Bearing wall connection at the foundation in superinsulated wall assemblies
I am in the planning stages for a home that I intend to build on a property in northern Maine (climate zone 7.) I see that there are a good number of discussions here regarding the best types of wall assemblies for very cold, zone 7 climates, and even specifically northern Maine…so I am grateful for all of the very beneficial ideas here, and will eventually likely settle on either a double wall system, or a system with exterior rigid foam insulation. I’m leaning towards a double wall system, super-insulated with compact-fill blown insulation.
Regardless though, I’m wondering about the connection of such a thick, super insulated wall assembly at the foundation, and how it is supported…as it seems that having the required structural sheathing on the inner wall of a double wall system would be best from the perspective of vapor condensation plane location…making the inner wall of the double wall system the logical choice to be the bearing wall.
However, in a doubled 2×4 wall assembly, at least 12” thick in total, erected over a standard 8” concrete foundation wall, it would seem to me that the exterior wall would have to be the bearing wall, so that weight is transferred through the wall to the foundation…requiring structural sheathing to be placed on that wall, rather than on the inner, non-bearing wall. The outer face of the inner wall would at best be aligned with the very inner edge of an 8” foundation wall. Whether joists rest on a mud plate or are hung with brackets from the inner face of the foundation wall, the inner wall will not bear directly over the foundation wall.
So…my question is:
A. Does the exterior wall in a double wall assembly of necessity have to be the bearing wall, and if not, what techniques are employed in either the construction of the wall or the foundation, to transfer weight through an inner bearing wall to the foundation?
B. In a wall assembly with a single stud bearing wall set towards the outside edge of a foundation wall, with multiple layers of rigid foam outside the sheathing, proud of the outer edge of the foundation, what support is provided for the additional material, if any, other than the fasteners used to attach it?
Thanks in advance for your comments.
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Replies
"in a doubled 2×4 wall assembly, at least 12” thick in total, erected over a standard 8” concrete foundation wall, it would seem to me that the exterior wall would have to be the bearing wall, so that weight is transferred through the wall to the foundation"
I'm guessing you think this because you're picturing the outer wall being flush with the outside of the foundation, which would put the inner wall cantilevered to the interior. Instead picture the inner wall fully on the foundation, with the outer wall cantilevered outward.
Do a search for Larsen trusses and Klinginberg walls for visual examples of how this works.
Trevor, thanks for the reply. Yes, that is precisely what I’m envisioning, the outer wall against the outer edge of the foundation wall, and the inner edge cantilevered (well, at least in a strict physics sense any weight bearing on the inner wall would be cantilevered relative to the foundation wall; but the wall itself obviously would be resting on and supported by the floor assembly.)
Yes, I’ve looked at Larsen trusses, but I’ve not heard of Klingenberg walls; I’ll research them now. With truss system, you’re not really building a true double stud wall. You’re basically just firring out much further than normal, to accommodate additional thickness of insulation, basically cantilevering the exterior plane out beyond the foundation wall. So how is that supported? Simply with the fasteners that connect it to the inner bearing wall?
Also, perhaps tangential but related (I just sit here thinking about these things, and doing internet research to try to figure it out) - I can’t quite wrap my head around how a thick or double wall assembly, super insulated solely with fiber/cellulose/Roxul, can properly dry, even with the air barrier condensation placed an appropriate distance within the wall, to keep vapor from condensing on the structural sheathing. At some point as the vapor continues traveling through the wall assembly, it WILL indeed condense. And if it condenses in any sort of loose or compact fiber/cellulose/Roxul insulation, won’t it just become waterlogged, heavy, compacted, thermal performance degraded...not to mention if it eventually comes into contact with other wooden components of the floor/wall/roof system? How does it dry completely to the outside? I can wrap my head around how multiple exterior rigid foam insulation layers work; the vapor would condense at some point while traveling through them, would fall back or down until it hit a drainage pane, and would then be directed out and away from the structure. But how could a loose fill insulation possibly work??
"You’re basically just firring out much further than normal, to accommodate additional thickness of insulation, basically cantilevering the exterior plane out beyond the foundation wall. So how is that supported? Simply with the fasteners that connect it to the inner bearing wall?"
As far as I know, yes.
"At some point as the vapor continues traveling through the wall assembly, it WILL indeed condense."
I'm pretty sure this is not true, but I'll let someone with a better grasp of the science explain why. But consider this, you have water vapor in your house all the time. Why is it not condensing on the interior walls? It will only condense if it hits a surface that is below the dew point temperature, and I think this holds true within the wall assembly.
"How does it dry completely to the outside?"
The outer sheathing should be vapor permeable. That's where the drying occurs. Anything inside the vapor barrier needs to dry to the inside. If you have your interior sheathing as your vapor barrier, you probably won't have any insulation on the inside of it.
> At some point as the vapor continues traveling through the wall assembly, it WILL indeed condense.
Of course (with either all fiber fill or with external foam). The question is if it does any harm.
> And if it condenses in any sort of loose or compact fiber/cellulose/Roxul insulation, won’t it just become waterlogged
Small amounts of condensate are harmlessly absorbed and dry later. The trick is to minimize the source of moisture and maximize drying. Things such as great air sealing, smart vapor barriers, plywood (or fiberboard) and rain-screen gaps help. Cellulose helps because it absorbs/distributes well, kills mold and reduces air flow.
Also...in addition to being very cold (climate zone 7), my property is in a quite humid area, averaging about 70-75% humidity, nearly year round, and it sees above average precipitation, both rain and snow. Does this change any of the basic principles of double wall/super insulated construction for me, such as the type or placement of membranes within the wall or roof assemblies?
Your climate zone can definitely affect your requirements. However, there isn't really such a thing as a humid, cold climate. Cold air just can't hold much moisture. For example, at -4F and 95% RH, there is the same moisture in the air as there is when it's 68F and 5%. So 75% RH at typical winter temperatures in zone 7 is actually very, very dry.
*Bing!*
That’s the sound of the proverbial light bulb coming on haha.
Thanks for the explanation of cold climate humidity...it seems so obvious now; I guess that’s why they call it *relative* humidity. ;)
Mr. Simmerman,
There are some important issues regarding moisture control in thick walls. For more insights than I can quickly provide here, try searching for threads on GBA and articles at Building Sciences. The short version of a potential issue for double walls, is moisture load ramping over several seasons.
A builder in Massachusetts (I believe ) monitored moisture levels in the sheathing of a double walled home which showed fairly high moisture content in exterior sheathing during the cold months. Fairly normal and the reason one assures adequate vapor relief to the outside world. A potential problem was discussed (or discovered) when readings were accumulated over additional years. Summer humidity levels were high enough to prevent adequate drying of the sheathing before the next cold cycle ramped up the moisture load again. Successive years of this cycle seem to push median moisture levels up as the sheathing could not dry to a high local humidity condition.
If I recall correctly, not every double wall house in every environment behaves this way. I would suspect that combinations of different styles of rain screen gapped siding and local weather variations could profoundly affect results. Despite living in a very favorably dry climate I opted for the "outsulation" approach with moisture control to the inside. This also puts all my sheathing and framing safely inside any condensation limit short of an indoor pool room. Which by the way is sort of the reverse scenario.
The concept of "drying" one way or the other and all the vapor rate discussions should be tempered with a visit to one of Martin's discussions of the magic air arrows so freely used in ventilation diagrams and elsewhere. Vapor diffusion is painfully slow compared to active ventilation when dealing with moisture loads. Imagine a swimming pool of clear water, then add a bottle of ink in at one end. The dense ink color will stay concentrated for quite some time and it might be a day or more before the whole pool has a tint. Add the same bottle of ink to a fountain and the fountain will be tinted evenly in minutes.
An inadequate air exchange situation for sheathing is one of the most easily achieved conditions. Stagnant air will load up with humidity and equalize its moisture capacity with the sheathing and then stop taking up more. Highly humid air can still only equalize the moisture load to some point relative to the sheathing that may or may not actually get to a "dry" condition for the sheathing. This is why the ramping effect can happen.
Obviously not every double wall house runs the risk and many contributors I'm sure will have data. However, you said you are building in a coastal environment which, even in the northeast, may increase your risk factor. Careful detailing of the siding would seem to be an important concern for your design conditions.
Aside from these concerns, be sure to investigate the ability of your insulation contractor to properly pack dense cellulose insulation. Drooping tops and missed fills are a frequent issue and a common complaint on the forum is unprepared or incompetent cellulose installers.
Robert,
You've raised lots of issues here, all of which are discussed at length in various GBA articles.
Briefly:
During the winter, the wall sheathing on a double-stud wall gradually gains moisture. (This isn't condensation -- it's sorption.) If the wall is designed well, the moisture dries to the exterior in April and May. The key to the long-term safety of any wall assembly to to ensure that the rate of drying exceeds the rate of wetting.
If damp sheathing worries you, use exterior rigid foam instead of a double-stud wall.
With a double-stud wall, either the interior wall or the exterior wall can be the bearing wall. That's up to you. If the interior wall is the bearing wall, either Larsen trusses or the Klingenberg approach are possible solutions.
If you like reading, I suggest you start with GBA's "Category" page with links to articles on walls. Here is the link: Walls.
Specifically, see these articles:
"How Risky Is Cold OSB Wall Sheathing?"
"Monitoring Moisture Levels in Double-Stud Walls"
"Two Views of Double-Stud Walls"
"The Return of the Vapor Diffusion Bogeyman"
"Double-Stud Walls"
"Exterior Rigid Foam on Double-Stud Walls Is a No-No"
"Lstiburek’s Ideal Double-Stud Wall Design"
Trevor, Jon, Roger, and now Martin,
Thank you for taking the time to read and reply...frankly I’m amazed at the willingness and helpfulness I’m finding here, even if the questions I ask have been asked and answered and discussed here before, multiple times and in multiple ways. Indeed I’ve read some of the articles and discussions you’ve linked to, Martin, and I will absolutely read the ones that I haven’t seen yet. Thank you!
On a tangent, is there a search feature for the discussion forums that I’m somehow missing? The articles and discussions that I’ve read, I’ve actually arrived at via web searches. I’ve tried just browsing through the forums - and gone down many rabbit holes about interesting topics completely unrelated to the information I’m looking for! ;) - but it doesn’t seem a very efficient way to hone in on a specific topic.
At any rate, thank you all again; your help and advice is very useful and much appreciated!
Robert,
Q. "Is there a search feature for the discussion forums that I’m somehow missing?"
A. There are two avenues to explore: (1) The GBA search box -- note that after your results come up, they can be further refined using the "Search Filters" box that appears in the left margin of the search result page -- and (2) Google.
Admittedly, for some purposes, Google works better than the GBA search box. For example, you might want to try this Google search:
"double stud walls" +"GreenBuildingAdvisor"
Ultimately, the best search box is the brain of an editor who is familiar with GBA's deep inventory of articles.
I've had similar questions as Robert Simmerman regarding the double stud wall and how it bears on the foundation. Especially troubling to me was IRC R404.1.5.2 (2012) that says the concrete foundation wall must be at least the thickness of the wall above. So if my double-studs enclose 11 inches, then it seems my foundation must be 11 inches thick or more.
Unsure, I paid a visit to one of our township plan reviewers (Lancaster Co, PA, climate zone 5 at the transition from zone 4) and showed him some drawings. He didn't seem worried at all about foundation wall thickness. So then I asked about doing a cantilever beyond the foundation edge. In my case I'd been entertaining 2" of XPS on the foundation exterior and thought it might be best if the framed wall cantilevered 2" outward and flush with the XPS. Of the several methods I proposed, the plan reviewer seemed to greatly prefer using a wider sill (image attached). I proposed additional blocking under the double-stud walls but he didn't seem concerned if joists were 16" O/C. I noted (and he agreed) that while this scheme might be OK for joists perpendicular to the foundation, for walls where the joists run parallel, the outer joist is floating in space unsupported by the foundation. Instead support would be provided by blocking resting on top of the foundation and fastened to the "floating" joist and the next joist inward.
Perhaps this shows my inexperience, but the plan reviewer only considered a wall as load-bearing if it carried a floor load or roof load -- I guess 'cuz the dead load of a wall is usually small by comparison.
Dennis,
I interpret that section as saying the foundation wall only has to be as wide as the load-bearing portion of the wall above. If you had a wall where the exterior framing took all the loads, and to satisfy a fanatical owner the wall ended up two feet thick, it makes no sense to require the foundation to be the same width.
One of the chief benefits of making the inner-wall load-bearing is that you can stop the floor framing flush with that wall, and by carrying down the outer-one unimpeded, reduce the thermal bridging, and eliminate the need to air-seal at the rim joist. The detail you show makes both insulation and air sealing a bit involved.
The problem of providing bearing for the inner-wall occurs more on houses using foundation walls and slabs for the main floor, than those with framed floors. A 10" foundation wall will easily accommodate a 12" double-stud wall.
Responding to Malcolm,
Thank you very much for your reply. Your interpretation makes sense to me, that the foundation is there to support bearing walls (supporting roof and floor loads) which says to me that other structures are usually light enough by comparison and thus adequately supported by standard framing practices.
I totally agree that ending the floor on the inner wall is vastly better for thermal bridging and I've thought of doing that. I think the newly attached drawing perhaps portrays the concept. But the question that absolutely plagues me is, if I have a double stud wall with roof trusses on top, which of the stud walls is the bearing wall for the roof? If both walls are the same height and tied together with a top plate, then I'd think the load will be shared between them, although probably not 50-50. So how can I consider the outer wall as the non-bearing wall?
Thanks again.
Dennis,
There are a few variations.
-If the inner-wall is load-bearing you may have to (as Robert is contemplating) move the sheathing to the inner-wall to resist shear, or if it is a two storey house, tie the structure together by extending just the subfloor out to the exterior wall.
- Depending on what insulation you are using, it may make sense to use separate sill-plates.
- The outer-wall might be balloon-framed.
- A variable-perm membrane can be attached to the outer-face of the inner-wall, which leaves it well protected over time.
Having the two exterior walls the same height isn't generally a problem (remember all the interior non-load-bearing walls are the same height too). When the roof load is applied the outer-one will compress immeasurably and the loads will follow the inner-one. the one instance this can be a problem is when the inner-wall has more rims and plates. These shrink more, and can leave the outer-one carrying the load. A simple solution would be to frame the outer-one 3/8" lower.