Flat roof to pitched roof – will venting work?
We are designing a house in Bozeman, MT – Climate Zone 6B.
Half of the house is a gable with attic truss which makes venting easy with fascia vents and continuous ridge venting.
The other half we would like to do a vaulted cathedral ceiling. Our trusses are deep enough to allow for 14″ of Loose Fill Insulation (Sheep Wool Insulation) (Well above code minimum) and maintain at least a 2 Inch Air Gap above the insulation for ventilation from Fascia to Ridge.
From what I can gather this would work except for one area above the kitchen where the pitched roof (9/12) transitions to a flat roof. My question is if we vent the soffit at the flat roof side and the ventilation gap is connected to the pitched roof ventilation gap, will the stack effect still work in this condition?
Air would enter the soffit vent, travel horizontally 6′ then up the pitch to the ridge vent. Would there be enough draw to the ridge for this to work properly?
I have been on a mission to exclude spray foam from our project so a non-vented assembly flash and batt would not be preferred.
I also don’t like the idea of doing above deck polyiso on just half of the house as the goal is to keep the two gable forms the same width and height. We are trying to keep foams, synthetics and plastics to a minimum with our build.
Any advice on how to make this cathedral ceiling (and flat roof transition) work would be greatly appreciated.
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Replies
tpowellpalm,
I'd have no reservations about using that detail. Just make sure you have a good warm side air-barrier, and if you are considering ceiling mounted lights, include a service cavity.
Malcom- Thank you for your response -
We will make sure that everything in the interior is well sealed.
Much appreciated
tpowellpalm,
One thing to note: Bozeman is still using IECC 2021 as their adopted code, which requires R-60 in roofs/ceilings. That requirement was reduced to R-49 in IECC 2024, but at the rate Bozeman adopts updated codes it might be a while till 2024 is adopted.
I'm not sure what brand of loose fill sheep's wool insulation you are using, but it seems like most brands claim an R-value per inch of 3.5-3.8, with some (probably very optimistically) claiming up to R-4.2 per inch.
At R-3.5 per inch with 14" of insulation, you hit R-49 on the nose. Code-minimum for IECC 2024, unfortunately not the version of IECC that Bozeman uses, which requires R-60.
If you assume R-4.2 per inch at 14" depth, you're still a bit shy of code-minimum (in Bozeman) at R-58.8.
It's hard to tell exactly, but throwing a scale on that screenshot looks like 16" framing (14" insulation + 2" vent channel)? You'll have to go deeper if you want to hit code-minimum, much less be well above code minimum.
Also hello; I work in Bozeman as well.
HI Paul-
Great to see a fellow Bozemanite on the platform.
You make a good point on the insulation values.
I guess I was looking at the Exception R402.2.1 that states you can install R49 if you have full depth insulation over the wall plate. (The uploaded section isn't totally representative of the detail but there will be full height insulation over the top plate). Now that I look at the code section however- It does say that it is for ceilings with attics (whereas this is a cathedral ceiling). Regardless you are right that this really isn't well above code minimum. But alas we are shooting for R60.
You have a good eye on the scale- The truss is shown at 16" but should be 18" deep. This would allow for 14-16" of fill insulation and 2-4" of vent space above. (Thank you for catching that)
The Sheep Wool Insulation we have specified is from Havelock Wool. They claim R4.3/Inch but probably wouldn't hurt to plan for a bit more insulation.
I was mostly gauging whether the venting would work over the flat roof portion. But sounds like so long as the vent space is connected continuously to the pitched venting gap to the ridge, we should be ok in this instance.
Much appreciated for your response.