Reversed Arctic Wall in Zone 7
To all,
In my looking for the best wall for my new build this summer; 1300 s.f., single story, Zone 7 far northern Minnesota 9,000- almost 11,000 HDD, 70 to 80s kind of humid summers and very cold (-30 to -40F) winters.
I am looking at the REMOTE wall, but recently I came across a site called Root River House. As I see it the house uses what I describe as a “reversed Arctic Wall” insulating technique using the Larsen Truss on the inside of the structure vs the outside. This intrigues me since it uses no foam. Since this is new construction I can adjust the build for smaller rooms.
The owner was nice enough to email me and answer a few basic questions. He reports he has had no issues with moisture in the walls during the 2 years he’s lived there. He lives in Zone 5-6. Not as cold as where I am going to be.
Has anyone ever tried the insulating technique using the Larsen Truss on the inside of the structure vs the outside? Looking to gain R30 to R40 in the walls.
Thanks!
Steve
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Replies
Steve,
Rather than being a "reversed arctic wall", I think it could be more fairly described as double wall construction where the outer wall is load-bearing and the builder has added braces at mid-span because he is worried about bowing.
There are countless variants on double walls, none of which use foam. That is probably your most fruitful line of enquiry.
Here is a link to a project by a contributor here using similar techniques:
http://www.kaplanthompson.com/_images/publications/09.06-jlc.pdf
Several other active posters, like Stephen Sheehy, have also used it on their builds.
I'm posting an image of the Root River House wall assembly to help GBA readers understand the type of double-stud wall that Steve is talking about.
The wall assembly is discussed on this page: Why use a Larsen truss wall?
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Steve,
For more information on double-stud walls, see:
GBA Encyclopedia: Double-Stud Walls
For more information on Larsen trusses, see:
All About Larsen Trusses
For more information on the Klingenberg wall, see:
The Klingenberg Wall
Thanks Martin,
The link you posted show an interesting disparity between what they originally intended and what they built. The drawing shows what I'd characterize as Larson Trusses, but in the photos of the walls, they have abandoned the cleats except for one in the middle of the studs.
I'm belabouring the point because I think it helps to conceptualize what they ended up with as a frame wall with all the components you would expect (top and bottom plates, studs, etc) as opposed to the individual trusses with two chords separated by plywood webs you generally associate with Larson Trusses. As far as I can see the only reason the vestigial cleats remain at mid- span is that the inner wall is framed from 2"x3"s. I'd suggest Steve move to a conventional 2"x4" framing and eliminate the cleats altogether - unless the walls are much taller than usual.
The difference I'm seeing between the "double stud wall" I've seen on the Internet and this is; on the root river home they incorporate a layer of OSB sheathing between the two walls. The double stud walls don't seem to have this. Am I correct?
Wondering if having the larger amount (9.5") of insulation on the interior of this OSB layer will cause moisture problems further out in the wall. The owner claims it has not bee a problem.
Steve
Steve,
For more information on double-stud walls with OSB in the middle, see these two articles:
Lstiburek’s Ideal Double-Stud Wall Design
The Klingenberg Wall
I will also be building a new home in Northern MN Zone 7 summer 2016. I have pretty much decided on exterior foam over double wall. I just don't quite feel comfortable with how cold my sheathing would get in the double wall, and where the dew point would be in certain situations (somewhere in my blown in insulation). I guess I could be swayed once I get my cost comparison.
Steve Vigoren,
As your fluffy insulation gets thicker, the amount of foam you need to avoid moisture problems also goes up. A lot of cellulose with a little foam can make the sheathing damper, not dryer. If you just want the sheathing a little warmer, it's safer to use mineral wool board on the outside, or to put the OSB in the middle as in the articles Martin links.
Response to Charlie, 2x6 wall with OSB sheathing and two layers of 2" foam on the outside, 1x4 cladding and horizontal siding. MN Code calls for R15 foam with 2x4 walls and R20 with 2x6 walls. I might even go with fiberglass between the studs as I can DIY it. Not a lot of super insulating going on in my area, even blowing insulation would have to be DIY I believe. I don't have an issue with loose blowing the attic, but packing the walls might pose a challenge.
Update on wall idea: This week I met with the designers of the Root River house. The wall they are suggesting is:
12" of Cellulose (R-40) wall. Bilt Rite on the exterior, then tar paper, then 3/4" furring, then cedar siding. The interior of the wall is covered with Intello, then interior finish.
The idea is to insure the wall breathes in both directions. Also the house was reduced to 1120 sq ft.one level.
Thoughts?
Steve,
I assume that you are talking about Bildrite (not "Bilt Rite"), a brand of fiberboard sheathing.
Your plan will work, as long as you are aware that fiberboard sheathing can belly outwards when the wall cavities are filled with dense-packed cellulose or blown-in fiberglass.
There are two ways to deal with the bellying problem: (a) brute force (forcing the bellies back and hoping they stay that way) or (b) two layers of furring strips (horizontal and vertical).
For more information, see Wall Sheathing Options.
Martin,
Yes, Bildrite is the brand of fiberboard we are using. They manufacture it about 2 hours away and it's readily available.
My insulation contractor is aware of this bowing issue. He was the one who brought it to my attention. We thought of using a more rigid type of wood siding than cedar, but it was cost prohibitive. He says if the insulation is blown in properly it shouldn't be an issue.
As a preventive measure, the cedar siding will be installed over furring strip, and bildrite before insulation begins. The furring strips should hold the fiberboard against the studs, plus it give the needed airspace for drying. some. We are going to use mesh on one side of the individual trusses to create a cavity for the insulation. This will hold the insulation horizontally and reduce pressure. The combination of this and the gap behind the siding should allow for and bowing.
Key to making that stackup work is a high permeance exterior sheathing. A more rigid (but more expensive) solution than fiberboard would be 3/4" exterior grade gypsum board such as Georgia Pacific's DensGlass, and mount the siding to furring, not the sheathing.
Mounting the siding to furring is also a critical detail to allow drying toward the exterior. Cedar clapboards tight to fiberboard sheathing could be problematic, since clapboards are nowhere near as inherently back-ventilated as shingles, and even that might not be enough. With 1x4 furring through-screwed to the framing with 3" pancake head timber screws 24" o.c. it would be fine to hang the siding on the furring. If the fiberboard bows a bit over time it's still not a problem.
The OSB sheathing interior to the 2x6 is also an important layer in the stack up. In the detail drawing it's labeled "air barrier", but it's functions also as a "smart" vapor retarder. Ideally at least 40% of the total R would be on the exterior side of the stackup in a Zone 7 climate, which would ensure that wintertime moisture doesn't accumulate in the OSB. With the fiberboard sheathing, cellulose cavity fill both sides and the rainscreen you can probably cheat that by quite a bit, but if you have choice, 40% would be very safe. With 5.5" of celluose on the exterior and 6.5" on the interior to get to the foot of cellulose that's about 46", which means there's some margin.
Response to 13:
Yes the siding will be mounted to furring strips for the reasons you've listed. We feel the bowing will be kept to a minimum with monitoring and proper installation and thus the more expensive materials won't be needed.
Regarding the use of OSB, the plan actually calls for the elimination of that middle OSB barrier shown in number 2. The feeling is any wood barrier...even a semi-permeable OSB barrier is a restriction to free movement of vapor laden air. The continuous air barrier of Intello on the interior behind 12" of insulation should stop air infiltration. Plus we are eliminating the cost almost $600 of OSB plus the labor to install it.
You actively WANT to build "...a restriction to free movement of vapor laden air..." into the wall structure, since that's the major problem. By putting that air barrier in the middle of the assembly it's well protected from damage during construction or later, and won't have 1001 penetrations from plumbing & electrical to meticulously air seal.
Putting an OSB layer in the middle also makes it easier to get the dense-packing right. With foot-thick cavities can be difficult to get consistent density and sufficiently high density to eliminate settling over time. It's a lot easier to get that right in 5.5- 7.5" thick cavities. It's not impossible to do 12", but it takes a dedicated contractor with experience doing walls that fat.
The stackup posted in Martin in response #2 shows no vapor retarders on the interior. If you're building with a smart vapor retarder like Intello Plus it will be safe to skip the OSB. Otherwise you'd have to detail the wallboard as an air barrier, and paint it with a vapor retardent latex primer to bring it's vapor permeance down. But putting the air barrier & vapor retarder on the interior there is more air sealing detail to get right, and it's integrity will usually degrade over decades of use from picture hanging nails, etc. The cellulose helps redstribute the moisture loads from those defects, but it's easier to keep internal OSB layers defect free.
Intello Plus isn't exactly cheap either. If the difference in cost of skipping the Intello and installing an OSB layer detailed as an air barrier instead is under $1500, I'd personally view it as cheap insurance to go with the OSB, which offers better assurance on the insulation installation quality, and is more robust as an air barrier over time. Clearly YMMV.