[Strawbale] Breathability of Wood Shakes and Roofing Felt

[Andrew Lund]

--- Rob Tom <ArchiLogic at yahoo.ca> wrote:

> On Fri, 10 Nov 2006 23:28:32 -0500, Andrew Lund
> <awl1400 at yahoo.com> wrote:
> 
> > cob/cordwood home with a post-and-beam frame.gable
> ends ...framed with  
> > 2x8s and attached a 1x3 "skip
> > sheathing" to the outside. We did the clay
> slip/straw
> > insulation mix, and slip-formed up the walls with
> it.
> 
> > That is, we would attach big sheets of wood to the
> > studs, pack the cavities with the straw/clay, then
> > move the forms up and continue to the top. This
> method
> > worked well, and we covered the inside of the wall
> > with an earthen plaster.
> 
> > We plan to attach wood shakes to the
> skip-sheathing
> 
> > concerned about the breathability of
> > the shakes as you are supposed to install strips
> of
> > tar paper between each row of shakes. Won't this
> > defeat the purpose of the breathable wood as a
> siding
> > material?
> 
> Andrew;
> 
> I'm not sure I follow your reasoning WRT concern for
> lack of  
> "breathability",
> assuming that by "breathability" you mean an ability
> to disperse moisture  
> that
> has gotten into the cob, presumably from the
> interior.
> 
> First of all, you have a 3/4" airspace between the
> cob and the backside of  
> the shakes, created by the 1x3 skip sheathing.

> Second, the 18" wide strips of  asphalt-impregnated
> felt (aka "AIF",  
> "tarpaper") would only lap onto the top of each
> course of shakes a  
> distance that leaves 2x the exposure between the
> bottom edge of the AIF  
> and the butts of the shakes. That hardly creates any
> sort of air-tight  
> membrane.
> 
> Third, the AIF that you use should be of the
> perforated variety.
> 
> #1 & 2 above would allow any liquid moisture to
> drain (assuming that the  
> backside of the AIF is a sort of drainage plane for
> any condensed moisture  
> exiting the cob wall) and #3 would allow
> vapour-phase moisture past the  
> AIF moisture barrier.
> 

Thanks for the insight. I just wasn't sure if "AIF"
was intended to be breathable or not. 

> That being said, I do wonder if the cob is providing
> any appreciable  
> insulation value at what is a critical area (gable
> ends) WRT building heat  
> loss.

Well, the straw-clay is MUCH less dense than cob. It's
basically just a damp straw that we pushed in by hand.
I've read from other sources that the R-value is
estimated to be from 2-3. Even if it isn't quite that,
it is a relatively small proportion of our building
envelope. I've read a lot of places that recommend 12
inch walls, but 8 was much more convenient for us.
Also, being that the insulation was installed loose,
any voids and gaps were eliminated, so that should
help cut down the air infiltration. Plus, I'm sure the
1 1/2 thick earth plaster we have has got to help a
bit...

Andrew


 
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