Off-ridge venting — Multiple rows?
Don’t know if anyone has run into this, but Lomanco has added requirements at some point that they want all exhaust venting in an attic at the same line. Have always done this in a general sense, where if I didn’t have enough space to place all vents on one row, I’d install another row below it. With the new wording, I contacted them and they insisted all of the vents have to be at the same height in the roof — essentially all on one row. The concern was that the lower row would convert to intake for the upper row. I get that, but haven’t seen that issue occur on many houses with multiple rows.
This can be a bit of a challenge with some hip roofs. He was pushing me toward power vents and turbines, which makes me suspect…
So what say all? Do off-ridge vents need to be at nominally the same height? Interested in the science, not the anecdotes.
Thanks for any feedback.
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Lstiburek writes: "Where the air leaves isn’t as important — whether it’s a ridge vent, or mushroom caps, or gables. " Not exactly what you asked, but related.
The operative word being "leaves" related to what Lomanco is saying could happen from an intake/exhaust standpoint.
Technically air could enter and leave different portions of a single vent - so what. It's all about pressure - and it's extremely likely that much more delta-P will be generated between the soffits and the ridge than between two closely spaced near ridge vents (meaning both ridge vents will be exits).
Here is the fundamental problem: Lomanco writes "Exhaust vents pull air from the easiest intake source...". This is absolutely wrong. Exhaust vents pull from all available intake sources (with the amounts depending on pressures and resistances).
If you installed a powered vent next to a passive vent, it would be wasteful (but would still pull air from the soffits). The delta-P across the powered vent might be large in relation to the normal soffit/ridge delta-P. So some (likely to be small (attic), maybe around 50% (cathedral), never 100%) of the air would follow the short path (passive vent to fan). But two similar passive vents in the same area is completely different.
David,
I don't know if it is due to our code containing language requiring venting at both the eaves and top of the roof, but I don't think I've ever seen roof vent anywhere but within a foot of the ridge - and have definitely never seen multiple rows. Is that a regional practice?
Yeah, I probably have a thousand or so homes in Florida I left behind with multiple rows. Code requires them in the top third, and if you use a 135 and place it within a foot of the ride, it'll rip off in any wind going over the ridge - not to mention that you essentially weaken the roof sheathing below standards for the APA.
Good points. I'll be interested to see what people say about your main question about multiple rows and air-flow. it's something I've never thought about.
Hi David -
I don't thing we know enough about soffit-to-ridge venting--especially in cathedral roof assemblies--so I have started to do some "wingnut" testing on this:
https://www.greenbuildingadvisor.com/article/wingnut-testing-soffit-ridge-roof-venting
What I am finding so far is that:
1. the two main driving forces for soffit-to-ridge venting is either wind or stack effect. So far in my limited testing, there is a big difference in these two drivers and how it translates into air flow.
2. The three-dimensional geometry--length of vent, depth of vent, and even width of vent--seems to make a big difference. Still too early for any conclusions, but the depth of the vent space seems to be a big driver: the deeper the better.
Stay tuned; I have blogs coming up where I will have more info.
Peter
Peter - this deserved a thank you and a reply, but somehow I missed it. So, belatedly, thank you. I've been following what's been written here and looking forward to more. I've come to the position to date (especially after a brief email exchange with Bill Rose) that there are so many variables, that its best to meet code, balance it, and forget about it. I'm keeping my rows of off-ridge vents, placing them on the same plane where its practical.