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 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

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.


"marson" wrote in message
ups.com...
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.

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

If you want super insulation at a cheaper price then double wall look a
SIPs structural insulated panel, you can get R 40 in a wall, double
wall might get to only R 28-30

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

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:

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

You say "they" are not interested in SIPs, probably from lack of
experiance is why. Its your building, your utility bill, research it, it
will be cheaper, quicker- easier to build, and more energy efficient.
Maybe you need a new "they"

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:

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?

"marson" wrote in message
ups.com...
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.


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/desig...rk/default.htm


Steve.

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

On 10 Sep 2006 14:22:12 -0700, "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?

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.


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.

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.


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.

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.

Hi,
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.

"marson" wrote in message
ups.com...


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?


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

marson wrote:
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".

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.


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.

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.


i brought up the idea of exterior foam, but they wanted to go with the
double wall. we'll see how it goes.

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



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

M&S wrote:

marson wrote:

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".


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.


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.


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.


i brought up the idea of exterior foam, but they wanted to go with the
double wall. we'll see how it goes.


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



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





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