[GSBN] Straw bale at high altitudes (and snow against bale walls)

Jacob Deva Racusin buildnatural at googlemail.com
Fri Jul 11 22:03:41 CDT 2014


Martin,

We've had a lot of snow pack up against our buildings.  I have noticed 
that in some places - particularly against north walls where it takes a 
really long time to melt - premature wear against the plaster can be an 
issue. This is common enough to encourage us to provide generous 
overhangs (24" min) and really strong separation of exterior plaster 
from grade (18-24" standard) as standard practice, although honestly 
that's as much to address splash back as snow pack.  I have also 
noticed, however, that there is enough heat loss from the walls to melt 
the snow directly adjacent to the wall - there is almost always a gap 
between the exterior wall surface and the snow bank, and that's 
obviously helpful.  The drying potential of the plaster in this region, 
from our limited but relevant testing in late winter/early spring during 
our testing program, seems to be adequate in nearly all cases, with 
higher MC% against the north wall where the drying potential in general 
is worse.  Not sure that counts as a formal answer, but our general 
strategy as been to get the plaster/bale assemblies up above most of the 
snow back, and not worry to much about it when the snow does build up.  
It's been working pretty well so far, to the best of our knowledge and 
testing.

In a related note, I've just been consulting with a contractor working 
on a wood-clad straw wall up here, who is dealing with premature failure 
of a skirt board against the inside corner of a bump-out on a north 
wall.  The leading hypothesis, after consulting a lot of pretty smart 
folks (including Derek Roff, on this list), is that the combination of 
snow pack and drip line from an inadequately-flashed bump-out roof is 
causing dramatic splash back at that location.  So, snow isn't just an 
issue of melting water, it also changes the grade profile during that 
magic window where it is raining/roof snow pack is melting and the snow 
banks are still there.  Apparently, if the theory is correct, this can 
foil some otherwise-adequate bottom-of-wall detailing strategies.  And 
if it can rot out wood in three years, plaster stands little chance...

My apologies to the metric-oriented members of the community, I am 
thoroughly illiterate in that regard.

Cheers,
Jacob

On 7/11/14, 7:57 PM, martin hammer wrote:
> Re: [GSBN] Straw bale at high altitudes (and snow against bale walls) 
> Hi Misha,
>
> In terms of moisture concerns for strawbale walls, elevation in itself 
> is not relevant.  What matters is climate (precipitation, temperature, 
> humidity, wind), and how it relates to interior "climate".  Higher 
> elevations do mean lower relative temperatures for the region (in all 
> seasons, 3-5 degrees F per 1000'), so indirectly it matters.  But very 
> low outdoor temperatures _beg_ for strawbale walls (just ask our 
> Canadian friends!).  As long as you employ normal good practices to 
> prevent relatively warm-moist interior air from condensing in the 
> strawbale wall, by using a vapor retarder on the inside face of 
> exterior walls (the IRC requires Class III vapor retarder (between 1.0 
> and 10 perms) in cold climate zones) and by sealing penetrations on 
> the inside face of these walls.
>
> I agree with what everyone else has said re: the foundation issues.
>
> Also the concern was raised about snow sitting against the bale walls. 
>  I've never been convinced one way or the other whether this is a real 
> or an imagined problem.  Thoughts / questions include:
>
>   * When the snow-wall interface is below freezing, presumably nothing
>     detrimental occurs.
>   * When snow at the snow-wall interface melts (because all snow is
>     melting or because the snow at the interface melts because the
>     wall surface is warm enough) is a space created that allows drying
>     of the plaster, and thus no harm?
>   * When the snow at the snow-wall interface melts, does it saturate
>     the plaster, which then wets adjacent straw causing degradation,
>     and/or does moisture in the plaster sometimes freeze and damage
>     the plaster?
>
>
> Does anyone have experience with snow against bales walls that answers 
> these questions?
>
> Thanks
>
> Martin
>
> */Martin Hammer, Architect
> /*1348 Hopkins St.
> Berkeley, CA  94702
> 510-525-0525 (office)
> 510-684-4488 (cell)
>
>
> On 7/11/14 8:40 AM, "Misha Rauchwerger" <misha.rauchwerger at gmail.com> 
> wrote:
>
>     I have a client that wants to build a straw bale house at 4000
>     feet in the Sierra Foothills.  They have been getting conflicting
>     information about the wisdom of building with straw at that
>     elevation.  In particular there is the concern about the effects
>     of moisture in the wetter months, and possible condensation inside
>     the walls.  They know about the need for big eaves, and permeable
>     plasters, but have been swayed against the idea from a local green
>     architect in town. Please direct me to any research, or anecdotal
>     evidence to support straw bale construction under these
>     conditions, or maybe there is valid concern.  I have only built in
>     the lower/dryer elevations on flatter sites.
>
>     They also share these concerns:
>
>     Their lot is sloped, so they would likely have to build a full
>     walk-out basement on the lower level, and the living space on the
>     upper level.  This means that the full lower level is built of
>     concrete (probably Faswall or Durisol); will they would have to
>     build out the lower walls to match the width of the straw bales?
>      How is this disparity in wall thicknesses usually resolved with
>     the least cost/impact?
>
>     - With a walk-out basement, is it possible/reasonable to do a
>     stepped-foundation on the lower level to minimize the amount of
>     concrete used?  Or does the mixed use of concrete and post/beam
>     and straw bale construction create unreasonable headaches in the
>     building process?
>
>     Thanks everyone for your comments,
>     Misha Rauchwerger
>     builtinbliss.com <http://builtinbliss.com>
>
>
>
>
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-- 
Jacob Deva Racusin
Co-Owner
New Frameworks Natural Design/Build

Author, The Natural Building Companion
Chelsea Green Press, 2012

(802) 782-7783
jacob at newframeworks.com
http://www.newframeworks.com
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