[GSBN] The Straw Stuff

Tue Mar 8 18:25:15 UTC 2011

Chris,

We've actually done the Straw Stuff several times but we use metal lath
stapled to the studs instead of the wood lath. I think the metal lath has a
better chance of containment in fire. We've mainly done this on interior
walls (to match the bale aesthetic) and not yet on a whole project or when
the client wasn't up for an experiment. I have seen that when we have had to
cut back into the lath that the straw stays put. We've also used a thinner
lime slurry on straw as well to try and mitigate mold growth and fire while
accelerating drying time. There did seem to be a bit of a reaction with the
lime and the straw so we haven't pursued this much farther. The time savings
is immense and I even have a straw clay mixing machine. Harry Hagaman, of
Green Design Systems was working on a precast wall panel system that used
2"x8" studs 4' on center and 2" x 4" welded wire lath with compressed straw
in between that he had patented and trade marked as Straw Wall. I didn't
here the results of any fire testing and he did win some clean tech awards
but for some reason I haven't heard of it coming to market. I always thought
the thermal issues of the thin wall and the conductivity of the stud made
for a limited system. I do like it in an offset double frame wall. Fire
blocking becomes an issue on tall walls but I think it is a good solution on
south and southeast walls if you want more insulation than the
mass/insulation ratio you get from strawclay and the window and door
frequency make bales awkward. It can also be used to form and fill interior
sculpture walls where Cob is either too heavy or costly and you don't
need/want more mass than a thick earthen plaster can provide.

Chug, if I understand you correctly, I'm concerned that you're only letting
the straw clay dry for a day (or even a few days) before plastering. We've
seen mold growth even up to a month later in hot dry California.

And to that question, I've never heard a decent explanation for why we see
so much more mold growth on straw clay but not cob with the same straw in
the same clay just different proportions. Is likely happening in both cases
just less volume of mold with less available mold spores and food (straw)?

OK this turned out to be more than I hoped at first, back to the mud.

Tim O-K (for now)

On Tue, Mar 8, 2011 at 8:52 AM, Derek Roff <derek at unm.edu> wrote:

> Like Bruce and Chug, I have some concern about increased fire risk.
> However, it seems likely that if you have stuffed the straw to the same
> density as a bale, then your fire resistance should be similar.  With the
> caveat that Bruce mentioned, of conditions changing dramatically if/when the
> lath burns and allows the straw to decompress.  I'm not sure that it would
> suddenly decompress, but it might.
>
> It would be interesting to build a test wall section, leave it for a couple
> of months, and then cut through the lathes.  That would give some idea of
> whether the straw would acquire a compression set and be fairly stable in
> its packed form, or whether it would still be eager to unpack itself.
>
> I'd like to hear more about how you determined the density of the stuffed
> straw.  I'm guessing that you found you could pack a bale of straw into less
> than a bale's volume in the wall.  Is that right? Chris, do you have any
> sense of how consistent the density was/is from edge to edge and top to
> bottom?  I would have imagined that the lathe would bow outward
> significantly, under the pressure necessary to reach bale densities.  If I
> understand your posting correctly, that didn't happen.
>
> How would you compare cost and time for stuffing straw in this way to
> filling a similar wall cavity with dense-pack cellulose?  "The Straw Stuff"
> is a very interesting experiment, and I'm glad that you shared your
> information with us.
>
> Derelict
>
> Derek Roff
> Language Learning Center
> Ortega Hall 129, MSC03-2100
> University of New Mexico
> Albuquerque, NM 87131-0001
> 505/277-7368, fax 505/277-3885
> Internet: derek at unm.edu
>
>
> On Mar 5, 2011, at 9:16 PM, Chris Magwood wrote:
>
>  Hey all,
>>
>> This past summer, we did a little experiment with our students that
>> seemed to work well, and I'm curious if anybody has tried it (or has
>> good reason to not try it!) before I go ahead and try it on a
>> slightly larger scale.
>>
>> We were building two walls with straw/clay, using a double stud
>> framing system and using thin wood lath on the studs as the form/
>> cage for the straw/clay. We then used clay plaster over the lath on
>> the inside and outside. As usual in our climate, we were under the
>> gun to get the straw/clay placed in time for it to dry thoroughly
>> before plastering.
>>
>> As we were doing so, I wondered if we might just use the same
>> framing and lath system and stuff the cavity with dry straw instead
>> slip-straw. So we built a sample frame and did just that. The wall
>> was 12-inches deep (as was the straw/clay wall). We placed about 18-
>> inches of lath at a time and then packed in the dry straw. We
>> measured the density of the original bale and we were able to get
>> the straw into the wall cavity at a slightly higher density than the
>> bale itself. We had pieces of lath cut to go across the wall to keep
>> the straw packed down at the intended density, but found we really
>> didn't need them. The whole thing went together really, really
>> quickly and plastered up nicely.
>>
>> So can anybody think of a reason why this might not be a good idea?
>> Yes, I know it's not as fun and natural as stacking bales... I'm
>> still a huge fan of doing things that way. But so many people are
>> building stick frames of one kind or another and then going to all
>> kinds lengths to notch, trim and otherwise jam bales into those
>> frames. This system fits a double frame wall really well, accepts
>> plasters really well and in general seems like a decent way to go
>> about using straw as an insulation. The thickness can be varied to
>> reach intended insulation values.
>>
>> Thoughts?...
>>
>> Chris
>>
>
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-- 
Tim Owen-Kennedy, Owner
Vital Systems, natural building & design
P O Box 751, Ukiah, CA 95482
www.vitalsystems.net
888.859.6336
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