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Posted by marson on September 10, 2006, 9:36 pm
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:
> marson wrote:
> > 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.
> >
>
> 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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