[Strawbale] OSB or Not? (rendering versus sheeting)

[Jeff Ruppert]

Hello Everyone,

There are many important reasons to not use OSB over bales.  But what is 
more important is to ask why do we use plaster?

1.  Plaster increases the insulating qualities of a bale wall 
many-fold.  It is not a trivial amount.  If air is allowed to circulate 
on and through the bales, the wall as an assembly will not insulate as 
well as if it is plastered.

2.  Plaster increases the fire resistance of bales exponentially.  
Without it you are risking losing your home from something as simple as 
a electrical short.  If the bales are not sealed with plaster they will 
ignite much easier.

3.  Plaster keeps rodents and insects out if detailed properly.  Without 
plaster rodents will have a much easier time moving about the walls and 
spreading within.

4.  Your plaster can be a structural component of your building.  By 
adding plywood or OSB you are not connecting the plaster to your bales 
and eliminating it as shown in many structural tests.  If plywood is 
used it will take the place of plaster on the bales to resist in-plane 
shear forces, but why spend more money than necessary?

5.  Plaster, when done properly and maintained will last for many years.

6.  You may want to consider a rain-screen type of assembly, but you 
will still need at least one coat of plaster on any exposed bales behind 
the screen for many of the reasons above.  If you are in a very wet 
climate, this is a preferred method on any type of construction, bale 
walls included.  Maybe this is what is meant by "OSB or Not?", but even 
if you use OSB or plywood, you still need to seal the bales with plaster 
behind the sheeting material.

Hope this helps.

Jeff Ruppert, P.E.
Kabul, Afghanistan