[GSBN] Sheathing and straw bales proposed

Ian Smith ian at lopezsmolensengineers.com
Fri Jul 18 23:48:13 CDT 2014

Hello GSBN,

This is my first time posting here... hope I'm doing this right.  Quick
intro of me:  I am a structural engineer that started working on straw bale
projects in 2003, with Jeff Ruppert.  Jeff is over in Paonia now, and I'm
still in Boulder.  Pretty sure I'm also still the Treasurer of COSBA
(Colorado Straw Bale Association), although we're on some sort of extended
vacation after hosting the ISBC in 2012.

Anyway, my wife and I have recently bought a small house in town and we're
hoping to do a strawbale addition on it.  The addition will be rectangular
with a gable roof.  We're thinking of building a wall system that would be
relatively experimental for us here, although I understand that others are
doing similar things in Vermont, and maybe Europe...?

I just wanted describe our proposed wall assembly and see if anyone has any
comments, etc.

The assembly would be, from interior to exterior:

"Conventional" 3-coat interior clay plaster with lime finish coat(s)
applied directly to straw bales, stacked on-edge.
2x4 (4x9 cm) wood studs at standard spacing with blown-in cellulose or batt
1/2" (1.25 cm) Structural wood sheathing where needed (structurally), and
fiberboard elsewhere.
Exterior finish A) On walls under eaves (south and north) - double building
paper and 2-coat lime plaster
Exterior finish B) On walls exposed more to weather - weather resistant
barrier, vertical wood nailers, and horizontal wood (or similar) rainscreen.

This wall assembly potentially allows for the complete construction of the
exterior walls - with finish and windows installed - and roof, before the
straw bales and stud insulation are installed.  Therefore, installation of
the straw bales can (potentially) occur at any time of year or in any
weather, since the exterior of the building is 100% complete.  No need for
temporary bracing.  It also provides some savings in labor since the need
for "normal" exterior plaster is greatly reduced (although additional,
expensive finish materials are needed).

Yes, the exterior side of the bales will have no plaster.  Although, it
could be possible to do a slip (thin) coat on each bale while stacking.
The batts or blown cellulose between studs would (theoretically) settle
tight against the bales, mitigating the potential for air movement there.
I assume the insulation would also contain a fire retardant.  Differential
settlement of the bales would be mitigated by tying the bales to the studs,
and providing proper support for the interior plaster at the foundation

Most of my concerns with this proposed assembly are focused on building
science and moisture issues.  I think I need to use the fiberboard
sheathing wherever possible to achieve the highest (overall) permeability
possible, but I'm not fully sure...  Also, are there any permeability
issues with the double building paper and lime plaster when applied
directly to the sheathing?  Should I maybe install a drainage plane behind
the plaster?

Let me know if you think of anything else.  Thanks very much,  (and Dirk,
congratulations on the big SB building!)

Ian Smith, P.E.
Boulder, CO, USA
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