Lost in the Roof Venting Sauce
Hey All,
I thought I had a good plan on roof venting but now I’m questioning about all of it. We ended up not getting a ridge vent on the house. The roofer didn’t like the layout when it came down to how the joists met the ridge, and said it wasn’t really compatible with the cut, so I’m back to trying to figure things out. Here’s what I’m working with:
– CZ 6 in the Idaho/Wyoming region. Lots of snow lately and pretty cold. Current conditions in the photo attached. Also in a forest and forest fires are certainly a concern.
– Two sections of roof. One is a 12/12 vault ceiling with 12″ I-joists. One side of that roof meets up with a porch overroof (pictured).
– The second section of roof is 8′ high trusses. Image attached
– No roof vent cut in the roof. Asphalt shingles
– Hoping to avoid using spray foam but if everyone here says it’s necessary, then so be it
– HVAC in the basement
I’ve read a lot and gone down multiple rabbit holes, but just confused at this point and hoping this group could shed some light.
Roof #1 – Vaulted Roof:
This roof section to be unvented. Current plan is to dense pack all of it, install smart vapor barrier on warm side, add 2×3 furring service cavity, and run electrical for lights in that cavity so as to not penetrate the air barrier. No recessed lights. Drying to the inside. Main concern would be vapor getting into this area. I know vapor diffusion ports don’t work in this CZ.
Roof #2 – Truss Attic:
My two ideas here would be to either A.) do an unvented assembly with a similar airtight ceiling. Fill it with a ton of loose fill insulation (R60-75). or B.) Vent from the soffit to a gable vent — but I’ve had a hard time figuring out if air will flow through that desired path appropriately.
Anyway — Given this info / layout, what would you all do? Understand that if things were different this would likely be easier, but this is what we’re working with. Considerations include wildfire embers, snow & ice dams, and vapor.
As always, thanks–
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Replies
Sean,
Nice house, beautiful view.
For the trussed roof: I'd go with your option B with loose fill insulation at the ceiling. Use a gable vent and several spot vents near the peak to exhaust the air.
For the cathedral ceiling: You can't use permeable insulation in an un-vented roof unless you have sufficient impermeable insulation above. I'd use options 3, 4 0r 5 from this link: https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
Thanks Malcom. Easy enough with the trussed roof and makes sense. For the cathedral, I had read that fibrous insulation in an unvented cavity was possible, but maybe too risky in our CZ. This video seemed to make it appear possible: https://www.youtube.com/watch?v=2LDGcVGH0RY (around 2:45). Perhaps spray foam is the only option.
Sean,
There are a couple of things in their approach that are suspect.
- From 52 seconds on they describe the installation of exterior insulation, including rigid mineral wool, but the section shows the joist cavities empty, and the permeable mineral wool sandwiched between two layers of sheathing. In that situation it is the upper sheathing that is at risk. The lower one they identify as "the sheathing:" is really just the ceiling plane. That approach would work fine with impermeable foam boards, but not permeable mineral wool.
- The assembly shown from 2:45 on has two problems. The first is it isn't code compliant. Outside of climate zones 1, 2, and 3, you can't use use permeable insulation in the cavities without either a vent space or rigid insulation above. The second is that in heat dominated climates the vapour drive is to the outside. The moisture that gets into the cavities and accumulates on the sheathing comes from the interior. It doesn't magically change direction and decide to start drying to the inside simply because you make the interior face of the roof assembly vapour-open. I'd call that wish fulfillment building science.
Understood -- thanks Malcom
I would build a mini attic at the peak of the bedroom cathedral ceilings. You can connect the attic space to the main house attic and also vent it with a gable vent on the other side.
Dense pack the rafters up to this mini attic and loose fill the attic space. The rafters are no technically vented but they can dry through diffusion to the attic space. This is pretty common in retrofit in areas with half story construction and holds up.
You can also do as above and instead of dense pack, insulate with batts in the rafters and leave the code required vent gap above. Simple soffit vents can now be your intake and the mini attic gable vent the exhaust. This is a fully code compliant option that is zero moisture risk. Make sure the baffles you use are permeable, my preference is to install baffles only at the soffit and use high density batts for the rest without any baffles.
Akos,
GBA ran an article on that assembly some years go (which I can't find). It seemed to be a good solution for those situations, although I do remember Martin pointing out it doesn’t meet code.
No knock on Sean who unfortunately finds himself in this bind, but getting pushed into a corner with few options for how to insulate a roof once the framing is complete is a constant theme here on GBA. I hope others starting projects take note and avoid the problem.
Why does the Roofer get the final word on the ridge vent?
It is a truss roof. There is no beam to get in the way. My guess is the carpenters did not leave a gap for the vent and the roofer did not have or want to use his saw.
If there is language in the vents instructions the precludes ridge vents on your roof OK show me but I don’t like them is just wrong.
And pick one of the 5 cathedral options from the article.
Walta
Walta,
As I understand it the problem isn’t in the trussed portion of the roof but rather the framing of the cathedral ceiling, which is supported at the ridge.
Malcolm, the truss design he shared is at the shed dormers. Like Walta, I don't understand why they can't do conventional ridge venting.
Mike,
Sorry - I meant to say “cathedral ceiling". I've corrected my post. I’m assuming the I-joists butt up against a flush beam.
Oh got it, that makes sense. I've been thinking about a parallel-chord truss roof with no ridge beam I've been working on so that didn't come to mind! It's hard to believe how many framers and builders don't automatically drop the ridge beam a couple of inches to allow for venting, or at least ask someone whether they should. I'm sure blame can be shared with architects and engineers.