Cob: why light clay

[Vicki and David Wicker]

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I will tell you our situation and explain why we have chosen a timber =
frame with light clay infill rather than a "regular" cob house. Opinions =
would be appreciated. We have dug out our place and are ready to pour =
footings but haven't begun actual construction yet.
We live in central Arkansas where the weather gets very hot and humid =
during the summer. Although we do get some cold weather in the winter, =
you can heat your house fairly easily with good solar design, good =
insulation, and a quality wood or pellet stove. However, staying =
tolerably cool in summer is a whole nother ball game. I have spent hours =
looking for ideas on passive cooling but there just aren't the options =
for cooling as there are with heat. My concern with cob, adobe, and =
rammed earth is that, unlike in the Southwest where the temperatures =
drop off significantly at night, allowing the house to transfer its heat =
back into the environment, in our area night time temperatures remain in =
the high 70s and 80s for days, with high humidity to boot. Our fear was =
that our thermal mass would heat up with little or no opportunity to =
release that heat back into the environment and we would essentially be =
living inside a brick over for several weeks of the year.
To try and minimize artificial cooling we have chose the following =
design. Our house will be set back into the hillside so that the back =
wall is entirely bermed with the front side walking out level. The front =
is southern exposure and will take advantage of passive solar. Berming =
in will give us the cooling effect of the earth, but I did not want to =
go the whole distance on an underground house. Also, since we are in the =
mountains, digging into the rock is expensive. Our cost to get deep =
enough into the hillside just for berming was $2000.
Because of the berm, the back wall will be block and the side walls will =
be blocked to about 3 feet. It would be very expensive in terms of =
blocks or poured concrete to create a wide enough foundation to build =
the "pure" cob walls. In addition, we plan to build a living roof. We =
chose a flat roof (well, slightly sloped) to minimize surfaces that =
would be heated up by the sun. We chose the living roof to try to =
utilize the cooling effect of plant transpiration. We chose timber frame =
to give us the strength to hold up the living roof and because my =
husband is an experienced timber framer, which gives us confidence on =
structural integrity.
As for the light clay infill, straw vs. chopped straw vs. sawdust, I =
assume people chop the straw so they don't have "hairy" walls. Why we =
are thinking saw dust over chopped straw is saving the extra labor of =
chopping the straw and saw mills are very prevalant in our area, so saw =
dust is readily available and cheap. We chose infill over making bricks =
with a cinva ram once again because of saving labor. We were fortunate =
to find a mortar mixer on its own trailer for about $900. We will mix =
the clay mix at the clay site, pull it to the wall site with our little =
tractor and pack it in the forms. Making bricks would require several =
more steps and moving the bricks several times.
Once the walls are up we plan to wrap the outside in a radiant barrier =
insulation, once again to try to keep our thermal mass from heating up =
in the summer. We haven't decided yet on an exterior covering. The =
excavators dug up a lot of rock so we are considering rocking the =
outside. Oh, and we're planning on an earthen floor, once again to try =
and gain some cool in the summer, with radiant tubing for winter heat. =
We plan on a hahsa to run our radiant heat.
We're forging ahead but still have our ears open to ideas, so if you see =
any major flaws in our plan, or have some suggtions for improvements =
we'd love to hear them.
Vicki and David Wicker

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<DIV><FONT size=3D2>I will tell you our situation and explain why we =
have chosen a=20
timber frame with light clay infill rather than a "regular" cob house. =
Opinions=20
would be appreciated. We have dug out our place and are ready to pour =
footings=20
but haven't begun actual construction yet.</FONT></DIV>
<DIV><FONT size=3D2>We live in central Arkansas where the weather gets =
very hot=20
and humid during the summer. Although&nbsp;we do&nbsp;get some cold=20
weather&nbsp;in the winter, you can heat your house fairly easily with =
good=20
solar design, good insulation, and a quality wood or pellet stove. =
However,=20
staying tolerably cool in summer is a whole nother ball game. I have =
spent hours=20
looking for ideas on passive cooling but there just aren't the options =
for=20
cooling as there are with heat. My concern with cob, adobe, and rammed =
earth is=20
that, unlike in the Southwest where the temperatures drop off =
significantly at=20
night, allowing the house to transfer its heat back into the =
environment, in our=20
area night time temperatures remain in the high 70s and 80s for days, =
with high=20
humidity to boot. Our fear was that our thermal mass would heat up with =
little=20
or no opportunity to release that heat back into the environment and we =
would=20
essentially be living inside a brick over for several weeks of the=20
year.</FONT></DIV>
<DIV><FONT size=3D2>To try and minimize artificial cooling we have chose =
the=20
following design. Our house will be set back into the hillside so that =
the back=20
wall is entirely bermed with the front&nbsp;side&nbsp;walking out=20
level.&nbsp;The front is southern exposure and will take advantage of =
passive=20
solar. Berming in will give us the cooling effect of the earth, but I =
did not=20
want to go the whole distance on an underground house. Also, since we =
are in the=20
mountains, digging into the rock is expensive. Our cost to get deep =
enough into=20
the hillside just for berming was $2000.</FONT></DIV>
<DIV><FONT size=3D2>Because of the berm, the back wall will be block and =
the side=20
walls will be blocked to about 3 feet. It would be very expensive in =
terms of=20
blocks or poured concrete to create a wide enough foundation to build =
the "pure"=20
cob walls. In addition, we&nbsp;plan to build a living roof. We chose a =
flat=20
roof (well, slightly sloped) to minimize surfaces&nbsp;that would be =
heated up=20
by&nbsp;the sun. We chose the living roof to try to&nbsp;utilize the =
cooling=20
effect of plant transpiration. We chose timber frame to give us the =
strength to=20
hold up the living roof and because my husband is an experienced timber =
framer,=20
which gives us confidence on structural integrity.</FONT></DIV>
<DIV><FONT size=3D2>As for the light clay infill,&nbsp;straw vs. chopped =
straw vs.=20
sawdust,&nbsp;I assume people chop the straw so they don't have "hairy" =
walls.=20
Why we are thinking saw dust over chopped straw is saving the extra =
labor of=20
chopping the straw and saw mills are&nbsp;very prevalant in our area, so =
saw=20
dust is readily available and cheap. We chose infill over making bricks =
with a=20
cinva ram once again because of saving labor. We were fortunate to find =
a mortar=20
mixer on its own trailer for about $900. We will mix the clay mix at the =
clay=20
site, pull it to the wall site with our little tractor&nbsp;and pack it =
in the=20
forms. Making bricks would require several more steps and moving the =
bricks=20
several times.</FONT></DIV>
<DIV><FONT size=3D2>Once the walls are up we plan to wrap the outside in =
a radiant=20
barrier insulation, once again to try to keep our thermal mass from =
heating up=20
in the summer. We haven't decided yet on an exterior covering. The =
excavators=20
dug up a lot of rock so we are considering rocking the outside. Oh, and =
we're=20
planning on an earthen floor, once again to try and gain some cool in =
the=20
summer, with radiant tubing for winter heat. We plan on a hahsa to run =
our=20
radiant heat.</FONT></DIV>
<DIV><FONT size=3D2>We're forging ahead but still have our ears open to =
ideas, so=20
if you see any major flaws in our plan, or have some suggtions for =
improvements=20
we'd love to hear them.</FONT></DIV>
<DIV><FONT size=3D2>Vicki and David Wicker</FONT></DIV></BODY></HTML>

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