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

[Reuben Deumling]

Wow. This is good stuff. I still like the Larsen Truss idea, but this
diversity is very stimulating. One question: How do these thermally
sophisticated wall-sandwiches
accommodate window and door openings? Are there any tricks when it comes to
the vapor barrier and the issue of thermal bridging at these interruptions?

Reuben Deumling


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