[Greenbuilding] Double-stud wall (was Re: Best bang for the buck in wall insulation)

Thu Mar 27 23:32:37 CDT 2014

I find it simpler as design concept to keep the structure as one element
independent of what I've been calling the 'sacrificial' parts or everything
subject to elements. In this way the structure is primarily protected. The
sheathing forms part of that structure and moves with it.  The sheathing is
definitely a wrb but shouldn't be the primary as its primary role is
structural. My preference there is having the primary wrb outside of an
exterior insulation layer (board insul). 

Windows, doors other penetrations are easily deformed by attachment to
structural sheathing - they used to have big leaky gaps between them and the
framing so they could float. Current nail fin attachments are overly
fastened and adhesive membranes do restrict floating) - big problem for air
leakage when windows get twisted - so not an ideal attachment point for
those elements.  

Window and doors and other surface elements can float nicely on exterior
insul board - not as carpenter friendly but at that point the carpenter hat
comes off and the 'enveloper' hat should be on.

On the subject of rock wool and cellulose. I've been involved lately in pre
1990 rehab projects and having to specify exposure contaminant controls for
worker safety and it is getting serious and seriously expensive. At this
point it is still primarily asbestos and lead concerns but dust exposures
from aged friable borax laden cellulose, fiberglass, and brittle
formaldehyde coated rockwool will soon be added the list. I think we need to
seriously reconsider loose fill insulations of any sort.

From: Greenbuilding [mailto:greenbuilding-bounces at lists.bioenergylists.org]
On Behalf Of Alan Abrams
Sent: March-27-14 2:29 PM
To: Green Building
Cc: archilogic at chaffyahoo.ca
Subject: Re: [Greenbuilding] Double-stud wall (was Re: Best bang for the
buck in wall insulation)

<Logically, the interior wall would be the one designated as load-bearing
even if only because doing so shortens the spans of flexural members like
joists and rafters. When we're talking about fat superinsulated walls, the
difference in span can mean a difference in one joist size.>

taking off from the perspective pasted above, one could quickly arrive at a
Larson type system, where you not only shorten the span of the floor system,
but eliminate it altogether beyond the bearing wall.

OTOH, if the premise is keeping the air barrier in a single vertical
plane--and factoring in the current code obsession with braced wall
systems--I still prefer keeping the air barrier and sheathing--and therefore
the outer stud wall, in a constant plane. all the other previous points
reinforce this--logical integration of windows and doors, combining air
barrier and drainage plane--not to mention--keeping it low tech and
carpenter friendly.

The point about reducing joist depth is well taken: there is value in
"resourcing down." But it is less significant if you're using wood I-beams.
A tree that could provide a nice 2x12 is best left in the forest, but a
little more OSB to take a TJI from 9 1/2" to 11 7/8" is less of an impact.

It's worth mentioning that this strategy derives from Building Science
Corp's design for the NIST Net Zero Energy Test House--a sublime piece of
work, IMO...

-AA

Alan Abrams
certified professional building designer, AIBD
certified passive house consultant, PHIUS

certified passive house builder, PHIUS
Abrams Design Build LLC
sustainable design for intentional living
cell     202-437-8583
 <mailto:alan at abramsdesignbuild.com> alan at abramsdesignbuild.com
 <http://www.abramsdesignbuild.com/> www.abramsdesignbuild.com

On Thu, Mar 27, 2014 at 9:11 AM, Clarke Olsen <prismoidal at gmail.com> wrote:

The problem with single top headers is that, when triaging the lumber, it's
hard to find something you feel can be trusted up there all alone... 2x3
sounds better then it is: most of them are crap. The other reason to make
the interior studs bearing is that you can keep the foundation back, leaving
a very generous overhang to drip past the foam that covers your foundation.
I used to think I was a hero for putting 2" of blueboard under the slab and
against the foundation, until I noticed how nice & cool it was in summer...
Clarke Olsen
clarkeolsendesign.com
373 route 203
Spencertown, NY 12165
USA
518-392-4640
colsen at taconic.net

On Mar 26, 2014, at 5:55 PM, RT <archilogic at yahoo.ca> wrote:

> On Wed, 26 Mar 2014 11:42:01 -0400, AA-man wrote:
>
>> here's where I'm heading ...*OVE stud wall
>> *a void, width determined by specific space heat demand analysis
>> *an inner frame wall
>> *the entire framing cavity filled with DP cellulose
>
>
> The above sounds very much like a double-stud wall section so one should
choose which of the two frame walls is load-bearing .
>
> Logically, the interior wall would be the one designated as load-bearing
even if only because doing so shortens the spans of flexural members like
joists and rafters. When we're talking about fat superinsulated walls, the
difference in span can mean a difference in one joist size.
>
> In making the interior wall loadbearing, there's no need for the exterior
wall to be a 2x6 OVE stud wall -- it can be a 2x3 or 2x4 wall with studs at
24" o/c, single top plates, no lintels over openings (assuming that we're
talking about low-rise residential building)
>
> One just needs to ensure that one allows for eventual shrinkage of
cross-grain framing members (ie sill & top plates, rim joists, lintels) in
the bearing wall so that the curtain wall does not inadvertantly become
loadbearing.
> --
> === * ===
> Rob Tom                                       AOD257
> Kanata, Ontario, Canada
>
> < A r c h i L o g i c  at  Y a h o o  dot  c a  >
> (manually winnow the chaff from my edress if you hit "reply")
>
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