Double stud/Bonfiglioli wall
A couple of questions please.
I see on the Rockwool site that they offer 8″ Comfortbatt in Canada. Does anyone know if it’s avail in the US? I know i could send them an email but since I’m here………….. 🙂
I’m a fan of the double stud wall because I’m in favor of taking care of the thermal bridge and of using as little foam as possible. I also like the idea of window and door details being more standard. I’m thinking that if you built a double stud wall and took an idea from the Bonfiglioli wall by attaching a strip of rigid foam to one of the walls causing them to butt up against each other. In other words 2×4 – 1″ foam strip – 2×4 for a wall depth of 8″. That would leave a nice 8″ cavity for the 8″ Rockwool batt.Â
What do you think?
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I suppose that could work, and might even have a little bit of an advantage if you can also work the binfiglioli strips in to the top and sill plates too.
If you just offset the studs in the inner and outer part of the wall, you can use standard batts for 2x4 walls in both the inner and outer parts without any fancy cutting. This gets you an overall R value for the wall just shy of R28 if you are using mineral wool batts, and that’s allowing for the reduced R value where the studs are.
Bill
Greg,
It's one of many variants that sounds quite promising. A similar one would be using 2" foam (or mineral wool strips) so that you could use standard batts for 2"x4"s in the inner wall, and batts for a 2"x6" wall on the outside. One advantage of that one is you could sheath the inner-wall and treat it as an insulated service cavity if you wanted.
The disadvantages are:
That Bonfiglioli strips are labour intensive and probably only make sense if it is being owner-built. Paying someone to add foam to plates and studs would soon get pretty expensive.
The question of how necessary some moisture buffering is in a double-wall rears its head. Filling the wall with something like mineral wool (as opposed to cellulose) may make them more susceptible to wet sheathing. Whether a rain-screen is enough to mitigate the problem, or using something high-perm like fibre-board, I don't know.
If you remember my original plans included sheathing on the outside of the interior wall and fiberboard or Densglass on the outer wall. I will be building in climate zone 4B.
I'm not worried about the labor as I will be framing this house myself.
I'm shooting for whole wall R value in the mid 20's.
Another thought Malcolm. In a predominately cooling climate isn't it more advisable to have the air barrier on the outside of the insulation?
Greg,
It's not location of the air-barrier that matters, it's where (and what) the vapour-retarder is.
It's my understanding that zone 4B doesn't require a vapor retarder.
Greg,
No you don't need one. I was commenting on your suggestion the the location of the air-barrier mattered. With an air-barrier it's the presence that provides the overwhelming benefit. Location is very secondary, although moving it to the exterior in a warm, humid climate helps with wind washing. With a vapour-retarder location is as important as presence.
Greg,
Reply to post #3
Yes I remember, and was suggesting a variant that brought you closer to what you had originally suggested, and avoided the problems of cellulose installation.
Another question. Outer 2x4 wall with strips attached. Will the strips be rigid (see what i did there?) enough to support netting?
You have to be careful with dimension lumber not to get confused between nominal and actual sizes. I'm pretty sure that an 8" batt is meant to go inside a 2x8 bay which would be 7 1/4" deep.
Isn't the whole point of the Bonfiglioli wall that it uses single studs and gives the performance of double studs at lower cost? Doing the Bonfiglioli technique on a double-stud wall sounds like overkill. With double-stud, almost always the most cost-effective way to get a higher r-value is to put the two studs further apart and put more insulation in between.
I’ve always thought of bonfiglioli strips as just a thermal break. The strips help to get your whole-wall R value up closer to the R value of the batts themselves so that the studs don’t rob you of insulating performance.
Bonfiglioli strips alone won’t be able to get up to the performance levels you get with double stud walks (or exterior rigid foam). Both double stud walls and exterior rigid foam let you get whole-wall R values up above what you could get with batts (or dense pack cellulose or anything else) in a single wall thickness stud cavity.
Bill
They list batts at 7.25" thick and 8" thick.
https://www.rockwool.com/products/comfortbatt/?selectedCat=comfortbatt%C2%AE%20downloads#Specifications&Sizes
I like the idea of the inner strip to break the thermal bridge but i'm not a fan of 3/4" wide strip on the inside.
If you don't want foam strips on the inside, you can always put it on the outside. Provided your walls are braced on the inside, you don't need the exterior sheathing directly attached to the studs.
You can go with 2x4 walls with 2" foam strips (or R9 ZIPr ripped to 1.5") and still be able to nail the plywood up with 3.5" nails.
With R23 batts, this would get you roughly an R22 wall and keep it within a standard wall depth so you can use standard doors and windows.
I've no experience with them, but have you considered a Mooney wall? It seems like it could fit most of your criteria...?
That, or as Bill suggested, simply offsetting the studs on a double-wall? That appears simple (no foam strip detailing) and is a decent thermal break. Bonus is you can layout outside wall to sheathing and inside wall to drywall.
I've looked at both and the Bonfiglioli wall does a better job of addressing the thermal bridge than either of them. Really, the wall I proposed is nothing more than the Bonfiglioli wall except it substitutes 2x4 for the 1x2's.
It's always interesting to compare likenesses of assemblies. I.e. this is 'like this' save for 'this'. The build details are oftentimes where the differences become apparent. Your wall doesn't sound like a bad idea...
I might argue it's more in likeness to a double stud wall with foam between framing (assuming you're building it out with plates, etc) but that doesn't really matter other than to change the perspective it gives on the methodology. It feels weird to put foam strips in a double stud wall to me, but phrasing it as Bonfiglioli with larger wood perhaps feels less weird...
When you compare the thermal bridging of offset double stud to the 2" of foam sandwich are you using: 3.5"wood>2" foam> 3.5" wood, vs: 3.5" wood>3.5" roxul?