Cob: why light clay

[goshawk at gnat.net]

We live in Middle GA and the hot humid, not dropping at in temp very much at night, climate has 
been a concern on my part. We lived in the house last year but not all the cob walls were done. 
They were a lot of openings and lots of air flow so there was never any problem with too much 
build up of heat. This year all walls will be built by summer so we will have a better idea of how it 
performs. What I've done so far to address the problem of heat build up is 

  1. wide over hangs on roof keep direct sun off a large portion of the wall. 
  2. the design of our house is such that there is a lot of mass inside the house that never gets 
direct sun light. 
 3. we are coupled to the earth via the floor, and again, very little direct sunlight hit these floors. 
4. Very few interior walls that block air flow, so air can flow thru the whole house. 

and on last thing. I do not chop my straw and yes when I finish cobbing, it has hairy walls, but I 
put a lime plaster over the top so the hairy walls connect to the lime plaster and the straw is buried 
at that point.  I've opted for lime plastered walls as the level of dust it too great for either my wife or 
my computers otherwise. 

Pat



On 29 Apr 01, at 2:59, Vicki and David Wicker 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
> 


Pat Newberry
www.gnat.net/~goshawk