double wall house construction details

i'm about to embark on a project involving building a superinsulated house with double 2x4 wall construction. The walls will be 9" thick and will be blown with dense pack cellulose. i envision the vb going on the warm side of the inner wall. i live in a very cold climate. i have several questions:
which wall is load bearing? i planned on making the outer wall load bearing, in order to shell up the structure, and adding a second wall inside after the building is dried in. but what little info i have been able to find talks about making the inside wall load bearing. this is from the canadians. anybody know why you would make the inner wall load bearing?
where the wall has roof trusses resting on it, is fire blocking required at the top of the wall? I'm thinking not because it is full of insulation. i'll ask my inspector, but wonder what you guys think.
anyone know of a website with some construction details? how to handle the windows has me in a quandry. i'm thinking of oversizing the ro's by an inch, and making a box of 1/2 inch plywood inside them (the window would then sit inside this box). this would allow me to foam the window to the box, and then caulk the vb to the box. trouble is, nailing off the window into the edge of this plywood box may be problematic. i'm also wondering if there would be value in putting a 9" rip of plywood on the top of the outer wall, expecially on the 2nd story to prevent convection from the wall cavity into the attic.
if anyone's been there done that, i'd appreciate some tips. also, please don't waste your time telling me i'm stupid for going double wall--the architect and owner came up with this...i just build em.
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i would use closed cell foam sprayed in place, it expands and would better fill all the gaps.
frankly i think the double wall is a excellent idea, but do install a air to air heat exchanger to bring fresh air in the building
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I have built a few of these. Bearing wall is outside you want it to sit right over the foundation. You just duplicate the window/door openings with the inside wall. The ply wood (or 1x10) is then used just like a regular jamb. Standard construction practices such as sill caulking are just duplicated for each wall. Yes, put blocking between the two walls at the top to seal wall cavity from attic. This is not only good for air infiltration but also for rodent control.

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marson wrote:

I wish I could remember the publication but I read an article about a similar home in NH. I believe it had 10" walls with FG but I cant seem to put my hands on it.
I would think your idea would be a bit counter productive to the super insulated home in that the box itself (ply) is a thermal conductor connected directly to the outside. This would conduct cold directly from the exterior wall into the super insulated window box you build. Moisture/condensation? Just off the top of my head it would seem an outside wall framed conventionally and the inner wall framed to accept a box of rigid foam rather than ply may be better with respect to conductivity. Installing nailers (as small as possible) between the two walls around the framed opening(s)would allow for fastening the foam and finish material with less thermal conductivity than a continuous ply box.
As a side note even though you said not to bother, why has the architect/homeowner not considered other options or provided you with these details directly? The main goal of double walled construction is focused. It is to try to completely eliminate thermal conduction via the studs and _then_ to carry more insulation. Simply carrying more insulation is not the reason to frame double walled. High R values can be achieved much more easily in other ways. To fully achieve this goal the issues of conductivity have to be carried out throughout the entire frame minimizing connections and conductivity between the two walls. Without doing so they are just wasting materials and resources. Sure, it will have more insulation, but if your going to do it, do it.
The wall they are talking about will result in what, an R30-R35 or there abouts at 2.5 to 3 times the cost or more? They could achieve the same R value with 2x6's 16" o/c and then two inches of ISO applied directly to the outside. The only place you would have R 26.5 (or so) would be at the corners where you would need ply. If they used 1/2" sheathing at the corners, the field could be wrapped with a layer of 1/2" ISO, followed by a complete wrap of 2" ISO resulting in an R38. This can be pushed even farther by stack framing 2x6's 24" o/c eliminating even more conductors (and $$). They could also eliminate the sheathed ply corners with Z bar or let-in, etc.. There was a great article in FHB a while back about this practice. A search of Tauntons site would probably pull it up.
Even with ISO prices where they are (about $1.10/sf for 2" around here) it would still be far cheaper than building double walled with the additional framing, trim, and related details. If the architect isnt addressing all of these conductivity issues to you directly with specific construction details he/she is probably defeating the very purpose they are trying to achieve.
Mark
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i like your idea of small nailers, though the problem i am trying to solve is how to seal the air/vapor barrier to the window. if i had my druthers, i would get a 10" jamb factory attached to the window, and tremco the vb to the window jamb, but unfortunately, the owners want sheetrock jambs. but i will think about using a 1/2" xps box instead of plywood. i think nailing the window fins in will be an issue. i am using a 3/4" rain screen detail--maybe i could hang the window nailers into the window opening by 1/2".
i'm not that concerned about thermal bridging around the window--after all the window is one big thermal bridge and in fact is only r-5. i guess if you really want a super insulated house, you'd get rid of the windows.
i brought up the idea of exterior foam, but they wanted to go with the double wall. we'll see how it goes.
M&S wrote:

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marson wrote:

My thoughts were to install the window in the outer wall RO just as usual and then frame your inner wall RO larger by the thickness of the sheetrock and ISO. Run your nailers out to the outer wall/window and simply treat the ISO as you would an extension jamb. You could seal the ISO to the window/framing (it may hit the framing once you allow for the sheetrock) with a 1/4" bead of touch n' seal or similar gun foam. After that you would just sheetrock and you could additionally caulk your J bead to the window.
This way your window sits in a conventional RO rather than one lined with ISO or ply. Your ISO box would simply be like a set of extension jambs.
The only other option I could think of would be to sh*t can the nailing fins and fasten through the jambs which may be your best option depending on your outer trim details and the rain screen.
Just for informations sake what rain screen are you using and why? We just finished a job using Colbond 3611R.

Yeah. These are the details that I was mentioning/wondering about. There is far more to doing it right than simply making a larger cavity for more insulation.

Hopefully for your sake it doesnt become a nightmare. My $.02 on it would be if the architecht and homeowner want to make bad decisions make sure your hands are clean and they know full well that you are simply following THEIR orders.
Would love to hear about it as it progresses, Mark

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M&S wrote:

Just as I sent that last post I thought of another option. You could frame your outer wall RO normally then run a 1/2" rabbeting bit around the inner edge of the RO. This would create a 1/2" rabbet which would neatly accept a sheet of 1/2" ISO. You could install your nailers to the edge of those rabbets and frame your inner RO accordingly. The only tricky part about this one is you would then need to very accurately set your windows in the RO as you would have to install some furing strips over the ISO to bump out the sheetrock so it hits the window correctly.
As for the vb, the ISO is a VB so sealing it to the window framing and then sealing your VB to the iso (tape/caulk) at the inner wall would be easy.
Mark
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marson wrote:

Have you/they thought about using a 2x10 floor plate & header with staggered 2x4s at 16" on centre? The end result gives a 9 1/2" cavity for insulation, while cutting the number of 2x4 studs roughly in half. In theory there is less heat leakage through the wall, however there is a solid header/floorplate to transmit the heat. I would suspect the overall heat loss would be slightly lower, but I can't give you any numbers.
<snip>
--
Carolyn Marenger


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carolyn wrote:

studs, not halve them. Even if using the same amount of studs, you'll have to place them 32" on center. Using half the amount of studs, you'll be 64" on center. Draw it out on paper to see.

--
Bill
In Hamptonburgh, NY
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carolyn wrote:

i did think of that. the problem i see is in building it. building/sheathing/tyvecking/strapping a wall flat on the deck is a great labor saver. that would be tough with 2x10 plates.
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marson wrote:

Consider putting the vb on the cold side of the inner wall. General recommendation has been (for a cold climate) to have 2/3 of the insulation on the cold side of the vb, so you should be ok. That would help with the matter of electrical and plumbing folks leaving all sorts of penetrations in the vb. They would have the inner wall space to do their work. Afterward, you can use 3.5" FG batts there before the sheetrock goes on. I wouldn't use FG in the bigger cavity, because especially as the wall gets thicker the density difference in the air next to the sheathing on a cold day vs. the warm air next to the inside is substantial, setting up small air currents within the FG batt and hurting its effectiveness, unless it is more dense. The blown cellulose, to 3 lb/cu.ft density, is said not to suffer from this problem.
I assume you will have something like XPS covering the foundation wall on a house like this. What will you be doing on the outer wall so that the siding goes over the outside of the XPS?
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Building Sciences uses a foam sheathed building for a Very Cold Climate but you might find some other useful info from their studies.
http://www.buildingscience.com/designsthatwork/default.htm
Steve.
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dude... steel comes with 8" exterior load-bearing walls. My company fabs snow and wind loads into the steel itself and then sends it to you without any cutting or welding on-site. Just bolts together. You can finishi out the home any way you want, but keep in mind, since none of the internal walls are load-bearing, one can move around internal partitions easy-as-pie. If you are looking for better insulation, wouldn't it make sense to build out of a non-purous material?
JG
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If you're adding insulation on an existing structure, adding on the outside means you're sacrificing less living space, and you don't have to mess with the utilities. The only reason I can think of to make the inner wall load-bearing on new construction is if the 12" of span it buys you means you can use smaller lumber for the joists and rafters.

The people who do straw-bale construction have a bunch of different strategies for doing deep windows, I'd be tempted to flare the opening between 15 and 30 degrees on the sides and top, and choose/build windows with internal insulated shutters.

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marson wrote:

I live in Alberta(pretty cold in winter) My house is R2000 spec'd. It's air tight and I have to exchange inside/outside air to breathe. Still this house does not have double wall. Where is your location? You have to talk to certified energy efficient house builder in your area. And get an inspection likewise who is conversant with the subject. By building a house like this I got some rebate from government.
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In the UK pretty much all of our houses are double skin. Either masonry/masonry or stud/masonry. Around the early 1900's people realised that by providing a gap down the wall the building became more resistant to the ingress of water. In effect the external leaf is a weatherproofing layer with the internal leaf providing the structure. If the external wall were to be loadbearing then all the joists would need to pass through the cavity to gain support increasing the risk of moisture ingress and damage and at the same time increasing cold bridging.
Makes sense to me.
Darryl
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