Vaulted Hip Roof – Venting Strategy
First, the resources are here have been so helpful. I’m tempted to ask this question, in spite of the several great articles on valuated hip roof.
Here’s my situation: I’m constructing an addition to an existing home in Zone 5 (Idaho). The project is height constrained to due a historic district. I was able to go 1’6″ above the existing house for a total height of 20’11”. I was also asked to match the roof form of the existing, which is a hip roof.
Rafter are 2x10s, 24″ OC with 5/8 OSB. There are six 2-ply 1-3/4″ x 9-1/4″ LVLs making up the hip beams. They collect toward the top, almost in a peak. No long ridge so to speak. My exterior walls have a 7′-0″ plate height, and at the peak, the height is 11′-3″.
My question: I really don’t want to spray closed cell throughout, especially in an un-vented approach. It gets incredibly warm here for several weeks a year. So, could I install baffles in the rafter bays up to approximately nine feet in height, and then create a sort of mini attic near the peak, that collects air from the rafter bays, and vent through proper ridge vents? The problem I see is that this strategy won’t collect air from each bay, especially those short runs near the hips, but I could possibly use something like a Cobra vent system in those locations.
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Replies
What you propose should work if you take some care. Make sure those dead ended rafter bays have oversized soffit vents and don't put any pot lights or speakers in them. You want the ceiling there well sealed.
Without a ridge, you'll have to use some box vents at the peak. Not a fan of the look of these but they do work quite well.
Those Cobra hip vents would also work to vent those bays which would eliminate any concerns plus might be able to get enough net free area to avoid box vents.
Thanks, Akos. Good to hear this has a chance. My alternate plan is to rely exclusively on SmartBaffles (2" air gap) and Cobra vents along the ridges and hips. I assume this may also work if I was careful?
I have one valley, which may be the one and only space that lacks ventilation. Unless of course there's an adequate way to provide ventilation in a vaulted scenario for the valley...
Eagle765,
Another alternative is to cover the rafters with House-wrap, and strap above that to form vent channels. If you leave a gap at the valley and hip-ridges you have continuous venting. https://www.greenbuildingadvisor.com/article/building-a-vaulted-high-performance-and-foam-free-roof-assembly
Wow. That's a really interesting approach. Thanks for sharing. Unfortunately my sheathing is already applied on top of the rafters so I'm left with ventilation options from the underside, or doing something built on top of the sheathing. Due to those historic district height limits, I only have about 2" of height remaining.