[Strawbale] OSB or Not?

Alex Cristescu alexdcristescu at yahoo...
Fri Dec 3 14:30:46 CET 2010

Dear All,
I am very thankful for the discovery of this forum. You are truly passionate and 
anybody can learn a lot from all of you.
I will start building a strawbale house near Bucharest - Romania next year in 
may and as you can see   in the attachment I have osb on the interior for 
structural reason and the easiness of execution. My question is: It is possible 
or NOT to plaster OSB with lime mortar on reed blanket? Anybody do that? Any 
tips and tricks? 

Respectfully yours,
Alex Cristescu  

From: asbn <asbn at baubiologie...>
To: European strawbale building discussions <strawbale at amper....muni.cz>
Sent: Wed, December 1, 2010 11:08:13 PM
Subject: Re: [Strawbale] OSB or Not?

Dear Michel, Jure and...

Why do you compare a (thick) layer of earth/clay directly affiched/sprayed
on strawbales with a (thin) layer of earth/clay on OSB?
If this plaster has the same thickness, the thermal mass is the same.

Let me try to explain, why OSB (or diagonal bracing with wood) is used:
a) for the structural strength of the wall, which has to be guaranteed (and
is much easier to calculate with the infill-system), otherwise your house
would not withstand the windforces and snowloads. You can make diagonal
bracing with metal (which is always a problem because of a possible
corrosion) or with a structural wood(board) or you can rely on the
structural quality of the sandwich-system earth-straw-earth(or lime). But I
would not want to explain or proof this to a building official...
b) second you need an airproof layer on the inside and a windproof layer on
the outside of the wall, especially if you want to build a low-energy or a
passive-house. These layers can be folies under the plaster (which are ­ as
OSB ­ nearly impossible to plaster directly without cement) or OSB - this is
the great advantage of the OSB-board: it functions as structural board AND
airproof layer (Dampfbremse) on the inside of the wall. Airproof in this
case means: it guarantees, that your (strawbale) insulation has the full
thermal quality and it reduces the moisture going in the wall (from inside).

We never plaster OSB directly, we use an installation-layer like 3,5 ­ 5 cm
Heraklith-BM (magnesite-bound), which is similar to Fibralith
(cement-bound). This board is an ideal plaster-ground and somewhere you have
to put all your installation (heating, water, power) in the wall. Some do
that in the straw (I never would do that), some say, they can do it just in
separation-walls or floors. I don't believe that.

So, this is the reason, why we have these layers: every layer has a function
to guarantee the structural (wind, loads), physical (moisture) and thermal
(full insulation capacity) of the wall.
You can change the materials (like OSB), but you have to find a material,
which has the same qualities.
I looked long for a subsitution for OSB (in a time, when OSB was not
available formaldehyde-free), but only found MgO-boards in China, which have
the same quality (but are too expansive if you add the transport).
Yes we built with diagonal wood-bracing, folies, reed or Heraklith-BM (as
plaster-ground) and earth-plaster in the beginning, but if you calculate the
price of the materials AND the work, OSB is much cheaper.
We want to build ecological houses in the best possible (ecological and
structural) quality to the smallest possible price, this is why we have
choosen OSB and Heraklith-BM and strawboards (the Czech ecopanely) as

This is - im my opinion - important, if you build for and sell a house to a
customer, if you do it for yourself, you can do all these experiments,
because you are not capable and can plaster cracks as often as you want.
But if you build a house as a carpenter or professional strawbuilder for
money, you have to be more careful, otherwise your company will not last
very long...

Mit lieben Grüßen
Herbert Gruber
asbn - austrian strawbale network
Österreichisches Netzwerk für Strohballenbau
3720 Ravelsbach, Baierdorf 6
Email: asbn at baubiologie...

> Hello all,
> As Jure, I am also living in Slovenia and planning a house, and workshop (
> together about 500 m2) to build next summer
> I took some time to think about the discussion going on last week about using
> OSB.
> I started off a little different, suggested by this forum, to find a
> timber-framer first and then an architect that is used to work with such a
> company.
> It turns out very good.
> Of course, timber framing for strawbale walls is specific, and I will have to
> provide the information to the architect.
> My feeling is that using OSB or similar large sheets with strawbale building
> can be avoided.
> Wouldn't the properties of strawbale walls be better, if just plastered from
> both sides?
> With a thin plaster on a OSB on the inside of the wall, there is not much
> thermal mass either.
> What would be your thoughts on this;
> The framing of the house on the inside is ideal for avoiding thermal bridges.
> On this framing (outside) I could nail thin strips of wood (30mmx15mm) with
> spacing of about 50 mm or more, and stag the bales against the strips.
> This gives a lateral support to the bales, and some grip for the plaster later
> on the inside.
> Then sew though the bales and fix the same strips of wood on the outside of
> the bales for later good grip of the plaster.
> This feels very good, but the plaster becomes maybe too thick (plus burlap or
> reed), and is maybe too complicated to do.
> Seen from other buildings, the bales are usually just squeezed between the
> poles of the frame.
> It looks more simple somehow, but are thermal bridges of the frame and bales
> not a problem here, or do the bales insulate enough to avoid troubles?
> Windows are also put into "boxes" of sheet material.
> What are the ways to avoid thermal bridges here, condensation of the bales
> around windows?
> Now being in the design phase, I would be grateful to get your suggestions and
> idea's.
> With greetings from snowy Slovenia!
> Michel.
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