Attic Assembly, Will Vapor be a Problem?
Hello,
I’m building my first house and am working on the attic/roof assembly and would like to make sure it is not going to rot out. I’ve read through a bunch of articles on this site about this issue and am still not entirely sure if my design will work so I wanted to pitch it out to everyone and see what they thought.
The build is in Climate zone 6a-7 in southern Alberta, Canada.
Rough drawings are below but the general concept is to have vaulted ceilings on the second floor with a small ‘attic’ running through below the peak. Vapor barrier leaders will be used to tie the vapor barrier from the second floor into the attic and bring the whole attic into the conditioned space. Roof truss provide more than enough space to reach above required R values and mitigate thermal bridging. My understanding is that the vapor barrier should prevent moisture from migrating through to my ridge and causing issues on my sheathing.
I’d expect that a vapor permeable roofing underlayment across a ridge vent may be necessary to allow any vapor that enters the assembly to escape from the ridge.
Can anyone comment on if they see any large design flaws in this assembly? Will I need a way to pull air from the attic space to reduce humidity?
Thanks,
Anthony
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Replies
vapor diffusion ports, which is what you are essentially proposing, do not currently work in your climate (and may never). There is a in depth article from building science corporation but they were unable to make that a safe assembly in anything over zone 3 (maybe they got it up to 4 but definitely not 6).
Hey Freyr,
Do you know of anywhere that describes why these are a problem in the colder zones? I've read about every article on GBA on cathedral roofs and, while they do agree with you, I can't for the life of me find anything that describes why.
Thanks,
Anthony
Anthony,
This article describes the problem in colder climates: https://www.greenbuildingadvisor.com/article/a-researcher-looks-at-insulated-roof-assemblies
Thank you!
Anthony,
What you have identified as a vapour-barrier on the underside of the trusses, is really primarily an air-barrier. In Alberta it's usually poly, and works well if diligently sealed.
Your roof venting needs to be below the sheathing, not above it. As freyr_design said: Any moisture that gets into your truss space has to be removed by air movement, not diffusion. So the sequence of your stack up from inside to out needs to be:
- Air barrier
- Insulation
- Vent Channel from eaves to ridge.
- Sheathing.
- Roof Underlayment (no need for it to be permeable, the sheathing can dry to the underside).
- Strapping (optional).
- Metal Roofing.
Thanks Malcom,
I have two follow up questions
1) Is there a way to ensure I maintain a continuos WRB if I am venting underneath my roof sheathing? It would seem to me like the WRB would need to break in order to allow air in from the soffits for this to be the case. In which case I might have to just tie my WRB into my air barrier with a leader over the second floor's top plate?
2) If my venting is occurring underneath the roof sheathing then it seems like I certainly need a roof ridge vent. But many articles have pointed out that this is not advised in my climate zone - Freyr pointed this out as well. Though I still don't understand why.
Thanks for your thoughts,
Anthony
Anthony,
1) A WRB (like Tyvek, or building paper) is there to stop bulk water from entering your walls, while allowing moisture to escape by diffusion. It's not usually used as the primary air barrier. If you are using the interior poly as your primary air-barrier on the walls, you can just attach it to the poly on the ceiling. If you are using your sheathing as your primary air-barrier, you can tie the two together with the strips over the top plates you refer to. There is no real advantage to the WRB being continuous.
2) I've never seen advice that suggests a ridge-vent is a problem in cold climates. Freyr_design advised against a vapour-diffusion port at the ridge. That's a different animal.
The assembly I suggested in post #2 is the most common one you see all around you in Alberta. A vented trussed roof. It is simple, easy to build, works well, and is relatively cheap. The only difference in your case would be the underside of your trusses are sloped.
Thanks again Malcom.
I guess my only question is just how does the air which is traveling beneath the sheathing leave the assembly? Is it common practice to leave the ridge open without a vapor permeable membrane on top? Like a vapor-diffusion port without the membrane? Is that what you mean by ridge vent?
A standard ridge vent does not have a membrane covering. It is an open gap covered by a cap like this: https://www.homedepot.com/p/Owens-Corning-VentSure-4-ft-Strip-Heat-and-Moisture-Ridge-Vent-with-Weather-PROtector-Moisture-Barrier-VS4W/205915464?source=shoppingads&locale=en-US&pla&mtc=SHOPPING-CM-CML-GGL-D22-022_010_ROOFING-NA-NA-NA-PMAX-5718363-NA-NA-NA-NBR-NA-NA-NEW-Pmax-CL3TestA&cm_mmc=SHOPPING-CM-CML-GGL-D22-022_010_ROOFING-NA-NA-NA-PMAX-5718363-NA-NA-NA-NBR-NA-NA-NEW-Pmax-CL3TestA-71700000103694446--&gad_source=1&gbraid=0AAAAADq61Uc0jth5jyFOU0Euiwd5bKKVB&gclid=EAIaIQobChMI_tn_jPHfhAMVuAytBh3MPQKiEAQYASABEgLFnvD_BwE&gclsrc=aw.ds
Anthony,
I think the articles you have read on vapour diffusion ports have thrown you off course. They are a very recent innovation which have been adopted as a useful strategy in warm climate zones. The overwhelming majority of roofs in North America, and I'd venture to say 99.99 of those built in Canada have always used eave to peak ventilation to move air through attics. Ridge vents, or box vents near the peak are what you will see on all the houses around you in Calgary. They have no membrane between them and the attic below.
Hopefully Malcolm answered all your questions but I would mention one thing: is very easy to transition you air barrier from outside of sheathing to underside of ceiling by laying a piece of your wrb (if that’s your air barrier) over top plate before truss install and integrating it with your interior ceiling air barrier (with tape or if it’s peel and stick with itself). Even if you just tape you sheathing joints you can tape a piece of intello to sheathing and lay over top plate. To protect it while placing truss you can use plywood or 2x on top.
Draw out a section and see how it all lines up.