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