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New Home in NW Ohio

Post in 'The Green Room' started by luckydog, Jan 12, 2012.

  1. luckydog

    luckydog New Member

    Joined:
    Jan 11, 2012
    Messages:
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    Loc:
    NW Ohio
    Hello. I am planning to build a new home this coming spring located in northwest Ohio. I have been researching the various techniques used for exterior walls, and have come here with some questions. I've narrowed my decision down to either a double stud wall (or a variation of the Larsen Truss) or a 6" or 8" flash and fill wall with the Mooney technique incorporated. During our winter season we can see extended temps in the low single digits and in the summer we can see extended periods in the 90's, so I want to build an efficient home for all seasons. The house will face south so we will also incorporate some passive solar techniques in building the house. Whether I use a double stud wall or Mooney wall, I plan to do the extra framing myself and not pay a contractor to do it (i am a union carpenter). We're contemplating using geothermal or radiant heat in the floor, but thats another question. I plan to use some sort of cultured stone on the exterior of the building.

    Questions:
    When building with a double stud wall using spray foam, like the one showed in building science.com's high r-value wall #10, is it wise to place a vapor barrier on the exterior side of the interior wall (as they show) if you spray the exterior wall with 2" of foam? For starters, it sure would be difficult to place the plastic on the exterior side of the interior wall, and secondly I thought I read somewhere that you did not want to incorporate 2 vapor barriers (the spray foam acting as one). This detail also shows densglass or fiberboard as the exterior sheathing. Why not use osb as exterior sheathing?

    When using a flash and fill wall, would 8" studs be a better option than 6", or just overkill? The Mooney technique seems like a very easy thing to do, however, if your going to spend the money to do a Mooney wall it almost seems like the same amount of wood could be used to erect a double stud wall....

    In your opinions, are both of these techniques overkill? Would a 6" flash and fill wall be sufficient?

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  2. DickRussell

    DickRussell Member

    Joined:
    Mar 1, 2011
    Messages:
    85
    Loc:
    central NH
    lucky:
    On the subject of radiant floor heat in a superinsulated house, I think it is a lot of expense and may not give you what you hope to get, if that is a "warm toes" feeling underfoot. This is especially true if the floor is a slab. A highly conductive material, like concrete, won't feel warm to bare feet until it gets into the upper 70s (Fahrenheit). If you run the slab at that temperature in a superinsulated house, it takes very little time for the whole room to go to that temperature, which will feel stifling. In conventional construction, where comparatively speaking the house is losing heat like a sieve, you can have the warm piggies feeling. In a superinsulated house you'd have to modulate the slab temperature down to the low 70s and keep your slippers on or just carpet the floor.

    On the cultured stone outside, follow any of buildingscience's recommendations on installation. Without that exterior foam,any maybe even with it, you'd want to have a mesh of some sort behind the stone to provide a ventilation and drainage gap. Brick and most such materials are "reservoir cladding" in that they can soak up a lot of rain in wet weather. When the sun comes out that moisture can be driven inward and create moisture problems within the wall cavity. The foam will provide a vapor retarder to slow down that inward vapor drive. Still, you probably don't want a sheet of polyethylene near the inside wall, as that will prevent inward drying completely. Except in parts of Canada, code doesn't require a poly vapor barrier, only a 1-perm vapor retarder, and then only in climate zones 5 and 6 I think it is. That can be achieved with a vapor retarder primer paint on the drywall. If you want a sheet material as part of your air barrier system, you can use Certainteed's "MemBrain," a so-called "smart vapor retarder." The permeability of that stuff goes way up when the air next to it is humid, so that you can get drying to the inside when needed. In general, while you need a good air barrier, to prevent air leakage, you don't want water vapor difusion "barriers" (less than 0.1 perm). You want to slow down the diffusion of water through the wall, so as to avoid spikes in moisture content of the wall structure, but you don't want to prevent diffusion entirely, because you want the wall to be able to dry in at least one direction for parts of the year. Double vapor diffusion barriers are indeed frowned on, as they eliminate drying from upset conditions.

    You also can get a good air leakage barrier on the inside by using the "Airtight Drywall Approach" (ADA). That involves the use of sealant or gaskets where the exterior wall sheetrock goes over the framing. You've undoubtedly seen this described elsewhere. You also can use electrical boxes designed to integrate with the air barrier (sheetrock or film material), such as those from Airfoil (http://www.airfoilinc.com/). I used those for my new house and like them. Using that addresses the purpose of having the air barrier on the outer side of the inner wall frame, which is to leave the inner cavity free for wiring penetrations without worrying about air leakage.

    My opinion on framing the exterior walls is what yours seems to be. It's just as easy to add a second stud wall as it is to do a Mooney wall, which I think is aimed more at retrofits of existing construction. That reduces thermal bridging even more and lets you make the insulation cavity whatever you want. To be sure, a 2x6 wall, well air-sealed (however you do that), and with a layer of foam outside the sheathing to reduce thermal bridging, will give you a very good wall. It's just not in the superinsulated class.

    As to use of OSB as exterior sheathing, I used plywood. Either material, when very dry, is a class II vapor retarder. As either absorbs moisture, its permeability goes up, but it does so more for plywood than for OSB. Also, OSB, when it gets wet, does not fare as well as plywood. Advantech does make a sheathing material, though.
  3. luckydog

    luckydog New Member

    Joined:
    Jan 11, 2012
    Messages:
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    Loc:
    NW Ohio
    If I were to use a 6" stud wall and use the flash and fill technique, would I still want a plastic vapor barrier under the sheetrock or would the 2" of spray foam be sufficient if applied correctly? What type and thickness of exterior foam should I be considering?
  4. DickRussell

    DickRussell Member

    Joined:
    Mar 1, 2011
    Messages:
    85
    Loc:
    central NH
    I find a good source of information on stuff like this is the Green Building Advisor. There was a thread on exterior insulation needed to keep sheathing above dew point at: http://www.greenbuildingadvisor.com...lating-minimum-thickness-rigid-foam-sheathing. Do a search over there on "flash and batt" and you'll find other good discussions. As has been pointed out, a flash coat of spray foam against the inside of the sheathing can seal up the sheathing/stud/plate joints, but it doesn't cover other places, such as under the wall plates, rim joist area, and wall/ceiling joints. Those have to be addressed during framing, with gasketing or liberal use of acoustic (non-hardening) sealant, such as from Tremco.

    As far as use of a poly vapor barrier on the inside goes, no, you ought to avoid that, regardless of whether you use spray foam toward the exterior. One nice property of a film like poly is that you can overlap and tape seams with pieces of poly passing behind the ends of interior partitions that meet exterior walls, and do the same where walls and upper level ceilings meet, to establish a very tight air barrier. The problem is that poly is the wrong material to use, except perhaps way up in Canada. Poly has much too low a permeability and prevents any drying to the interior at times. This can be a very real problem in at least two situations. One is if you have a water-retentative cladding such as brick on the outside; when the sun beats on the brick after a rain storm, there can be a tremendous inward vapor drive, and the poly forms a barrier to any subsequent drying to the interior. The other is in summer during periods of very high humidity, with A/C on inside; that too can lead to condensation on the back side of the poly, especially if the thermostat is set too low. For the same reason, vinyl wallpaper should be avoided. If you like the use of a flexible sheet good for establishing an air leakage barrier, you can use MemBrain. It will permit drying to the inside during periods of high humidity yet perform as a class II vapor retarder when the air is dry, as in winter. I used that on our new house for all the above reasons. The alternative is using ADA techniques to establish the air barrier and just vapor retarder primer on the drywall (0.5 perm). That will meet code. Actually, just a couple of coats of latex paint provides sufficiently low permeability as to avoid problems of interior humidity moving too fast into the wall cavity, provided you have first addressed air leakage.

    [Edited 5:29 to add "primer" toward the end]
  5. begreen

    begreen Mooderator Staff Member

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    Messages:
    46,908
    Loc:
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    Dick. Thanks for the informative and detailed posts. I've learned a lot from them. Are you in the industry or just exceptionally well informed and a good explainer?
  6. DickRussell

    DickRussell Member

    Joined:
    Mar 1, 2011
    Messages:
    85
    Loc:
    central NH
    BG, I'm a retired (mostly) chemical engineer. Superinsulated houses is a subject I've been engrossed in on my own since the early 80s. It's been partly hobby and partly obsession. As I got closer to retirement, I spent even more time perusing every bit of information I could find on the subject and following a number of related internet forums. It's amazing what is available for reading. A lot of the info is readily verifiable by simple calculation, and some just makes good sense qualitatively, based on the principles involved. To be sure, there is a lot of misinformation but usually that doesn't stand up to criticism from others on a form well attended by those who do know their stuff. Some of the sites I can recommend are

    www.greenbuildingadvisor.com
    http://forums.jlconline.com/forums/index.php
    www.buildingscience.com

    As a result of my in-depth self-taught education on the subject, I wound up (I believe) much better versed on building science issues than most builders and nearly all other homeowners.

    A couple of years before retirement, we laid out the floor plans for the new house, using software (I used Home Designer Pro, a subset of the full Chief Architect package). In parallel with that, I worked out the exterior shell I wanted to have and did the energy model for it, using just a spreadsheet. Finally, after some of the structural aspects were specified by an engineer to provide drawings for the building inspector, we got underway in May of 2010. We had no real hope of finding a contractor who knew how to build a superinsulated house and get it right, so we just went with someone who understood that the house would be different from anything he had done before and that we would have to work together to get it right. I retired as the project began and was onsite daily. The crew learned a lot about building a house like this, and I learned a lot watching the actual construction. A few things I did myself, some of it working after they left each day and on weekends. It went quite well, I thought. However, I do have to wonder how the typical homeowner ever really gets the energy efficient house he wants, without being as informed and involved as I was.
  7. begreen

    begreen Mooderator Staff Member

    Joined:
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    Messages:
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    Loc:
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    Good to have you aboard, we are kindred souls and haunt the same reference sites. I would love to retire into a project like that.
  8. luckydog

    luckydog New Member

    Joined:
    Jan 11, 2012
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    Loc:
    NW Ohio
    So tell me this.....I'm building a double stud wall that will end up a 12" cavity.....2x4 studs on the outer wall with plywood sheathing and cultured stone/vinyl siding on the exterior. I'm going to spray the sheathing with 2" of closed cell spray foam and fill the rest of the cavity with cellulose, then hang drywall over a 2x3 stud wall on the interior. Do I want a vapor barrier on the inner wall with the closed cell on the exterior wall?
  9. DickRussell

    DickRussell Member

    Joined:
    Mar 1, 2011
    Messages:
    85
    Loc:
    central NH
    No, even without the vapor retardency of the foam on the sheathing, you really ought to avoid a vapor "barrier" (<0.1 perm, such as polyethylene) on the inside. Your building code may require a vapor retarder layer (1 perm or less), but that can be either a vapor retarding primer paint (0.5 perm) or MemBrain (1 perm with dry air, much higher with humid air).

    You still want to do a thorough job of air sealing, more than just where outer wall studs meet the sheathing. Have the crew apply a line of acoustic sealant on the subfloor before they stand up the wall frames. Use it also anywhere there could be leaks at joints between building materials. Make the rim joist area is properly sealed. Use electrical boxes made to facilitate air sealing on exterior walls. If you do a good job of keeping air flow (leaks) out of the exterior walls, you don't have to worry about molecular diffusion, as that is really a slow process. Just a few perms of resistance to diffusion is sufficiently low to avoid problems, without being so low as to prevent slow inward drying during those times you need it. The wood framing and especially that cellulose insulation can safely absorb a lot of moisture at some times and slowly release it at other times, as long as you slow down the rate at which moisture diffuses into the wall, yet no so much as to prevent slow diffusion back out later.

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