Bonfiglioli Wall with Exterior Insulation
5Stud
| Posted in Energy Efficiency and Durability on
Don’t worry, it can easily turn into a double stud assy!
Climate zone 6 (not too many km’s from 7a)
I will be building a ~700 sq ft ADU and my goal is to use as little material as possible while still getting R40-ish walls.
Nothing groundbreaking here, just a bonfig with exterior insulation.
I like the idea that I can use off the shelf 7,25″ Rockwool or dense pack.
Is there a condensation concern?
Is it just too fussy? (I know, lot’s of extra steps but I work cheap)
Thanks
Phil
NOTE: exterior OSB all edges blocked and taped.
I have left out vented rainscreen and vapour membrain but they will be there.
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Replies
Phil,
We can of course go down the rabbit hole of comparable alternatives, but if you are fine doing the fussy work, what you suggest will yield a well functioning wall with no condensation concerns.
Just wait until I mention 6 mil poly!
Phil,
Do you use the same foundation detail with the embedded 2"x4" ladder on the top in Manitoba that they seem to favour in Alberta?
Yes, or 2 x 6 for the rich people!
It can be a troubling detail if the concrete crew doesn't fill the whole space. Windy!
I am mulling over mono slab vs 1.2 metre stem wall so no pt ladder for this project
The reason to use outsulation in a wall assembly is to avoid thermal bridging, so installing rigid foam on the inside of the stud is doing nothing of help, and a lot of extra work.
Its a lot easier to frame an R40 wall assembly with 2x8 R25 DP cellulose and 3" R15 rigid foam or 4" R16 Rockwool. No poly in CZ6!
I appreciate that Armando,
I know it is easier your way.
I am doing 9 foot walls so a 2x6 stud is half the price of 2x8.
Was sort of teasing about 6 mil!
Intello if dense pack because it is cheaper than mesh+vapour
I'll save my poly for ceiling.
Don't understand exterior insulation + bonfig. Redundant. I would do one or the other.
Patrick,
Maybe the compromise solution to avoid using 2"x8"s, but still have the deep cavities, would be to fur out the studs with 2"x2"s, rather than the foam and plywood?
As of this writing (2023-04-07), pre-discount, a 2x6x10 is $10.28. A 2x8x10 is $11.27.
If 2x8s are pricing out TWICE 2x6s, it's time to find another supplier.
(Note: These are not special prices. In fact, Home Depot's website price is better than my pre-discount lumberyard price.)
The same two pieces at our Home Depot up here have more than a $5 difference in price. Not double, but not insignificant.
2x6 x 104 5/8" = $10.98
2x8 x 10ft = $17.85 (can't get studs)
So I misspoke about double.
Sure, this is a redundant thermal break but why is that a bad thing?
R value of 2x material is 1.4
2x8 7.25 x 1.4 = R10.1
2x6 bonfig 5.5 x 1.4 = R7.7 + R5 (foam) + R1 (plywood) = R13.7
If my studs are 18% of the wall, this is a measurable improvement of the whole wall R value
5stud,
Isn't the comparison at the framing around R-26 vs R-30, if you include all the layers? The question is whether that difference over 18% of the wall is worth the work and complication to you.
Lets look at these walls (this all assumes 20% framing factor), ignoring air films and sidign/dry wall R value. R4.2/inch MW insulation.
As proposed (Bonfig with 3" GPS) ~R38.
2x8+3" GPS ~R34.
Double stud 2x4 + 0.5" foam+ 3" GPS ~R37.
If you look at the energy use difference between any of the above, it is essentially noise.
Even a simple 2x8 wall with no exterior insulation is good enough for such a small structure. The cost of energy use between this and anything higher R value is on the order of $10/year.
Malcolm,
Not sure how you got 26 vs 30.
Using the quick and dirty R times percentages and ignoring the exterior insulation I get 24.78 vs 25.43 using R28 Rockwool.
This does seem to be a waste of time!!
And thanks for chiming in Akos. I agree, noise.
5stud,
I was using your numbers, but adding in the exterior insulation and incidental layers. Your numbers and mine are only for a rough cut a the R-values at the framing. Akos' numbers are whole wall.
You could also use two 2x4s inline on a 2x8 plate and use 1/4 foam between the studs. I'm not sure about your market, but here 2x4s are about half the price of 2x6s still, if your concerned about money. I would think this would be easier, faster and cheaper to fab and install than the Bonfig wall since the thermal break isn't a concern. I would also consider your time. Is it worth going through all the trouble to get a thicker wall with smaller framing members and foam? For a 700 sq ft adu I'm assuming the extra cost would be about$700-800ish more for using 2x8s. With smaller framing and foam you would add time to building the wall for a marginal R value increase. Just something else to consider.
Thanks Bwiemels,
These are good points.
I knew this could turn into a double stud wall!
I'd add that 1/4" foam in the middle of the wall is probably not doing anything; a 1/4" air gap would work nearly as well or maybe better than foam, without the fussiness. Dead air is R-5.6/in and without a large temperature differential across the space, and without air flowing through the assembly, not many Btus are moving around.
When you say 1/4" air gap, are you talking only about MW?
Can I dense pack this assembly as is?
I'm talking about Bwiemels' suggestion of using 1/4" foam between the studs. According to people who know more about installing cellulose than I do, gaps up to 1 1/2" usually don't need to be filled, so a 1/4" gap can be ignored.
Thank you Michael that's good to know about the small air gap! It sounds like if you are considering something like this batts would be the best choice. If you dense packed the cavity I would think you would potentially need netting or the foam strip to keep the insulation in the cavity to get the proper density. My dense packing experience is minimal and I have never dense packed a double stud wall. Maybe the 1/4" wouldn't make a difference for achieving proper density?
Bill Hulstrunk, who knows more about cellulose and has probably trained more cellulose installers than anyone else, told me that with double stud walls, gaps up to 1 1/2" don't need to be netted to get the proper density. Depending on the situation, that may be pushing the limits but 1/4" should be totally fine without netting. If you do dense-pack a cavity that deep, be aware that you need higher density than typical to avoid long-term settling. Somewhere between 3.5 and 4.0 pcf is the right range; many installers seem to stick to the 3.0 to 3.5 pcf range because it makes installation easier but subject to long-term settling.
Having built a house with the bonfig wall, I'm hesitant to recommend it in new construction. It has a great potential when you remodel a house with a 2x4 wall, but when starting with a blank sheet of paper there are better options.
As already pointed out, the exterior insulation takes care of breaking any thermal bridge so then you're really just looking for total R value. I'd think that you could and should shift that focus to the windows, attic, or general air sealing. Those are the places that would make a bigger impact anyway.
Wasn't there an article about diminishing returns on wall insulation recently?