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Re: GSBN:Fwd: CASBA_ Pea Gravel
There are several excellent points already made here:
- that prescriptive requirements, whatever they might be, are typically
limiting and not always appropriate, especially in the hands of
designers or code officials with limited understanding of particular
systems, materials or methods of construction,
- that in this case, we actually don't have research and testing
results to rely on for this,
- that there are a number of competing concerns wanting to be addressed
and probably an even greater number of theories and strategies
competing to meet them,
- that including intent is critically important in writing codes,
though missing more often than not in most codes (this should not be a
deterrent to us getting it right going forward),
- that there are at least a couple of methods (with some variations)
that predominate now - pea gravel or something equivalent and rigid
- and of course, there is the first point, that David Arkin and company
have a particular problem at present and have asked for some help...
The language "There shall also be a drainage plane between the straw
and the top of the foundation, such as a one inch layer of pea gravel."
The question of intent is a complex one, and I can only say that that
was not language that I would have used - in part because it isn't
correct - this is not a drainage plane, but more accurately would have
to be called a drainage zone - that is, a separation between the straw
and the top of foundation.
The question is what to do about this - how to provide information that
would both help with David's problem and guide us in the next evolution
Over the years I've gone back and forth a good bit on how best to
address this issue. John makes the point that moisture moves around
inside assemblies both as water vapor and liquid water (hopefully the
latter only rarely and in very limited amounts - though enough water
vapor in the presence of cold-enough condensing surfaces can produce
plenty of liquid water). Not included in what follows are the other
environmental issues surrounding the use of foam and various other
moisture barriers and so forth. Figuring out what's going on with
moisture and how to most effectively deal with it is what I'm focusing
on, but I don't discount in any way the importance of being conscious
of what we're using to achieve those goals and what their impacts are.
I just don't have time or space to go into all of that now. In general
I tend to favor the lowest tech, lowest impact EFFECTIVE strategies,
thinking about what else might be at stake if what you're focused on
fails to perform as required.
Here are the concerns/issues that I have thought most about...
*First and most obviously, we've always thought it important to raise
the bales above the floor level (for either slab-on-grade or top of
foundations that are at the same elevation as the floor) or where
exterior slabs or floors are at the same elevation (such as porche
slabs - we've seen porch slabs which sloped back toward the house with
bales sitting on a moisture barrier at floor level - why did we see
this? Because we were called to help remediate the resulting rot at the
bottom of the wall as the bales got wet every time it rained). This is
both to keep the bottom bales dry during construction and to deal with
plumbing disasters, overflowing bathtubs or toilets, etc. but also has
the benefit of providing structural attachment of netting or mesh,
tie-downs, etc. and solid backing for plaster grounds, mesh, and
flashing on the exterior and plaster grounds and baseboard trim on the
*Rising damp (terminology from the UK, which I think is perfect because
it describes what is going on with soil moisture coming up into
buildings and assemblies by both capillary action and the movement of
water vapor), which necessitates the inclusion of moisture barrier of
some sort at the top of the foundation or on the slab (in the case of
slab-on-grade construction). This deals with moisture coming up into
the wall assembly.
*Condensation caused by cold foundations or slabs. This is what led to
the two current practices of either having pea gravel or foam between
the sill plates or whatever method is used to raise the bales above the
floor or foundation. I have a basic preference for foam insulation for
the energy conservation benefits and for separating the bales from a
cold condensing surface. But along with these benefits come the issues
the John raised concerning having an impervious material against the
straw. So if liquid water is in the wall and gets down to the bottom of
the wall, it could sit on this foam as liquid and cause problems. I
know that some people have devised a way to cut sloping slots in the
foam every few inches so that any significant amount of liquid water
would drain toward the exterior plaster where it could get out or have
some chance to dry. The idea of the pea gravel was to have support for
the bale without creating a surface for water to collect on at the bale
interface and to also have a capillary break to keep water that might
accumulate at the top of foundation level (on top of the moisture
barrier) from wicking up and wetting the straw. And of course this also
allows more movement of water vapor in the entire assembly - which is
either good or bad depending on how much and where the moisture is
coming from and where it's going.
*Desire to have adequate strength for seismic areas and height of
elevation above top of foundation/floor level has led to the use of
4x4s (roughly 3-1/2" x 3-1/2" or 89 x 89 mm) for sill plates to the
interior and exterior edges of the bales. This actually allows for
different strategies, including, if one wanted to do so, a combination
of foam at the bottom and gravel on top of the foam. This could be
further refined to have the foam insulation fill the full height of the
sill plates for the inner half of the space between the plates and the
pea gravel fill the upper space on the exterior side. You could put a
layer of 2" thick insulation across the whole space at the bottom and
then a second piece of 1-1/2" thick insulation on the interior half of
the space, filling the rest with pea gravel. This would give you pea
gravel for drainage where it's most likely that liquid water would
occur while providing good insulation to the overall assembly.
And obviously, there's are many other possible strategies and
materials. Years ago I suggested using a clay-based oil absorbent
material (it's the same as kitty litter - the stuff used in cat boxes,
only you can get it in much larger, less expensive quantities) instead
of pea gravel as the fill between sill plates since it has the capacity
to absorb and chemically bind a fair amount of water. Of course at some
point it would reach its capacity to hold moisture and they be much
like other fillers.
Perhaps the design I suggested above - with the combination of foam and
pea gravel would satisfy the building official. Eventually we'll need
to get this sorted out.
That's my two and a half cents for today...
From: John Swearingen john.skillfulmeans@...
To: GSBN GSBN@...; david@...
Sent: Wed, 12 Dec 2007 9:10 am
Subject: Re: GSBN:Fwd: CASBA_ Pea Gravel
We've supported the use of pea-gravel or some other broken plane, not
because we think the wall will be drenched with water, but based on our
observation of vapor migration and accumulation through bales. Here's
- Vapor is continually moving around inside a bale wall as
conditions change. Many observations have been made of considerable
movement of moisture within a bale wall, and significant
humidity between the top and bottoms of a wall.
- It's easily observed that as moisture migrates through a bale it is
prone to stop and accumulate at any impermeable surface, and reach a
concentration where rotting will occur. This is why we avoid vapor
on the bale sides. The same conditions can occur at the tops and
- Bear in mind, also that the sills are at the bottom of the wall
where the most wetting occurs from direct rain and rain splash from
ground. Some of this moisture will initially migrate to the bottom
wall by gravity, especially in conditions of cold rain. As David
wont to say, good boots, good hat, etc.
- Some moisture may come from the footings: even though there might
some vapor barrier in place, it is often imperfect due to foundation
- Leaks do occur around windows and cracks, and gravel does provide
path for dissipating moisture.
- Accumulated rot in one place, the bottom of the bales, could
compromise the integrity of the wall.
I understand that many of us are very, ah, fixated, on R-values and
buttoning up our coats. I would not trade increased insulation for
cautious vapor management. We typically use two 4x plates of wood,
good for at least R-10---slightly low but not so far from optimal for
horizontal insulation when located low in the building shell, in our
climate. We have also used, in colder climates, a scheme we learned
Ken and Polly, simply alternating strips of rigid insulation with
of pea gravel, so you can get another 4-5" of rigid down there and make
Gore a little happier...
John "Gore-Techs" Swearingen
On Dec 11, 2007 9:37 PM, Jeff Ruppert jeff@... wrote:
David and Anni (or Catherine et al),
I completely agree with your questioning of this requirement. In our
environment, we use all sorts of materials between the sills, with
IMHO, this brings up the ever-present issues related to what we
Any mention of specific materials makes things like this prescriptive,
not open to performance qualifications, nor the ability to prove
since no accompanying explanation of intent is present. Any future
attempts to codify bale construction techniques should have
intentions. There are too many environments and situations that will
arise conflicting with any codification of our work.
The recent work by Martin Hammer and everyone else in the past has
wonderful, but I really think we can do better and qualify our intent,
just as other parts of the code include. There are accompanying text
intent for code structures in Canada (B.C.) and whole sections of the
I.B.C. that have intent documents.
Anything we do with regards to codes will serve us better by including
our intentions so those of us in questionable circumstances can
from any contributing work in these areas.
Catherine Wanek wrote:
> I am forwarding this inquiry from the California Straw Building
> Association (CASBA) list from award-winning architects David Arkin &
> Anni Tilt. Perhaps this will stimulate discussion on the GSBN
>> From: "David Arkin, AIA" david@...
>> Date: Tue, 11 Dec 2007 17:21:39 -0800
>> Subject: CASBA_Mem Pea Gravel
>> Fellow CASBAnauts (CASBAnuts?):
>> We haven't used pea gravel between our sill plates for some time
>> using rigid insulation instead. My opinion is that if a bale wall
>> enough water in it to need 'drainage' (from any source), those
>> are goners.
>> The building official in Sonoma is citing it's recommendation in
>> SB332 as a good enough reason to require that it be there. SB332
>> says, "There shall also be a drainage plane between the straw and
>> top of the foundation, such as a one inch layer of pea gravel."
>> anyone know of the intent of this?
>> More importantly, does anyone know of any testing as to why this
>> might (or might not) be a good idea?
>> Your thoughts (and/or Holiday Greetings) always appreciated,
>> David and Anni
>> * * * * *
>> Arkin Tilt Architects
>> Ecological Planning & Design
>> David Arkin, AIA, Architect
>> LEED Accredited Professional
>> CA #C22459/NV #5030
>> 1101 8th St. #180, Berkeley, CA 94710
>> "There is no way to peace. Peace is the way."
>> â?? A. J. Muste
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