Sanity check: No roof vents at all in a “vented” attic?
I’m building a small house in Phoenix AZ. The attic shall be unconditioned; have no ductwork; and have an insulated floor of blown cellulose at R-50 to R-60.
“Unconditioned” traditionally means “vented” but I am very resistant to putting vents in my roof, and the ridge doesn’t suit itself to a ridge vent very well (quite short plus super-wide ridge board).
What I will do is install quite a few soffit vents — haven’t yet calculated just how many sq ft of air flow that is. Presumably just having soffit vents means zero actual air flow unless the wind is blowing just-so. Maybe it’s not worth even having those?
What I’m looking for is a sanity check that this isn’t foolhardy. I’ve read Martin’s “All About Attic Venting”, which suggests that the reasons for venting are mostly debunked myths. But… going the non-vented route does typically mean “conditioned” and this isn’t that.
Thoughts?
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In dry climates you can often get away without ANY roof venting (at least until the roof develops leaks during the Arizona monsoon season. :-) )
With soffit-only venting it will work considerably better than with none.
Since IRC 2018 in zone US climate zone 2B (= Phoenix ) it still meets code to skip the soffit vents and go with a diffusion vent near the ridge- zero air flow, good drying capacity.
https://www.greenbuildingadvisor.com/article/vapor-diffusion-ports
See section R806.5, subsection 5.2 for the relevant code prescriptive details:
https://codes.iccsafe.org/content/IRC2018/chapter-8-roof-ceiling-construction
With soffit vents also work well with box-vents well below the ridges (better in dry, not-snowy or windy climates like Phoenix) in lieu of ridge venting, if the appearance is acceptable, eg.
https://cdn.treehouseinternetgroup.com/uploads/photo_gallery/large/202598-5bca93bea0ba2_joan-after-box-vents.jpg
Ah, interesting -- I had never heard of diffusion vents before. It looks like they are exclusively for conditioned spaces, according to the linked article, so that wouldn't apply to my situation. But I am going to keep those in mind when I re-do the cathedral ceiling in my main house.
Box vents are ubiquitous here in Phoenix -- that and turbines. I've never even seen a ridge vent here, although I'm sure they must exist on some of the newer homes. Those are 100% what I picture when I say I'm "very resistant to putting vents in my roof" ;-)
The 1/150 rule would mean either 5 of 10 sq ft of soffit vents to meet code. I'm not sure which of the two numbers is accurate since the insulated space in the attic will be roughly 800 sq ft but the actual full attic goes over the extensive patios and increases the under-roof space to about 1600 sq ft. My reading of R806.5 doesn't make it obvious.
Make the vent area 1/150 and code allows you to put it where ever you want. You can combine with some shingle vents (eg, Lomanco Deck-Air) near the ridge. Always air seal the ceiling well.
I think increasing ventilation area, but not specifying the location is not a great idea. An imbalance between intakes and outlets could lead to de-pressurization of the attic.
Or in this case (soffits only), some pressurization, which might be helpful in Winter and harmful in Summer (due to the house being a cold stack). But AFAIK, this is another often referenced "fact" where the data isn't publicly available. Ie, who knows how significant this is.
Moist air rises, so at least some vents near (< 3') the ridge makes sense to me.
Oh, I've never seen that style of vent before. Those are far nicer looking than the standard boxy pimples I typically see on roofs. Four 10ft sections (one on each side of my hip roof) would give me 2.5 sq ft of exhaust. Hmm... very very interesting.
kurt,
Arizona is a lot more forgiving of roofs than most climates, but I'd at least provide the ventilation required by your code.
As a general rule I think it's a mistake to make assemblies that don't include the possibility that they won't stay as well constructed as they were initially. Good air-sealing does remove a lot of the need for ventilation, but if the roof experiences even small bulk water intrusion, or the air-sealing is compromised over time, the ventilation may be enough to stop either being a problem. Assemblies that rely on perfection are asking for tr0uble at some point.
Malcom, that is a great rule of thumb! I do tend to gravitate towards the "I will just guarantee that it'll be water-tight forever" and "not one molecule of air will make it from my conditioned space into the attic"... but even if I did do it at the beginning, the chances that it will stay that way forever is almost nil. Having a "backup" makes excellent sense, regardless!
Kurt,
One of the curses of designing and building in a small geographic area for several decades is you get to see (and often have to correct) the mistakes you made. Because of that I've ended up much more risk adverse than I probably otherwise would be.