Cob: why light clay

[Chris Holmes]

You might want to take a look at information on Earth Shelter housing from the 
Rocky Mountain Institute, www.rmi.org

It seems one of the most important  things that you need top consider is 
maximizing the thermal storage capcity of the earth around the building so that 
it will continue to absorb heat during the entire hot season, and be able to 
cool off during the cool season. RMI's research indicates that heat energy will 
tend to move through the ground at about 1 meter a month.  How long is the cool 
season???  You may be able to come up with a solution to increase the 
dispersion of heat out of the ground during the cooler season.

Remember that the walls at the back and sides should not be covered with 
cuboards or the like, as they will prevent thermal transfer to the earth.

To increase the cooling I have two suggestions that may help.  
1)Ensure that the overhang on the front of the building is at least 3-4 feet. 
And maybe add a second exterior shelf 1 ft wide at 3 feet off the ground. 
This will make certain that there is enough shading.
2)Earth pipes for ventilation. Concrete drainage tubing with a 2ft diameter can 
start from 30 feet behind the building and be sunk into the ground under 
excavated fill.  Allow the tube to pass along the back wall of the building and 
cut holes into the back wall every 2 feet- connectin to the earth pipe.  This 
should create a nice cooling passive ventilation. The concrete tubing is also 
quite inexpensive. I saw this work quite well in a new building in england.




Chris Holmes,

On Apr 29, "Vicki and David Wicker" <macmastr at cswnet.com> wrote:
> 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>
> 
> 
> 
>