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

Wed Jul 12 14:41:28 CDT 2006

>
>> The reasoning behind this wall system was (1) to isolate the wiring  
>> on the warm side of the vapor barrier, the thinking being that the  
>> wires and the outlet holes compromised the insulation (especially  
>> batts), (2) that the finished wall
      was the same thickness as 2x6, allowing standard window/door trim,  
(3) the foil-faced foam was a reflective
      surface, thermal break, vapor barrier, and R-5 to boot, (good  
value from minimum foam use),(4) to enable vertical
      drywall in 8'+ ceilings. I did say that if I were to do it again,  
I would frame with 2x6. I agree that the double vertical
      wall gives a thermal break sans foam.
      Clarke Olsen

>> 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.

>   On 7/12/06, Robert W. Tom <ArchiLogic at yahoo.ca> wrote:
>> 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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