Building and Construction - Insulating a double-wall: how and when?

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Insulating a double-wall: how and when?

Dick Russell

2006-03-07, 11:21 am

I have a question concerning installation of wall insulation.

In one double wall design for a superinsulated house, the outer wall
bears the roof load, while the inner wall bears the interior ceiling
and floor loads. The vapor barrier is wrapped around the inner wall
frame before it is raised to a vertical position, so that the vapor
barrier is on the cold side of the inner wall, leaving room for
installation of plumbing, wiring, and drywall without making holes in
the vapor barrier. The two wall frames at any level are tied together
above the top plates with 1/2 or 5/16 inch plywood, for support and
firestopping purposes. The exterior sheathing is applied to the
outside of the outer wall, and covered with housewrap and siding. The
cavity between the sheathing and vapor barrier is filled with
insulation, which some prefer to be blown cellulose. Before the drywall
is applied to the warm side of the inner wall studs, the space between
those studs is filled with fiberglass insulation. The finished assembly
has the vapor barrier less than 1/3 the way across the total
insulation, high total R value, and minimal thermal bridging.

Now the question: how and when is the insulation in the cavity between
sheathing and vapor barrier installed? It can't be installed until the
exterior wall is closed and the windows installed, to avoid rain
damage, so that means installation from the inside. While fastening the
vapor barrier to the cold side of the inner wall frame provides a tight
seal and excellent protection against migration of interior moisture
through the insulation in winter, it means blocking off access to the
larger insulation cavity before the insulation is installed. The only
thing I can think of, for the case of a two-story house, is to make
small slits in the vapor barrier just below the top plate, blow in the
insulation, and tape the slits. The same technique would be used to
completely fill the spaces below windows. This kind of insulation is
blown to a certain density, resulting in slight compression, so as to
avoid settling over time. I have to wonder about the pressure of the
insulation against the vapor barrier and whether the vapor barrier will
give too much. I suppose that could be alleviated by supporting the
vapor barrier with some sort of mesh material stretched first over the
inner wall frame.

I think the how and when question wouldn't be there for a single-story
home. In that case, the plywood firestopping connecting the tops of the
inner and outer walls could have predrilled holes. The insulation could
be blown in from the top, accessed from the attic space before the
attic insulation is installed. The holes in the plywood would be
covered after filling the wall cavity. Filling spaces beneath windows
might be awkward from the top plate. While the cavity beneath a window
in a double wall design would be accessible from the sides, since the
framing and headers are only in the wall assemblies, getting the
insulation under the window from an attic access strikes me as an iffy
proposition.

I'm most interested in comments and suggestions from those well-versed
in the application of blown insulation. Envisioning a wall system that
gives good performance once completed is one thing, but the practical
aspects of building the wall in the first place are something else.

Eunoia Eigensinn

2006-03-08, 7:21 pm

Dick Russell wrote:
> I have a question concerning installation of wall insulation.
>
> In one double wall design for a superinsulated house, the outer wall
> bears the roof load, while the inner wall bears the interior ceiling
> and floor loads. The vapor barrier is wrapped around the inner wall
> frame before it is raised to a vertical position, so that the vapor
> barrier is on the cold side of the inner wall, leaving room for
> installation of plumbing, wiring, and drywall without making holes in
> the vapor barrier. The two wall frames at any level are tied together
> above the top plates with 1/2 or 5/16 inch plywood, for support and
> firestopping purposes. The exterior sheathing is applied to the
> outside of the outer wall, and covered with housewrap and siding. The
> cavity between the sheathing and vapor barrier is filled with
> insulation, which some prefer to be blown cellulose. Before the drywall
> is applied to the warm side of the inner wall studs, the space between
> those studs is filled with fiberglass insulation. The finished assembly
> has the vapor barrier less than 1/3 the way across the total
> insulation, high total R value, and minimal thermal bridging.
>
> Now the question: how and when is the insulation in the cavity between
> sheathing and vapor barrier installed? It can't be installed until the
> exterior wall is closed and the windows installed, to avoid rain
> damage, so that means installation from the inside. While fastening the
> vapor barrier to the cold side of the inner wall frame provides a tight
> seal and excellent protection against migration of interior moisture
> through the insulation in winter, it means blocking off access to the
> larger insulation cavity before the insulation is installed. The only
> thing I can think of, for the case of a two-story house, is to make
> small slits in the vapor barrier just below the top plate, blow in the
> insulation, and tape the slits. The same technique would be used to
> completely fill the spaces below windows. This kind of insulation is
> blown to a certain density, resulting in slight compression, so as to
> avoid settling over time. I have to wonder about the pressure of the
> insulation against the vapor barrier and whether the vapor barrier will
> give too much. I suppose that could be alleviated by supporting the
> vapor barrier with some sort of mesh material stretched first over the
> inner wall frame.
>
> I think the how and when question wouldn't be there for a single-story
> home. In that case, the plywood firestopping connecting the tops of the
> inner and outer walls could have predrilled holes. The insulation could
> be blown in from the top, accessed from the attic space before the
> attic insulation is installed. The holes in the plywood would be
> covered after filling the wall cavity. Filling spaces beneath windows
> might be awkward from the top plate. While the cavity beneath a window
> in a double wall design would be accessible from the sides, since the
> framing and headers are only in the wall assemblies, getting the
> insulation under the window from an attic access strikes me as an iffy
> proposition.
>
> I'm most interested in comments and suggestions from those well-versed
> in the application of blown insulation. Envisioning a wall system that
> gives good performance once completed is one thing, but the practical
> aspects of building the wall in the first place are something else.

Dick;

I've built a few double-studl homes.
Typically the inner wall is loadbearing with 2x4 @ 16" o/c. It carries
the floor loads and roof loads.

The outer wall or curtain wall is either 2x4 or 2x3 studs @24" o/c.

First the inner loadbearing wall is framed with just a single top plate
and squared-up, then polyethylene sheet laid over the studs leaving
about a foot overhanging all around the perimeter and then the double
top plate installed pinching the poly between.

OSB sheathing is then applied over the poly.

The curtain wall is framed directly on top of the bearing wall and then
suspended with temporary spacer blocks (ie 2x4 if you want a 4" cavity,
2x6 for 6" cavity etc) and then the plywood plates (1/2") are attached
at the top and bottom plates and around openings. The blocks are
removed and the cavity between the OSB and the curtain wall framing is
insulated followed by the insulation in the curtain wall and then the
exterior OSB sheathing is attached and the wall tilted up.

The walls usually stand up without any bracing required but slap on
some anyway.

Once tilted up, the walls are at least 2/3 insulated and the wall air
barrier is done so the house is already better insulated and air-sealed
than most conventional homes that are 100% complete. The poly is backed
up by the structural OSB sheathing so it isn't very susceptible to
damage during installation of services in the bearing wall cavity. I
used 6 mil black poly for the job since it is inherently UV-resistant
and any dings show up readily against the OSB background.

Dick Russell

2006-03-09, 11:21 am

Eunoia, in the wall you describe, I'm curious about a few things.
Perhaps you can comment.
1. It appears that you have two layers of OSB, one on each wall. I am
guessing that sheathing is on the inner wall at least in part for
structural bracing of the frame, it being the load bearing one. This
would add to the cost, and also make the wall heavier. Do you use a
crane or just enough people to lift it upright?

2. Does the assembled wall rest on the sill, or do you have the floor
joists and band header out to the edge of the foundation, with the wall
on top? If the latter, do you do anything about insulating the band?

3. Insulating before the wall is raised vertical must mean that you
have to work quickly, getting the walls and roof up, windows in, and
siding on before it rains. How do you deal with the timing of
construction with respect to rain?

JerryD\(upstateNY\)

2006-03-10, 9:21 pm

>>In one double wall design for a superinsulated house,<<

Seeing that most of the heat loss in a house is through the ceiling, I can't
see using a double wall in a house.
The greatest heat loss through the walls would be through the windows.
Doors would be the next biggest loss.
I'll bet there is only 10%-15% of the heat loss through the walls
themselves.
Spending all that money trying to cut the heat loss from 15% to 10% makes no
sense to me.

--
JerryD(upstateNY)

the outer wall
bears the roof load, while the inner wall bears the interior ceiling
and floor loads. The vapor barrier is wrapped around the inner wall
frame before it is raised to a vertical position, so that the vapor
barrier is on the cold side of the inner wall, leaving room for
installation of plumbing, wiring, and drywall without making holes in
the vapor barrier. The two wall frames at any level are tied together
above the top plates with 1/2 or 5/16 inch plywood, for support and
firestopping purposes. The exterior sheathing is applied to the
outside of the outer wall, and covered with housewrap and siding. The
cavity between the sheathing and vapor barrier is filled with
insulation, which some prefer to be blown cellulose. Before the drywall
is applied to the warm side of the inner wall studs, the space between
those studs is filled with fiberglass insulation. The finished assembly
has the vapor barrier less than 1/3 the way across the total
insulation, high total R value, and minimal thermal bridging.

Now the question: how and when is the insulation in the cavity between
sheathing and vapor barrier installed? It can't be installed until the
exterior wall is closed and the windows installed, to avoid rain
damage, so that means installation from the inside. While fastening the
vapor barrier to the cold side of the inner wall frame provides a tight
seal and excellent protection against migration of interior moisture
through the insulation in winter, it means blocking off access to the
larger insulation cavity before the insulation is installed. The only
thing I can think of, for the case of a two-story house, is to make
small slits in the vapor barrier just below the top plate, blow in the
insulation, and tape the slits. The same technique would be used to
completely fill the spaces below windows. This kind of insulation is
blown to a certain density, resulting in slight compression, so as to
avoid settling over time. I have to wonder about the pressure of the
insulation against the vapor barrier and whether the vapor barrier will
give too much. I suppose that could be alleviated by supporting the
vapor barrier with some sort of mesh material stretched first over the
inner wall frame.

I think the how and when question wouldn't be there for a single-story
home. In that case, the plywood firestopping connecting the tops of the
inner and outer walls could have predrilled holes. The insulation could
be blown in from the top, accessed from the attic space before the
attic insulation is installed. The holes in the plywood would be
covered after filling the wall cavity. Filling spaces beneath windows
might be awkward from the top plate. While the cavity beneath a window
in a double wall design would be accessible from the sides, since the
framing and headers are only in the wall assemblies, getting the
insulation under the window from an attic access strikes me as an iffy
proposition.

I'm most interested in comments and suggestions from those well-versed
in the application of blown insulation. Envisioning a wall system that
gives good performance once completed is one thing, but the practical
aspects of building the wall in the first place are something else.

Eunoia Eigensinn

2006-03-11, 5:21 pm

Dick Russell wrote:
> Eunoia, in the wall you describe, I'm curious about a few things.
> Perhaps you can comment.
> 1. It appears that you have two layers of OSB, one on each wall. I am
> guessing that sheathing is on the inner wall at least in part for
> structural bracing of the frame, it being the load bearing one.

Correct.
But first of all, I should mention that I haven't used stud framing for
walls in over a decade.

This
> would add to the cost, and also make the wall heavier. Do you use a
> crane or just enough people to lift it upright?

Straight wall runs are seldom more than 40 feet and they can be lifted
without a crane or wall jacks. Yes, they are a bear but not
unmanageable.

>
> 2. Does the assembled wall rest on the sill, or do you have the floor
> joists and band header out to the edge of the foundation, with the wall
> on top? If the latter, do you do anything about insulating the band?

It depends. On a house with no basement, the stud walls sit on the sill
plate atop a short stemwall, the curtainwall overhanging the sill plate
by 2". The foundation stemwall will have at least 6" of exterior
insulation (EPS or XPS) on it so the cantilever isn't readily
perceptible or odd-looking.

Where the wall sits on a floor deck, the traditional rim joist will be
at the outside edge of the bearing wall, with a parallel rim joist out
at the edge of the curtain wall, with blocking as required. The space
between the parallel rim joists in insulated. The connnection between
the poly sheet of the upper and lower storeys shoud be made with Tyvek
rather than poly since it is a continuation of the air barrier only.

>
> 3. Insulating before the wall is raised vertical must mean that you
> have to work quickly, getting the walls and roof up, windows in, and
> siding on before it rains. How do you deal with the timing of
> construction with respect to rain?

Rain management (fancy term for tarps and stuff (stuff" being anything
that works to keep the materials dry: glass, sheet steel, plywood
etc.).

Often, I've combined the doube-stud walls with exposed timber framing
on the interior so that the T&G lumber flooring is the finish floor and
ceiling. Obviously, you would not put this down unitl after the roof is
on and the doors and windows in. Timber joists would be have been oiled
before installation and would have a protective wrap to keep them dry.
Temp decking would be used for a working surface.

Eunoia Eigensinn

2006-03-11, 6:21 pm

JerryD(upstateNY) wrote:
>
> Seeing that most of the heat loss in a house is through the ceiling, I can't
> see using a double wall in a house.
> The greatest heat loss through the walls would be through the windows.
> Doors would be the next biggest loss.
> I'll bet there is only 10%-15% of the heat loss through the walls
> themselves.
> Spending all that money trying to cut the heat loss from 15% to 10% makes no
> sense to me.

And insulating the ceiling only makes no sense to me.
A super-insulated home means just that. Every component is designed to
minimise undesirable heat loss/gain, and that includes a competent
air-sealing strategy.

I see that you're upstate NY Jerry. I'm north of you in Ontario.

There were entire weeks in January and February that I didn't require
any auxiliary heating at all and so far this month, none at all.
Can you say the same ?

SteveF

2006-03-11, 8:21 pm

Put up a mesh, stick the hose through to blow in dense pack cellulose. Then
cover with vapor barrier and drywall.

http://www.regalind.com/regal_wall.htm

Steve.

"Dick Russell" <richard.russell@shawgrp.com> wrote in message
news:1141744075.061214.259460@j33g2000cwa.googlegroups.com...
>I have a question concerning installation of wall insulation.
>
> In one double wall design for a superinsulated house, the outer wall
> bears the roof load, while the inner wall bears the interior ceiling
> and floor loads. The vapor barrier is wrapped around the inner wall
> frame before it is raised to a vertical position, so that the vapor
> barrier is on the cold side of the inner wall, leaving room for
> installation of plumbing, wiring, and drywall without making holes in
> the vapor barrier. The two wall frames at any level are tied together
> above the top plates with 1/2 or 5/16 inch plywood, for support and
> firestopping purposes. The exterior sheathing is applied to the
> outside of the outer wall, and covered with housewrap and siding. The
> cavity between the sheathing and vapor barrier is filled with
> insulation, which some prefer to be blown cellulose. Before the drywall
> is applied to the warm side of the inner wall studs, the space between
> those studs is filled with fiberglass insulation. The finished assembly
> has the vapor barrier less than 1/3 the way across the total
> insulation, high total R value, and minimal thermal bridging.
>
> Now the question: how and when is the insulation in the cavity between
> sheathing and vapor barrier installed? It can't be installed until the
> exterior wall is closed and the windows installed, to avoid rain
> damage, so that means installation from the inside. While fastening the
> vapor barrier to the cold side of the inner wall frame provides a tight
> seal and excellent protection against migration of interior moisture
> through the insulation in winter, it means blocking off access to the
> larger insulation cavity before the insulation is installed. The only
> thing I can think of, for the case of a two-story house, is to make
> small slits in the vapor barrier just below the top plate, blow in the
> insulation, and tape the slits. The same technique would be used to
> completely fill the spaces below windows. This kind of insulation is
> blown to a certain density, resulting in slight compression, so as to
> avoid settling over time. I have to wonder about the pressure of the
> insulation against the vapor barrier and whether the vapor barrier will
> give too much. I suppose that could be alleviated by supporting the
> vapor barrier with some sort of mesh material stretched first over the
> inner wall frame.
>
> I think the how and when question wouldn't be there for a single-story
> home. In that case, the plywood firestopping connecting the tops of the
> inner and outer walls could have predrilled holes. The insulation could
> be blown in from the top, accessed from the attic space before the
> attic insulation is installed. The holes in the plywood would be
> covered after filling the wall cavity. Filling spaces beneath windows
> might be awkward from the top plate. While the cavity beneath a window
> in a double wall design would be accessible from the sides, since the
> framing and headers are only in the wall assemblies, getting the
> insulation under the window from an attic access strikes me as an iffy
> proposition.
>
> I'm most interested in comments and suggestions from those well-versed
> in the application of blown insulation. Envisioning a wall system that
> gives good performance once completed is one thing, but the practical
> aspects of building the wall in the first place are something else.
>

ConcreteGuy

2006-03-27, 6:21 am

Sorry I can't let this comment go....Its wrong...

You never heard of the Canadian R 2000 houses but what I read
here...In the courses you would take to learn how to build one you
would learn that 40% of the cost for home heat is because of air that
leaks out of it...Air you have paid to heat...Heat goes to cold...This
is a fact... CMHC have many training booklets you can get on all these
areas on building design,etc...They are one of the links below...

You see more insulation above the ceiling because its a large open
area and easy to add it there... Go to this web site and ask for info
on the R 2000 program and how to build these walls you have asked
about...

http://www.chba.ca/

Check these links for more information...

http://www.chba.ca/Links/index.php

Lansing Small ( Past President of Cape Breton Home builder's
Association )

On Sat, 11 Mar 2006 00:58:34 GMT, "JerryD\(upstateNY\)"
<jerry@righthere.com> wrote:

>
>Seeing that most of the heat loss in a house is through the ceiling, I can't
>see using a double wall in a house.
>The greatest heat loss through the walls would be through the windows.
>Doors would be the next biggest loss.
>I'll bet there is only 10%-15% of the heat loss through the walls
>themselves.
>Spending all that money trying to cut the heat loss from 15% to 10% makes no
>sense to me.

LinkBot