[Strawbale] OSB or Not?

RT ArchiLogic at yahoo...
Wed Dec 1 22:04:27 CET 2010

On Wed, 01 Dec 2010 12:10:00 -0500, Michel Van Mulders  
<michelvanmulders at siol...> wrote:

[<snipped> for the sake of brevity. Full text of message and thread can be  
seen at

> 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?

My short answer would be "Not" (to the question re: OSB) and "Yes" (to the  
"just plastered both sides".

Even with houses whose walls are not made of SB, I would not use OSB  
anywhere on the interior of the house inside the plane of the air barrier  
simply because OSB stinks. (Smell = off-gassing of likely unhealthy VOCs)

Further, OSB is a poor substrate for plaster.

Aside from the issues of off-gassing and unsuitability as a substrate for  
plaster, if the bales are not encapsulated by wet-applied plaster, the  
lack of direct contact between the straw and the OSB-as-air-barrier  
encourages convection to occur in the bales which results in a significant  
reduction in the effective thermal resistance of the SB wall assembly.

> 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.

First of all to clarify some terminology.

I understand "timber frame" (TF) to describe a framing system that uses  
large dimension timbers (ie smallest cross-sectional dimension typically  
greater than 150 mm) which uses mortise & tenon and dove-tail joinery to  
connect the framing members, joints locked with wooden pegs or wedges.

Since this type of joinery requires skilled persons to execute properly  
and to execute properly requires careful attention to detail, all of which  
requires more time, the labour costs will be high.

IMO, it would be a shame to bury this type of a frame inside of a SB wall  
and would be best left exposed to the interior so that it may be fully  

I understand a "Post & Beam" (P&B) frame to be one in which most or all of  
the joints are simple butt or lap joints, usually relying upon metal  
mechanical fasteners for joint integrity. The framing members may be solid  
timbers or built-up from small dimension lumber. This sort of a framing  
system requires less skill to assemble and goes together much quicker than  
a TF and as such, the cost will be lower than that of a TF system.

I understand a "Light Framing" (LF) system to be one in which all of the  
components are comprised of milled, small-dimension softwood lumber whose  
breadth is typically only about 38 mm, individual members usually closely  
spaced with centre-to-centre spacing typically being 400 to 600 mm ,  
usually with a thin (10 to 12 mm thick) sheet diaphragm (typically 1200 x  
4800 mm) applied to the faces of the assemblage to facilitate load-sharing  
and provide some in-plane shear resistance.

I see little point in using a P&B frame for the exterior walls of a SB  
house from either a viewpoint of aesthetics or one of structural  
integrity. Both of these would (IMO) be best embedded in the SB walls to  
take advantage of the bales' potential to provide in-plane racking  
resistance and also to eliminate the problem of trying to maintain the  
integrity of the interior air barrier.

Actually, I'm not particularly fond of the idea of using an  
exposed-to-the-interior TF for the exterior walls of a SB-walled house  
either because of these last two points (ie no contribution to racking  
resistance from bales, problems maintaining continuity of air barrier  
across framing members).

If large dimension timbers are desiredfor their aesthetic value, I think  
that a hybrid of the two systems would be best ... embedded LF in the SB  
exterior walls, exposed-to-the-interior TF or P&B
for interior columns and floor and ceiling framing.

Ideally the LF members for the exterior wall framing would be spaced  
framing members (essentially a parallel chord truss or "Larsen" type  
truss) with each chord of the truss at each extremity of the SB wall.   
This truss configuration of relatively small dimension chord members  
(typically 38 x 64mm or 38 x 89mm ) not only makes for very strong  
(out-of-plane lateral resistance) framing members while using a minimum of  
wood, it also provides useful points of attachment at both interior and  
exterior surfaces of the wall.

And since the LF members are embedded in the bales, the bales provide the  
otherwise-flimsy (in the plane of the wall)  LF members with lateral  
support/buckling resistance so that the centre-to-centre spacing can be  
increased from the "normal" 600 mm C-C spacing that one would typically  
use in a non-SB LF wall.

And since most exterior walls will include windows, doors and corners, the  
jambs and corner posts would be the logical locations for the LF hybrid  

(And I think that the lath strips in Michel's proposal would be  
susceptible to creating the same potential convection problems that would  
occur with OSB sheathed bales.)

=== * ===
Rob Tom
Kanata, Ontario, Canada
< A r c h i L o g i c  at  Y a h o o  dot  c a >
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