zone 7a interior wall retrofit for increased insulation suggestions
My 1961 home is in zone 7a(Edmonton, Alberta, Canada) The house is stucco exterior, 2×4 walls 16″oc, R12 insulation(yellow paper backed fibre) – There is no option for me to add exterior insulation as the house is already too close to the property line. I am planning to remove the interior plaster and drywall, remove the insulation, air seal the cavity with caulk and canned foam, then rebuild with 2×4 roxul R14, cover with durafoam expanded polystyrene, untaped-so as not to create 2nd air barrier and provide thermal break and then build another 2×4 wall with Roxul R14, cover with 6mil polyethylene vapor barrier and 1/2 inch drywall. Any ideas would be helpful, thank you. Would I be better just building two 2×4 walls with R14 tight against each other and just having one with vertical insulation and the other horizontal to provide thermal break?
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It's fine to have multiple AIR barriers, so I would tape the seams of the rigid foam if you go that route. What you want to avoid are multiple VAPOR barriers.
I'm not sure that rigid foam layer gains you much here. It would probably be easier to build this as a double stud wall, with the studs of the inner wall staggered with respect to the studs of the outer wall to help cut down on thermal breaks. If you build the walls tight together, this would let you fit double the insulation, which would typically be R15 for a mineral wool batt that will fit in a 2x4 wall. That would get you R30 for the whole wall -- not bad! If you were to add a layer of 1" EPS between the studwalls, that would add another R4.2 or so, for about R34 f0r the entire wall.
Staggering the studs but installing them vertically is going to be easier to build than trying to build a horizontally aligned studwall on the interior.
Bill
Thanks for your input, Bill. Considering the weather recently dropped below -50 degrees celsius - every little bit of R value counts!! I reached out to the foam manufacturer and they said the foam when taped and sealed is a vapor barrier. My understanding from the NBC 2020 9.25.4.2.1 is that foam with vapor permeance less than 060 ng acts as a vapor barrier-this foam is 30 ng. Based on this information I think I will move the 1"EPS so it is located behind the drywall and acting as the air/vapor barrier.
We did this with a 9000 sq/ft commercial retrofit. 2x6 walls with existing fibreglass. 2" of EPS was installed on the inside, then a layer of reflective vapour barrier, 2x3 horizontal furring, then 5/8 Type X. All electrical, network etc. ran in the warm cavity created by the 2x3s resulting in very few envelope penetrations. That wall system worked amazingly well at the very low temps we see here. There were nine ecobee stats in the building so I was able to gather some very comprehensive stats on building performance.
Pics are from a renovation 4 years after completion as I added windows to a film studio area so you can see how the wall was done. We did not want to remove the exterior finishes on the building, but the inside was a complete gut.
The architect I worked with on this project uses a similar wall for new construction, but will spec 4" outside CE now in combination with this wall system. This was 9000 sq/ft heated with dual redundant triangle tube boilers. Heating costs for the entire building (which also had an insulated main slab over-pour) were under $200 on the coldest months I kept data for. Average temps for those two months was -20C. That's heating about 9000 square feet.
I would try to make this into a standard stagger stud assembly on separate plates, which will result in good performance and not unreasonable detailing or costs. Sealed interior sheet material as air and vapour barrier will eliminate the need to air seal the stud cavities.
I assume there is exterior sheathing?
Since your in Edmonton look at the work of Peter Amerongen (https://www.linkedin.com/in/peter-amerongen-155a7540/), Butterick projects https://butterwick.ca/ and the site https://www.retrofitcanada.com/ which has a number of deep energy retrofit case studies.
There is also some hands on training coming to NAIT in Feb that will teach you all about the issues your are discussing.
https://www.passivehousecanada.com/events/ab-hybrid-150p-nait-feb-2023/
For purely air sealing you can look at areobarrier.
https://www.ecosynergy.ca/
The City of Edmonton may have adopted the same zoning bylaw allowance as Vancouver and Calgary in that you can put up to 12" of insulation into your side setbacks. Insulating outward is always easier and often less costly then insulating inward.
Creating a double stud dense packed cellulose wall to the inside would be a good option as long as you exterior cladding and sheathing has a high permeability rating so you don't trap moisture in your walls. Stucco, depending on its thickness is not very permeable.