SB: Re: (Loopy's) Rainscreen Claddings

[Rob Tom <ArchiLogic@yahoo.ca>]

10/24/2002 1:22:43 AM, "Dale Banks" <loopy@homernet.net> 
wrote:

>2x6 rough-cut 24" on center. 
>offset the studs 2" to the outside of the bottom and top plates,
> bales will be laid flat and notched 4" to fit into the studs and lay 
>even with the outside edge of the top and bottom plates.
>  In between each stud we will put a 1/2" coat of earth plaster, 
>providing an air barrier. 
>horizontal siding affixed to the studs, leaving a 1.5" gap between the
>siding and the plaster

Loopy;

When your post made its first rounds (loops ?), the first question I 
asked myself was:  
              "Why bother with the bales in that design ?" 
but that  wouldn't be a kewl thang to ask on a strawbale list, so I 
didn't. I suspect that many others may have asked the same question 
and didn't respond for the same reason, so it's not like people are 
ignoring you Loopy  (I hope).

Generally-speaking, in order for an air barrier to be effective at its 
intended purpose, it needs to be continuous.

In the above scheme, there will be discontinuities in each stud bay 
unless you wrapped the backside of  all framing members (which you 
intend to notch into the bales) with some sort of air barrier membrane 
material (likely Tyvek ?)to make the plaster-as-air barrier continuous 
(which would BTW, also help to make the plaster more effective as a 
drainage plane and bug barrier.

Failing that, I don't think that it'd be unreasonable to suggest that 
[(a total crack width of 1/16" per stud bay) X  (stud height+bay 
width) = ] ~6 sq inches leakage area per stud bay would develop 
once the plaster dries out.  

That would have the effect of compromising the effective R-value of 
the insulation significantly. How significantly ? You can do a web 
search or a search of  this List's archives and find the test data for the 
first  of the Oak Ridge Nat'l Lab SB wall tests for a rough idea of the 
magnitude.

Since there would be a wooden cladding on the outside in your 
proposed design, it seems that the only things that bales would 
contribute is a lumpy interior wall surface and possibly an R-value 
somewhere around that of a batt-insulated 2x6 stud wall (likely less, 
due to the less-than-ideal air barrier).

You already have the 2x6 studs. If one were to insulate the stud 
cavities in the normal fashion, add some insulating sheathing and 1x3 
furring strips, [ie no bales], one could easily achieve a superinsulated, 
rainscreen-clad wall in half the wall thickness and 1/3 the labour (ie no 
bales to notch, one less surface to plaster.)

However, if  "it's-gotta-be-bales-for-Loopy" then I think that it might 
be more effective (air barrier-wise), entail less work , provide better 
support for the siding, consume less space and use ~10% less lumber 
in the process, to apply a continuous layer of plaster over bales laid 
on edge and use 2x4 studs @16" o/c, set outside of the plaster skin 
(ie no notching of bales.)

While this might be okay for the purposes of a rainscreen cladding, it 
doesn't make much sense otherwise since the loadbearing stud wall 
being on the outside requires that things like floor joists and rafters 
span an extra 3 feet or so, requiring the sizes of the framing members 
to be *at least* one size larger than they would have been if the 
bearing were placed at the inside of the bales.  Also, the studs being 
at the outside of the bales negates the possibility of using them to 
create chases to carry services (ie plumbing, electrical) or provide 
support for things like wall-hung cabinets. 

        ~~~ * ~~~
         Rob Tom
  Kanata, Ontario, Canada
<ArchiLogic@CHAFFyahoo.ca>
(winnow the "chaff" from my edress in your reply)

Please visit http://www.theHungerSite.com daily






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