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

[Brian Waite]

Dear All,
I hesitate to join in such discussions because my only experience is with my 
own strawbalehouse, and that doesn't follow any of the rules. However 
building experience and common sense says that Jeff is right on all aspects. 
I would just like to add that the space between bale and OSB would create a 
chimney effect in the sad event of fire starting that would allow rapid 
spread and acceleration of combustion. One thing that makes strawbale 
buildings more resistant to fire than a normal timber framed house is that 
there are, usually, no such cavities.
Re. the exterior rain protection, I cover all of the (lime rendered) bales 
with tiles and use the chimney effect to draw air over the exterior render. 
I also leave the exterior lime rough "as sprayed" to present more surface 
area to the afore mentioned draught.
By the way all my lime render and plaster was done with one only sprayed 
layer and after three coats of limewash the interior is nearly as smooth as 
gypsum plaster - certainly smoother than I anticipated.
Brian
www.strawbalehouse.co.uk



----- Original Message ----- 
From: "Jeff Ruppert" <jeff.ruppert at gmail...>
To: <strawbale at amper....muni.cz>
Sent: Monday, December 06, 2010 12:19 PM
Subject: Re: [Strawbale] OSB or Not? (rendering versus sheeting)


> 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
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