Skinny Bonfiglioli Retrofit: Air-Sealing and Insulating
Here’s the situation. I’m renovating an old house in Climate Zone 6 (boarding zone 5) with the following existing wall assembly (from the outside in).
1. Vinyl siding
2. 1/4″ foam board (not sealed)
3. Original wooden clapboard siding
4. Original building paper (not in great shape)
5. Board sheathing (shiplap)
6. 2×4 wall 16″ O.C. (studs are actually 3-3/4″x1-5/8″)
6+. Dense pack cellulose
7. Plaster and lathe on the inside
I currently have one room gutted and am trying to figure out a straight forward strategy for air sealing and insulating with minimal usage of foam. After posting related question on this site, some late night scrolling through the forum archives, and calculations for total wall assembly R-values I’ve arrived at the following idea.
What if I furr out the existing 2×4 wall with strips of foam and furring strips, a la Bonfiglioli method, use a combination of canned spray foam and building paper (felt or tyvek) to air seal the board sheathing, add mineral wool batts to the stud bays, and finish the interior with an air-tight drywall approach?
Furring the studs will bring me up to a traditional 2×6 wall thickness and provide a thermal break, while the building paper, spray foam, and drywall will form an decent air barrier. Combined with new Low-E casement windows, I’m getting an effective R-value of 13.1 for the entire wall assembly, vs. an existing effective R-value of 7.1 (ignoring the drywall and everything outward from the board sheathing).
In my application, this solution would require less than 2 full sheets of 1″ foam (polyisocyanurate?) and a few cans of spray foam to nearly double the performance of my wall assembly, which seams like a fairly effective application to me.
Does anyone see any major pitfalls to this approach? I can work around the small loss of floor space due to the increased wall thickness.
More details for those interested.
Previous question: https://www.greenbuildingadvisor.com/question/air-sealing-during-gut-renovation-of-a-single-room
Bonfiglioli wall: https://www.finehomebuilding.com/project-guides/insulation/breaking-the-thermal-bridge
Assumptions and calculations
Wall Areas (sq-ft)
A_studs = 51 sq-ft
A_bays = 210 sq-ft
A_windows = 29 sq-ft
A_total = 290 sq-ft
R Values per-inch
Wood = 1.2
Batts/Dense Pack = 3.8
Existing Condition
R_studs = 3.75*1.2 = 4.5
R_bays = 3.75*3.8 = 14.25
U_windows = 0.50 (assumed, circa 2000 vinyl inserts)
U_total = ((A_studs/R_studs + A_bays/R_bays + A_windows*U_windows)/A_total)
= 0.140
R_total = 1/U_total = 7.1
New Condition
R_studs = 4.5 + 1″x5 + 0.75″*1.2 = 10.4
R_bays = 5.5*3.8 = 20.9
U_windows = 0.25 (from manufacturer)
U_total = ((A_studs/R_studs + A_bays/R_bays + A_windows*U_windows)/A_total)
= 0.077
R_total = 1/U_total = 13.1
Change in rate of heat transfer (energy loss/gain to/from outside)
(7.1/13.1) – 1 = -46%
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Replies
That sounds like a good approach to me.
Thanks for the confirmation Michael, I'll stop worrying about this and just get on with it.
Also, for anyone else reading this post, a more in-depth version of this discussion was had last month (I just hadn't found it until I posted my own version of the same question).
https://www.greenbuildingadvisor.com/question/bonfiglioli-wall-for-remodel-in-2023
I think you're on a good path. The only thing I would think about is whether the studs are plumb and flush, when I hear "old house" I think crooked lumber. If you're going to be hanging cabinets you really want the wall to be plumb. Because the foam is somewhat squishy the Bono wall is tricky if you're also trying to shim the wall flush.
I just did a kitchen with pretty much the same wall construction, 6A. It required sistering 2/3 studs to sort out about 1” out of true walls. I did not allow these to actually contact the existing studs to achieve a thermal break. We did have closed cell spray foam applied for air sealing. This is the best way to deal with this wall type (extremely leaky) in cold climates with respect to efficiency in both time and thermal performance.
Ship lap sheathing will have literally thousands of air entry points over 8-10 feet of wall and you will end up with a questionable air seal at the exterior, instead of the interior, which is where you want it in zone 6. Adjacent wall areas will still be extremely air permeable if they are left original. It's a lot safer to just use closed cell spray foam to both air seal and insulate. I know you're trying to reduce foam, but in this case I think the benefits would outweigh the costs (including environmental).
These wall systems do quite well in 6A with respect to longevity (often with little to no insulation) likely as they can dry very easily to the outside. Air sealing a partially renovated wall system with ship lap sheathing from the inside is next to impossible unless you use closed cell spray foam. This way at least you can air seal the wall section that you have access to. If you do the staggered 2x3 wall with low/no contact to original studs, then use closed cell spray foam (I never DIY spray foam), you have air sealing, structural rigidity and thermal bridging all addressed. You'll see varying opinions on this, but after doing renos over about 20 years, FLIR images before/after and comparing adjacent walls, it makes the most sense when it comes to thermal efficiency.
I've done what you're planning in terms of spray foaming gaps in sheathing and then tyvek on the interior side before the rockwool goes in. It's a bit of labour and not a method I would pay someone to do, but it was great as a DIY project. I got an oscillating tool to make it easy to cut the foam flush to the sheathing after the foam had cured.