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Double stud wall assembly

Gerald Blycker | Posted in PassivHaus on

I am building my dream house outside of Seattle in climate zone 4C with 4,615 heating degree days. It will be a net zero energy home built to Passive House standards. A 10 kW solar array will generate all of the energy the house uses with enough left over to power an electric car for 3,000 mile/yr. With solar panels now selling for $1/watt I can buy the panels, inverter, wire and mounts for under $20,000. Even in Seattle’s rainy climate, this system will pay for itself in six years if I install it myself and in 11 years if I hire professionals. But I digress….

My dilemma has to do with the wall assembly. I want to have at least R-45 for the walls and want to stay away from petroleum products (no foam). Here is my plan from exterior to interior.

siding
furring strips for rain screen
WRB
Densglass
2×6 stud with cavity filled with dense-pack cellulose
OSB as air barrier
lots more dense-pack
2×4 stud
gypsum

Why am I using Densglass, OSB and gypsum? Originally, I was thinking of just using OSB on the outside of the 2×6, lots of cellulose, 2×4 and gypsum. I’ve seen this assembly described in several Fine Homebuilding articles and on GBA but a Passive House builder told me that I would have water issues with the OSB because it would drop below the dew point and water would condense on it. I pointed out to him that practically every house uses OSB as the exterior sheathing and asked why they don’t have condensation issues. He told me that the standard code-built house is losing so much heat that it keeps the exterior OSB above the dew point or when water does condense, the heat leaving the house is able to dry out the OSB.

So, here’s the rationale for my assembly. My goal is to allow the wall to dry to both the inside and outside. The Densglass has a much higher perm rating than OSB to allow drying to the exterior. About 1/3 of my insulation is outside of the OSB to keep it warm (above the dew point). The OSB acts as my shear wall and my air barrier. With the air barrier protected in the middle of the wall, it will never get punctured. The space between the walls eliminated thermal bridging and allows me to create as high R value as I want. Yes, I realize the diminishing returns of adding more insulation, but space and cellulose is cheap. In a world with a perfect insulator (no heat transfer) I would be able to heat the house in October, turn the heating system off and it would stay warm all winter. I want to get as close to this point as possible, while still being practical.

I’ve modeled my assembly with a free version of WUFI and it seems to perform well. WUFI is software developed by the DOE Oak Ridge National Laboratory that allows you to model the heat and moisture transport through building assemblies and creates a video showing how the temperature and moisture content in various wall components changes throughout the year. Just out of curiosity, I ran another model for the wall assembly with OSB on the exterior. Sure enough, WUFI showed the water content in the OSB and entire wall rising throughout the winter, drying a bit in the summer, but not enough to lose all the water it gained. Each year, it gained more and more water showing a wall that is doomed to rot.

There are advantages and disadvantages of every wall assembly, but is mine crazy? One drawback is that the insulation crew needs to come out twice, but that would also be true if I used TJI’s as Larsen Trusses (which are more expensive and still have a thermal bridge).

I would appreciate any feedback or suggestions on a better wall assembly.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Gerald,
    Your proposed wall assembly should perform well.

    The GBA website has many articles on the topics you bring up. Here is one article that you may be interested in reading: How Risky Is Cold OSB Wall Sheathing?

  2. Floris Keverling Buisman | | #2

    Cellulose is affordable per R and so are 2x4's.

    Using cement sheathing is a good idea to enhance the outward drying capacity of a double stud wall assembly. BLDGTYP used this approach and http://www.foursevenfive.com products in their most recent project which is described with their exterior details methods here and this blogpost for interior details.

    You could push it even further and use SOLITEX MENTO PLUS as your WRB and skip the exterior sheathing. See attached detail.

    Guidelines for foam free assemblies can be found in ">this blogpost which go well with our construction details which provide other options (like using a I-joist or larson truss for your exterior insulation cavity). My WUFI modelling has shown that increasing the insulation on the exterior of your OSB will be beneficial to keep the OSB's water content in check. Please feel free to contact me at info (at) foursevenfive.com

  3. Jerry Liebler | | #3

    I've been struggling with double wall design issues for some time. Here are the issues I've found.
    What are you setting the walls on? Which wall bears the roof load? Roof loads need a clear path to the foundation. If the inner wall is the load bearing one ( only one should be load bearing as assuring a load split is near impossible) exterior foundation insulation can be used. If the inner wall is load bearing moving the OSB to it's outer face will both provide needed bracing and put more insulation outside it for better condensation immunity. Don't be tempted by the slight gain in thermal performance offered by staggered studs as the condensation on the sheathing will be worse regardless of where the sheathing is located within the insulation. There will be some load on the non load bearing wall, (it's own weight with cladding etc.) this too needs a clear path to the foundation.

  4. Floris Keverling Buisman | | #4

    It does require some thought to figure out how to make the exterior wall carry its own (and the sidings) weight. An I-joist bolted into the 2x4 interior wall or a Larson truss can take care of that. If an free standing 2x4 wall is desired, then you will have to resort to either extending the floor to carry that wall, example of double stud foundation detail (thermal bridge free/airsealed)

    or use a steel angle (which will cause a small thermal bridge if there is a conditioned basement - not a problem with a crawlspace (see BLDGTYP detail at bottom of blogpost

  5. Jerry Liebler | | #5

    Floris,
    The illustration sure punts on the structural issue of how to support the outer wall
    "Pack joist openings with batt insulation
    brace with blocking"
    The sub floor extending out to the outside is a minor thermal bridge that adds loads of lateral stability. and makes building and erecting the double stud walls much simpler.
    I plan on implementing essentially the illustrated detail but the " blocking" will be a piece of vertical I joist screwed to the rim board/ & floor truss at each floor framing member. and the subfloor will be nailed to both flanges of the I joist This is in effect a Larsen truss pony wall without top or bottom plates. I also plan on putting a strip of FRP on the bottom of the pony wall as a " critter guard".

  6. Albert Rooks | | #6

    Gerald,

    I've seen that assembly built a few times here in Western Washington. The dense glass is a good choice though I like woodfibre better. Regardless of which high perm sheathing you choose, consider "bibbing" the cavities before you blow them. Air spaces from settling are to be avoided. Most installers are not equipped to dense pack blindly.

  7. Jerry Liebler | | #7

    Gerald,
    When you say r 45, do you mean r45 cavity insulation or r 45 whole wall accounting for the wood?

  8. Gerald Blycker | | #8

    I was referring to R-45 as being the R value of the entire wall. We have not yet finished with the PHPP analysis though, so I might change the R value if needed to meet Passive House standards. As far as I'm concerned, the more insulation, the better.

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