Minimizing the Risk of a Wall Assembly
I’m building a house in Zone 6 in Ontario Canada. Unfortunately, I only started reading Martin’s articles on risky walls, all the Q&A here and any of Joe Lstiburek’s articles I can find on BSC recently, and am a bit too far into construction drawings and permits to make any major modifications to wall assembly thickness. Indeed, the Code “Compliance Packages” here specify a risky wall construction with R22 + R5CI or R19 + R5CI. A non CI wall can meet code with just R22 stud wall insulation.
I was planning on adding R5 exterior CI just like many of the builds and renovations in my area until I learned about the risk. Changing the walls to double stud wall or using 2 to 3 inch CI would require too much rework and city plan approval at this point; not to mention long project delays.
One option to de-risk the R5 was to use CDX sheathing with a drainable WRB and then 1″ open faced GPS over the WRB. Weep holes can be added at the bottom of the GPS for drainage if necessary.
My question(s) are will this really help the risk, but more is it really worth all the trouble to get an extra R5 on the walls? My architect is in the not worth it camp. He feels I should spend the money and effort elsewhere such as air sealing or windows. The minimum code for windows is U 0.28 or effective R3.5. I could get high performance windows of U 0.15 or R7 instead.
If I forgo the CI on the walls, the default wall will be a 2×6 stud wall with R22 cavity insulation, CDX with WRB or regular ZIP.
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Replies
How difficult would it be to upgrade your planed outsulation from the .75” R5 to 2“R14?
Walta
Allan,
You may have read this already, but this is a pretty comprehensive overview of the subject: https://www.greenbuildingadvisor.com/article/rethinking-the-rules-on-minimum-foam-thickness
Since you have no Canadian answers, I'll take a crack at it, and only in the context of Building Science, since I don't know the Canada Building Code...
In our CZ6, which I think it can be pretty close to yours, we need to install R11.5 CI on a 2x6 wall and R7.5 CI on a 2x4 wall with vapor retarders Class III, or ≥R5 on a 2x6 wall and ≥R3 on a 2x4 wall with vapor retarders Class II.
It maybe worth while for you to read 2021 IRC Table R702.7(1)Vapor Retarder Materials and Classes.
Using CDX sheathing with a drainable WRB or rain screen is always a great idea in any climate zone. I hope my answer helps.
Edit... Ups, by the time I wrote my answer here, you had a couple. Sucks typing with two fingers 😁
2nd edit: Air sealing should be priority #1 regardless what else you do. Always!
Thanks for the replies. Walta, I would have to change wall thickness which would change interior layout a bit, not a preferred option, my site plan is locked with the city.
Malcolm: yes I read that article and Joe's article on double vapor barriers. We still put 6mil poly before drywall here. Without going too far off topic, these articles have prompted me to choose vapor permeable in both directions. CDX with Siga Majvest 200 WRB and Siga Majrex in place of the poly inside. I think this makes the wall safer in winter and summer. So does the article imply that R5 is safer with this construction?
Armando: Thanks for the info, I'm about 500m from the shore of Lake Ontario so a bit more moderate and yes, probably very close to your zone CZ6. Cladding will be full bed stone and wood lap and shake siding. I'm also debating with my architect a rainscreen for cedar shake siding; Obdyke Slicker or pressure treated plywood strapping.
I've attached a summary page of the compliance packages for Ontario Code FYI, you have to pick one, no mixing and matching
I can't follow what wall you intend to build. But +1 on also adhering to one of the options in the 2021 IECC code if you are going to use continuous exterior foam.
With no exterior foam, a wall can be fine with proper attention to the perm ratio.
Additional things that generally help: air sealing on both sides, smart retarders on the interior side, plywood on the exterior and higher perm foam (like unfaced GPS or EPS).
Thanks for the references to IRC Table R702.7. Reading through those I found Joe Lstiburek's article BSI-026 They all laughed.... I don't know how I missed finding that one earlier. It would appear using the R5 is acceptable. So the proposed wall construction will be from inside:
drywall/Siga Majrex/batt insulation/ CDX/drainable WRB/R5 GPS/rainscreen/cladding
Joe's article is attached. Hope is OK since it was downloadable.
“I would have to change wall thickness which would change interior layout a bit, not a preferred option, my site plan is locked with the city.”
It is hard to imagine that your setbacks from the property lines are so tight that expanding the outside of the building 1.25 inches in every direction without changing the interior would violate your zoning requirements. If that is true it sounds like you are dancing on the head of a pin and let’s hope the surveyor is on site when they set basement forms.
It seems unlikely the city inspectors will be on site to see and or notice the foam is thicker than planned before it is covered with siding and unnoticeable to anyone without a 50 foot tape measure.
Walta
With 18" setbacks, 1.5" is a lot of space. You never want to be at the point where your walls are over the setback, it only takes one neighbor to come out with a tape measure. Than, it is either committee of adjustments or moving the wall. Neither option is fun.
I understand that some projects make field adjustments that do not comply with approved plans or zoning by-laws in hope that an inspector doesn't notice or require an after the fact variance if caught. I would rather plan everything well and build to comply rather than plan to exceed allowances.
Anyone with additional thoughts about the effort and expense to add R5 rigid CI versus putting it towards air sealing, windows or other upgrades?
> drywall/Siga Majrex/batt insulation/ CDX/drainable WRB/R5 GPS/rainscreen/cladding
Another cost/benefit tradeoff to consider would be to eliminate the rainscreen.
It's not too hard to calculate the cost vs energy savings of more R value. But it's hard to put a value on "wall is more robust against moisture".