What is the best way to construct a 2-3 foot overhang on a cathedral / gable design?
We are building a near passive house in East Burke, Northern Vermont with a gable roof and a partial cathedral ceiling…… This provides some challenges…
The design includes 2×6 blown fiberglass with 4″ exterior polyiso for walls (R 45). The cathedral includes (interior to exterior) 2″ polyiso + 2×12 blown fiberglass rafter bays (R 60) to roof sheathing. The ceiling will have loose fill fiberglass to R80. The overhang will attached on the exterior of the foam on the non gable sides; but an overhang 2×6 ladder is planned for the gable end with exterior 4″ foam coming into the ladder.
Any ideas on other ways to approach the cathedral/ gable issues of a highly insulated house?
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Replies
Phil,
I'm not sure what your question is. You seem to have decided how to proceed, but I guess you want feedback on your choices.
Here is some feedback: You can't build an unvented cathedral ceiling in your climate zone (climate zone 6) with only 2 inches of polyisocyanurate. The minimum R-value of the foam layer in this type of construction is R-25 for climate zone 6. That means that your polyiso layer must be at least 4 inches thick, not 2 inches. More information here: How to Build an Insulated Cathedral Ceiling.
Thanks Martin
This is a plan at this stage and looking for comments. See attached file. There is a single air barrier planned (the foam taped and sealed). Questions are:
a) Are there issues with the overhang ladder (thermal bridging) and air barrier?
b) Are there potential moisture issues with the air barrier sitting on the inside?
c) Although not on the diagram clearly, venting is planned exterior at this stage; ? add 2" more of foam instead....
d) Would you or anyone else have better suggestions (ie 4" foam exterior to the rafter bays...)?
Thank you.
Phil,
It looks like you forgot to attach the file.
Sorry. Thought I did.
Phil,
Your cathedral ceiling insulation violates most building codes. If you want to fill the rafter bays with fluffy insulation (it looks like you want to use blown-in fiberglass), then you either need to include a ventilation channel between the top of the fiberglass insulation and the underside of the roof sheathing, or you need to install R-25 rigid foam above your roof sheathing. Your illustration shows the rigid foam on the wrong side of the assembly -- and, as I mentioned before, it needs to be at least R-25.
All of this is explained in "How to Build an Insulated Cathedral Ceiling."
Thanks
Plan is for a ventilation channel above (not noted on diagram). Any thoughts on the overhang ladder?
Phil
Phil,
If I'm reading your sketches correctly, it looks like your overhang ladder will work. Just make sure that you know where your air barrier is, and that you have air-barrier continuity at the top of the gable wall (where the gable wall meets the roof insulation and roof assembly air barrier).
Great Martin. Got it. We are working on that. Back to the venting issue per your musings article which I read again (3rd time). If the air barrier is well laid in a well insulated cathedral (ie R60 or more) what is the risk of sheathing condensation however thick or thin the foam component? And might there be advantages to putting the foam interior vs exterior which was mentioned as an option early in the article but then not commented on? Or could you safely put 2" rigid foam either side of the rafter bays?
Phil,
Q. "If the air barrier is well laid in a well insulated cathedral (ie R60 or more) what is the risk of sheathing condensation however thick or thin the foam component?"
A. If you use air-permeable insulation like fiberglass, it's very easy for moisture-laden interior air to reach the cold roof sheathing. Many builders think that they can stop this type of air movement, but it is a rare builder who succeeds. The stack effect is working against you. All it takes is a few cracks near the ridge of your roof (the exit holes), and a few cracks in your ceiling (at partition top plates or wiring penetrations), and the stack effect begins working against you. (Of course, if you have a recessed can light, you might as well put up a sign near it that says, "air pathway.") The air exits at the ridge, pulling warm air from inside your house into your roof assembly. Your roof sheathing is cold, and that is where the moisture accumulates.
Q. "And might there be advantages to putting the foam interior vs exterior which was mentioned as an option early in the article but then not commented on?"
A. If you have a vented roof assembly, there can be an advantage to installing interior rigid foam (if the alternative is no foam anywhere). The advantage is that it's always a good idea to interrupt thermal bridging. But if you want to build an unvented roof assembly, you have to put the foam above the air-permeable insulation, not the other way around. Not only will your roof perform better with the foam in the right location, but it is the only way to comply with the building code.
Q. "Could you safely put 2 inches rigid foam either side of the rafter bays?"
A. If the roof assembly is unvented, there is only one safe way to do it. And in your climate zone, 2 inches of foam isn't enough. You need 4 inches.
Phil,
Wide overhangs can be a challenge at the corners because the overhang is greater on the diagonal from the corner of the house to the corner of the roof. I have overhangs of about 42” measured horizontally on my house, and would use that feature on a future house. I have scissors trusses, so the top of chords are 2 X 4s, and they extend as rafter tails for the eave overhang. They are adequate for the 42” overhang.
I used a gable ladder built with 2 X 4s. However, the effective overhang cantilever is greater than 42” when measured from the corner of the house to the corner of the roof, and the corners seemed a little weak. So I installed roof support brackets on the eaves at each corner. They do not run diagonal to the corner, however. They simply run parallel to the trusses and add support to the rafter tails closest to the corners. That extra support strength at the corner rafter tails makes the diagonal overhang to the roof corners stiffer. So it is an indirect way to add support to the corners of the roof.
In your design, the bottom two rungs of the ladder are lacking the support of any rafter tails at the gables, so those two rungs would seem to not contribute any support. One way to extent support into the corners is to double up the sub-fascia framing members, so they have greater stiffness to support a cantilevered load at the corners. You would double up those sub-fascia members extending back over several or all rafter tail and ladder rung ends, so they pick up support from several of those framing features.
Also, It would help to have a rafter on top of the gable that provides a tail as close to the corner as possible. Likewise, place a ladder rung on top of the eave wall, so it is as close to the corner as possible.
Phil,
Perhaps your eave detail doesn't show it, but it is not clear how ventilation air enters the eave, or how it can reach the underside of the sheathing.
Also, in an article about ice dams, Joe Lstiburek described "Thermal Plumes": warm, rising air heated by the sun striking dark siding that can accumulate under deep overhangs. I suspect East Burke has a lot of snow on (well insulated) roofs for extended periods, so you should read this article and decide if insulation extending beyond the building line to the fascia makes sense for you.
http://www.buildingscience.com/documents/insights/bsi-046-dam-ice-dam
Thanks. Great articles and points... This is referenced and really good as well.
http://www.thermalsealexperts.com/pdf/air_venting.pdf
I just realized that the drawings do not show the fully ventilated attic (1.5 story with partial cathedral then flat ceiling with R80 and fully ventilated above)..... on this design.
Martin,
In Post #5 of this thread you said "then you either need to include a ventilation channel between the top of the fiberglass insulation and the underside of the roof sheathing" . Then referenced "How to Build an Insulated Cathedral Ceiling." so I read that and found:
"If you prefer, you can locate your ventilation channels on top of the roof sheathing rather than under the roof sheathing. You can create 1 1/2-inch-high ventilation channels above the roof sheathing with 2x4s installed on the flat, with the 2x4s located above the rafters, 16 inches or 24 inches on center. Although this approach is less fussy than installing vent baffles underneath the sheathing, it usually costs more, because most types of roofing require a second layer of plywood or OSB on top of the vent channels".
With proper air sealing of the interior NO CAN LIGHTS etc doesn't ventilation on the TOP side of the sheathing work as the article suggests or is the article wrong? With standing seam metal roofing topside ventilation channels would add very little cost and allow all air permeable insulation to be used.
Jerry,
You're right -- the language in my article was more precise than the language in my response on this page. The ventilation channels can be either directly under the roof sheathing, or directly above the roof sheathing -- your choice.