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

[Robert W. Tom]

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>