[Strawbale] Breathability of Wood Shakes and Roofing Felt

[Rob Tom]

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.

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.

ie At the lowest density possible for a straw/clay mix (~300 kg/m^3 (or  
~18.7 lbs/ft^3), where the straw is just barely coated with clay and  
lightly placed (a mix that would be suitable for use in an attic) the  
R-value is about R-1.44 per inch.

For a mix that would be suitable for a wall application, the density would  
probably be closer to 40 lbs/ft^3 (and quite likely more), so the R-value  
would be something like R- 0.7 per inch or less, meaning that for a cob  
thickness formed by 2x8's, the nominal R-value would be something like R-5  
or less, before accounting for the thermal bridges created by the 2x8  
studs which would further lower the effective R-value of the wall,  
depending upon the stud spacing.


===* ===
Rob Tom
Kanata, Ontario, Canada
<archilogic at chaffyahoo dot ca>
winnow the chaff from my edress in your reply