are in the position to contend with reluctance and resistance from our architect. Our support system comes in the forms of GBA community and YouTube videos. You and Malcolm's willingness to spare your time means a lot to us. Thank you!
I think I generally understand your suggestion on the transitions. If you don't mind, may I come back for some review after I've had a chance to digest and consider the transitions you mentioned? I will do more homework and limit my questions as not to waste too much your time. We fully embrace the concept of using standard construction methods to achieve PGH efficiency. We just don't understand construction enough to know when it's smart and when it's an overkill.
Thank you again so very much!
Building a home is hard, if a supplying a bit of support makes it easier, I'm glad to provide it. Feel free to post if you have further questions.
Thank you, Akos! We definitely appreciate that you answered the question we should have been asking :)