Yep! Cathedral ceiling: To vent or not?
Okay, so I have tried to do all my homework before asking the question but I seem to be confusing myself more with every article and question I read including Martin’s “All About Attic Venting” article. Simple 4/12 cathedral roof with no intrusions (hips or the like) on a new construction home. Climate Zone 6 with high snow load potential but infrequent. Originally I was going to do a metal roof but value engineering is pushing me to shingles and my dilemma. The rafters are 2×14 TJIs so I have some room but I am aiming for upwards of R-60. Here are the two scenarios I am contemplating.
Top Down on these. Sorry I don’t have time for drawings
1:unvented
shingles
underlayment (does this need to be vapor permeable?)
OSB sheathing attached directly to rafters
6-7″ closed cell foam
remainder filled with loose fill or batt
drywall
2:vented
shingles
underlayment
OSB sheathing attached directly to rafters
1/2″ OSB or 1″ rigid foam attached to the flange of the TJI (the Justin Fink method) to give 1 3/8″ vent
7″ spray foam
remainder filled with loose fill or batt
drywall
I am leaning towards vented for ice dam security and shingle warranty. Here are my concerns. I read somewhere that if this is vented, I should allow vapor to flow all the way through the system to the vent. Which means no closed cell foam and the rigid foam should be perforated and not sealed at the edges, so I’m told. Doing this makes hitting R-60 a challenge in the space I have. It also means that my air barrier is the drywall. As important as the air barrier is here as everywhere, it seems very fragile and difficult to construct. All it takes is one macrame plant hanger hung from drywall hook to compromise the whole assembly. My belief is that as long as I maintain my 51/49 insulation permeance to keep condensation off the underside of the spray foam, it should breath to the inside and all will be happy.
The unvented assembly has some nice advantages. Easier to assemble, cheaper and more insulation (marginal but its there.)
Or maybe I should put a solar powered vent in each bay, skip the insulation all together and call it good.
Thanks in advance for any and all constructive input!
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Replies
Bevan,
You have gotten tripped up by a widely repeated falsehood: "I read somewhere that if this [assembly] is vented, I should allow vapor to flow all the way through the system to the vent."
This is false. I have addressed this red herring in several articles, including the 2011 article titled How to Build an Insulated Cathedral Ceiling, in which I wrote, "Builders should not encourage the migration of water vapor through a cathedral ceiling."
Because the falsehood is persistent, I addressed it again (and more extensively) in my 2013 article titled All About Attic Venting, in which I wrote (under the heading, "Confused thinking that needs to be debunked") the following:
“Although I have listed the four most common explanations for attic ventilation requirements, it’s important to mention a fifth explanation — one that is particularly muddled and confused. My nickname for this explanation is, ‘Your ceiling is a safety valve.’
“This explanation is entirely divorced from any understanding of building science. ...
“The ceiling-is-a-safety-valve theory encompasses several misconceptions. Here are two of them:
• The purpose of attic vents is to help lower indoor humidity levels. If you encourage moisture to flow through your ceiling assembly, you will improve conditions inside your house.
• Because your attic is vented, you need to feed a continual stream of moisture towards the attic vents so that the vents have something to do.
“Of course, these ideas are misguided. Ideally, your ceiling should include a thermal barrier that separates the warm, humid, interior air from the cold, dry, attic air. You don’t want to encourage any moisture flow through that assembly — whether by air leakage or by diffusion.”
Of your two choices, the vented approach is preferable, because it lessens the chance of ice dams.