Cape Cod Cathedral Section Insulation
I know, another Cape Code insulation question. Zone 4A Southwest MO.
Attic has R-60 on the floor with roof vents
Devil’s Triangle (DT) has R-60 on the floor and R30 on the room (knee) walls with the floor joist cavities sealed up under the knee walls.
No soffits or any venting for the DT.
Right now the cathedral portions have R-19 in the 2×6 cavities.
They stick out like a sore thumb in the winter since snow/frost doesn’t stay in those areas for long.
Wanted some validation (or correction) on my attached sketch before I start in on it this fall. I don’t plan on adding soffits or gable ventilation to the DT but I do want to maintain the air movement the R-19 currently allows.
To summarize the drawing:
1/2″ vent channel
R-30 rockwool (I have plenty of this leftover from the attic project since I had to buy two pallets)
Furring out the 2×6 rafters with 2″ XPS and a 1x (3/4″)
The drawing is looking down into a cavity. I have 5 sections of this to do (as seen in the photo). Two in each bedroom – due to dormers splitting them. 1 in the stairwell.
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Replies
Consider all the insulation in the world is useless without an effective air barrier that can keep the conditioned air indoors. Cape cods are notoriously for being very very leaky and incredibly difficult to seal up short of a gut rehab.
Have you done a blower door test on your home?
Have you read this article?
https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
Walta
Hey Walta. Thanks. I actually have a print out of that article on my desk :) And I know - kind of stupid of me to ask if option 1 is a good choice when well, it's option 1...
We had a blower test done around 12 years ago via our electric company. I just pulled out the paperwork and it says:
"...reading for your house is 3,9,33 CFM's. Recommended measures should reduce the reading to 2,753 CFM's. Continuous mechanical ventilation is recommended if reading goes below 3,885 CFM's."
So basically swiss cheese wrapped in siding.
I need to have it done again as that was when the crawlspace was vented and the access hole was just covered with an ill-fitting piece of plywood. Plus the attic access was just a piece of, also ill-fitting, drywall over the hole.
Since then I've put R-5 XPS over the entire exterior with the plywood sheathing gaps foamed first and then the XPS seams taped, crawlspace encapsulated, attic redone (gutted and air sealed), new windows, new doors, etc. Basically the focus has always been on efficiency as the first couple of winters here were miserable. So I have to assume, or greatly hope, that it's had a significant reduction in air leakage.
The CFM number is helpful but not useful by its self. Did they happen to do the math and calculate the useful ACH-50 number? If not, the instruction are at the bottom of page 2 in this link
https://energyconservatory.com/wp-content/uploads/2017/08/Test-Results-and-Sample-Test-Forms-Guide-.pdf
Seems to me you would get more results from air sealing than insulation.
Continuous mechanical ventilation is overrated in my opinion especially in drafty old houses.
Walta
Thanks for that doc. I hadn't ever looked into that since at the time I knew there were some major leak points.
All of that should be taken care of now (I need to call my coop and see if they offer a discount for a re-test). Actually, these cathedral portions are really the last thing I need to do that is a noticeable failing. Anything else that could be done would really have limited benefit.
That is a lot of work for not much gained.
I would leave everything as is and add a layer of rigid on the inside over the existing ceiling and new drywall over the rigid. Much less labor and will work just as well.
I know you aren't wrong and I won't argue it, only justify it to myself ;)
The only work to me is the mud work. I hate taping, mudding, and texturing.
I enjoy the demo, foam, and rockwool part. And painting.
Since mudding would have to be done either way, I might as well do the whole bit.
Definitely wouldn't be doing it if I was paying for anyone else's labor. This is my hobby/vice.