Cob: why light clay

Sun Apr 29 02:59:57 CDT 2001

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=2>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.</FONT></DIV>
<DIV><FONT size=2>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.</FONT></DIV>
<DIV><FONT size=2>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.</FONT></DIV>
<DIV><FONT size=2>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.</FONT></DIV>
<DIV><FONT size=2>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.</FONT></DIV>
<DIV><FONT size=2>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.</FONT></DIV>
<DIV><FONT size=2>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.</FONT></DIV>
<DIV><FONT size=2>Vicki and David Wicker</FONT></DIV></BODY></HTML>