[GSBN] Straw bale at high altitudes (and snow against bale walls)
Ian Redfern
ian at adobesouth.co.nz
Sat Jul 12 06:37:42 UTC 2014
Good evening Misha and All,
Here are a few comments from a moist, humid and wet climate where we have
had a week (7 days and nights) of cyclone force rain [70 to 100 mm (3" to
4") a night] and driving winds (100 + km/hr) ! ! ! ! Where wide eaves
are my design norm with a LEAST width of 1200 mm (4 ft) and most of 1500 to
2000 (5 ft to 6' 6'')
On a clients house where we built these 1500 (5') wide eaves the lime wash
on lime plaster (massaged well into the straw) has come through this well
the rain was driven to window head height (7') so these walls were well
washed !
The primary considerations are for rain to be deflected by the wide eaves,
contact rain and washing water to drain freely away, and good air flow
(strong wind here) to facilitate drying ! Re the question of snow melt I
have no direct experience with straw walls however at 7000 ft in New Mexice
we relied on the step up from ground to floor to prevent this becoming a
problem with timber clapboard and framed houses again the maxim on wide
eaves or verandahs proved their worth in reducing the snow bank against the
walls.
Where the eaves (drip line) is small and especially for 600 (2 ft) or less
we often specify a rain screen on a drainage/ventilating cavity (1/2
"minimum) to provide this primary deflection my concern with this solution
is to ensure the cavity is maintained during the snow and subsequent thaw
cycle and is not filled with ice.
I challenge the "green architect" to substantiate the contention of least
eaves on the shady side of the dwelling I recommend to clients and
wherever extend these to become wide verandahs or wider still for breezeway
car ports (roofed over only) to shield the cold wall.
I am not sure this adds much to your discussion
Regards
Ian
www.adobesouth.co.nz Ian Redfern
Adobe South
A: 5 Lancewood Rise, Onerahi, Whangarei
P: 09 436 4040 M: 027 490 2324
E: ian at adobesouth.co.nz
From: Jacob Deva Racusin <buildnatural at googlemail.com>
Reply-To: Global Straw Building Network <GSBN at sustainablesources.com>
Date: Saturday, July 12, 2014 3:03 PM
To: <GSBN at sustainablesources.com>
Subject: Re: [GSBN] Straw bale at high altitudes (and snow against bale
walls)
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 ³climate². Higher elevations do mean
> lower relative temperatures for the region (in all seasons, 3-5 degrees F per
> 1000¹), 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. I¹ve
> 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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>
>
>
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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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