[Greenbuilding] [BULK] Re: Double Stud walls (was re: How green is cellulose infill? / thermal bridging?)

Tue Apr 10 20:59:03 CDT 2007

	I've added a layer of foam to both the inside and outsides of
buildings as a retrofit, and I would recommend the outside method if you
can swing it.  Going outside covers more territory, inside you end up
doing a major interior remodel, or skipping key areas like T-walls.
Also your trim never fits right, leading to that well known phenomenon
where the carpenter's vocabulary deteriorates to just three or four
choice words.  Interior remodels are usually more bother and mess than
exterior remodels.  Unless you have a good reason to gut the house
anyway.  

You can achieve, with foam at least, up to 1.5" of added foam without
strapping the walls with furring.  For more insulation, foam between
furring studs and then over the top of them.  Or Roxul, or whatever you
are using. 

Don't worry so much about the vapor barrier issue.  Use a drainage plane
under whatever siding you choose, and try to make your wall airtight.
Be careful about window details, the extra lip under windows is a good
scoop for water entry.  Or install new windows and attach them to the
new, thicker outside wall in the conventional manner, and build out some
deep window trim inside.  That actually would result in the tightest,
and most waterproof, installation.   

Lawrence Lile, P.E., LEED AP

-----Original Message-----
From: greenbuilding-bounces at listserv.repp.org
[mailto:greenbuilding-bounces at listserv.repp.org] On Behalf Of Paul Newby
Sent: Tuesday, April 10, 2007 9:05 AM
To: greenbuilding at listserv.repp.org
Subject: [BULK] Re: [Greenbuilding] Double Stud walls (was re: How green
is cellulose infill? / thermal bridging?)
Importance: Low

Picking up on this half-year-old thread...

I'm reinsulating my house, which was built in the
1970s with 2x4 studs, and it seems that adding an
inside 2x4 layer on 24" centres in a double-stud wall
configuration is the lowest cost approach.  I plan
to use a double layer of R14 Roxul where possible
and polyurethane foam within the inner layer in
areas where the old wall can't be effectively
insulated with Roxul (headers, corners etc.)

The alternative approach I considered is to foam
the outer layer for nominal R24, then strap the
walls horizontally with 2.5" lumber and add a layer
of R9 Roxul over a vapour barrier.  This has the
advantage of isolating an inner service layer,
provides a higher level of insulation and saves
interior space, but is more expensive, and
I also have some concern about condensation over
the outside framing, particularly at the headers
where most of the insulation would be inside the
vapor barrier.

Comments?

Paul N.

Robert W. Tom wrote:
> On Tue, 11 Jul 2006 21:26:04 -0400, Clarke Olsen <colsen at taconic.net>

> wrote:
> 
>> I have built double walls like this: 2x4 framing, filled with
>> insulation, then covered inside with 1/2" foil faced foam, which is
>> then strapped with 2x4's flat. All the wiring takes place in this
1.5"
>> space, using 4"x4"x1.5" boxes. Taping the foam joints will make it a
>> vapor barrier.
> 
>> For a really well insulated house, I would frame with 2x6's @ 2' oc.
> 
> 
> I don't follow the reasoning for strapping a 2x4 wall with 2x4's laid
flat.
> 
> Those same 2x4's stood on end and flipped 90 degrees:
> 	- are capable of being a loadbearing wall
> 	- are capable of providing a cavity to hold R-12 or better
insulation
>          thereby eliminating the need for the expensive foil-faced
foam  
> insulation
> 	- are capable of providing wiring and plumbing chases
> 	   (to carry any-sized electrical boxes)
> 
> etc.
> 
> Nor do I follow the reasoning behind going to the trouble of making a

> double-stud wall if all one is going to do is insulate it to less than

> R-30.
> 
> Years ago when I was still making stick-framed/double-stud walls,
(this  
> was wa-a-a-y back in the Dark Ages, pre-Windoze OS, pre-WWW ...when
people  
> still wrote letters on paper and mailed them in envelopes and  
> non-technerds like me were struggling with DOS, Fortran, BASIC etc.
when  
> trying to use these infernal computing boxes ) ... the minimum
criteria  
> for a double-stud wall would be a nomimal R-40.
> 
> ie (from inside-> out )
> 	(i) drywall
> 	(ii) 2x4 bearing wall (studs at 16" o/c) insulated to nominal
R-13 (ie  
> 3.5" Roxul)
> 	(iii) vapour diffusion retarder/continuous air barrier membrane
> 	(iv) structural sheathing
> 	(iv) continuous layer of R-13 insulation, no framing in cavity
> 	(v) 2x4 curtain wall
> 	    (studs @24" o/c, centres not aligned w bearing wall studs)
> 	     insulated to R-13
> 	(vi) exterior sheathing
> 	(vii)AIF or housewrap
> 	(viii) 3/4" ventilation airspace
> 	(ix) rainscreen cladding
> 
> Typically the stud walls were built on top of each other on the floor

> deck... curtain wall propped up on temp blocks until 1/2" plywood
plates  
> were applied to the top plates of the bearing and curtain walls  
> (essentially creating a box beam out of the top plates) and the outer
two  
> thirds insulated before tipping the walls up. Once stood up, the
heavy,  
> wide walls would stand and stay straight without any temp
props/bracing.
> 
> Since the VDR/continuous air barrier membrane (I preferred black 6 mil

> poly) is on the outside of the bearing wall and backed-up by
sheathing,  
> with at least R-26-worth of insulation on the outside, the bearing
wall  
> could be left uninsulated so that services could be installed in the  
> "normal" fashion without having to futz with air-sealing details and
the  
> shell would at that point, already be better insulated than most tract

> housing so that even in the middle of winter, the interior would be
warm  
> without any auxiliary heating systems in place.
> 
> And since the poly was black, any accidental punctures or tears as a  
> result of installation of services would be readilly visible against
the  
> light-coloured sheathing behind it and easily repaired.
> 
> Then all services were installed, the bearing wall would be insulated
and  
> the drywall applied, again in the "normal" fashion, without any
futzing  
> with air-sealing since it was done as soon as the walls were tilted
up.
> 
> Another advantage to the above technique is of course that interior  
> renovations and access to services in the bearing wall can be
accomplished  
> without any concerns about compromising the integrity of the air
barrier.
> 
> Back then (when trees were plentiful and dinosaurs still roamed the  
> Earth), it was less costly to do a double stud wall in the above
fashion  
> than it was to do a 2x6 wall and slap rigid foam onto it to bring it
up to  
> superinsulation levels.
> 
> 
> 
> === * ===
> Rob Tom
> Kanata, Ontario, Canada
> <ArchiLogic at yahoo.ca>
> 
> 
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