dead ends in attic ventilation channels- close off and stuff them, drill side vents to next rafter?

Hi,
I’m pretty new to home owning, and VERY new to fixing and building stuff, and I LOVE this GBA site as a learning resource since I discovered it recently.Most of the contractors and trades I have asked about stuff or done work for me usually just give me a “it doesn’t really matter” type of answer, and it’s been great to get some better information and perspective from GBA columns and. QandA .
We bought a 1965 cottage a few years ago in Nashville ( zone4 I think?) — it’s basically a cape cod I think … (steep roof , attic with dormers and 4 ft knee walls) After we had owned the a house for a couple years, last year the attic ceiling paneling started to warp and sag ….Long story short , I had to gut the attic completely and get a new roof ,complete with entirely new roof deck,EVERYTHING was rotten- roof deck and insulation full ofwater and mold, It turned out there was basically no ventilation … soffit had no vents at all , and ridge vent had been stuffed with insulation and gable vents completely closed off behind more cheap paneling,…. it is clean now, new roof , and I am putting it back together with proper ventilation. No knee walls going back in, it’s going to be all conditioned space cathedral ceiling.
I have installed overlapping Provent chutes from peak to soffit in all the rafter bays, and furred out the 2×6 rafters with 2×2 pine to make room for R-23 5.5′ Rockwool -next , thanks to knowledge acquired here on this site, I’ll try to figure out a DIY air seal and wind block at top plate.
My question today though has to do with the space from soffit that dead ends into dormer window with no escape route to ridge vent, and then of course the space where the dormer stops and roof line resumes …. which leads to the ridge vent, but has no air intake from soffit of course.
I confirmed my concernthat these create an obvious ventilation problem reading some cathedral ceiling info from Martin H. in various places on this site , but if there was a clear solution in there somewhere I think I missed it ….
I realize the answers for the two spaces may be. different, but it seems like the only options. I can think of are :
1-seal off the rafter bays below the dormer windows and fill entirely with every ounce of R value I can pack in there and try to make it airtight?
2– drill a bunch of holes in the rafters below dormer window and try “MacGyver” some airflow from soffit vent out into the adjacent rafters that DO flow to the ridge?
Then … above the dormers ….
1-drill MORE holes to let air IN from adjacent rafters,?
OR
2-stuff full of insulation and seal?
3- allow an air channel with provent chutethat starts from a dead end to terminate at open ridge vent?(possibly to let any moisture get out the top?)
I’m going to attach pictures of both spaces …and if it means anything-any good answer doesn’t have to be fast Or easy…. I’m not a builder or a flipper,this is just for my own house and it only has to happen once.If it takes 2 days to work it out , I have the time!
Thanks in advance for any help or ideas!
-bob
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Replies
It sounds like you have the sheathing on already. If not, you can install horizontal 2x strapping on top of the rafters (and below the sheathing) so air can move horizontally to the next unobstructed rafter bay.
Otherwise, the safest approach is to insulate those dead-end bays the same as an unvented cathedral ceiling. Spray enough closed cell foam against the roof sheathing for condensation control, and fill the rest of the space with a batt. In climate zone 4, at least 31% of the total R-value has to be in the closed cell foam layer to protect the roof sheathing from condensation.
Since water vapor tends to migrate upwards, the rafter bays vented at the top only might be okay with being treated like the fully-vented portions of the roof. It is a bit of a risk though, but you can minimize the risk by making the ceiling as airtight as possible. Drilling a few holes might help if the rafters are structurally oversized, but I wouldn't really count on much air flow, especially if it has to go through more than one rafter.
Frank,
“ Drilling a few holes will help if the rafters are structurally oversized, but I wouldn't really count on much air flow”
Yeah, early on I tried that in an attempt to vent a poorly performing hipped roof, and saw no reduction in the moisture accumulation at the top.
Totally agree Malcolm. If a rafter bay with some holes does stay dry, I'd attribute that mainly to other factors. It's good you highlighted that sentence because I really meant to type "might help", although "wouldn't hurt" would probably be more accurate.
I think your options would be an unvented assembly in these areas or something like a vapor diffusion vent. While vapor diffusion vent is not allowed in your climate zone, it may work in a small portion of the roof. Also keep in mind you could spray foam just this portion of the roof and have a vented assembly everywhere else.
Bob,
You are much more likely to have moisture problems in the lower rafter bays that end at the dormer than the ones above which are open to the ridge vent, because as Frank said, although the upper ones don't benefit from air movement, the moisture usually accumulates at the top - where it can escape at the peak.
On the lower area I would install a continuous vent at the dormer end-wall. They are much like half a ridge vent. Here is one example: https://cansave.ca/lomanco-ventilation/omni-wall/
wow, I think you earned a" mic-drop" with finding this prefab vent thing for under the dormer window Malcom!The question now I guess is why didn't my roofer know about this? haha!
did everybody else know about these things too?
I'll send info to my roofer and. see if he could retro fit this in without having to undo too much finished thing shingling and. see what it would cost.
Thanks everybody for the feedback.
bob,
There are a few similar vents. Your roofer may find some easier to retrofit than others:
https://www.metalera.com/Products/Details/Metal-Era/Hi-Perf/Hi-Perf-Ridge-Vent-Sloped-Roof-Meets-High-Wall
http://www.roof-2-wall.com/