Thermal and structural cob

Hi all,

I'm getting to the cob part of my project.  I can't quite figure out where it is best to use thermal cob and where it is best to use structural cob.  I figure there must at least be some structural cob near the surface of all cob construction - sort of like thermal cob is the filler and structural cob is the casing.  Is that the right idea?  If so, how thick should the structural cob be?

Thanks,
Jean-Paul

Hi Jean Paul;  
What I call thermal cob , is cob mixed with short pieces of straw. Making it insulating.   This is used under the pipes in your mass, to keep the heat from transferring to the ground.

To me, structural cob is sand and clay. This is used over your pipes to form a clay tunnel in the event your pipe rots out. And then it is used as filler around all the rock in your mass.


A final layer of thermal cob , maybe 2" thick s then applied on top to help hold the heat in the mass.


I should ask ;  is your bench going to be all cob? Or are you encasing it with another material?

An all cob bench usually gets a final finish coat to keep down the dust and help with abrasion.  

Hey Thomas,    I think you might have that backwards. ;)  I've always known the term "structural cob" to mean cob with straw added so that it provides the tensile strength or structure that thermal cob  doesn't have.....and "thermal cob" relating to cob (without straw) that is able to withstand much higher temps that straw would just burn out in otherwise.

Look forward to seeing some pictures Jean-Paul when you get a moment.

Well Gerry ;
Did you notice my post about admitting I Might be getting old ?
I have this horrible illness... called CRS disease , you may have heard of it.
So ahh yes  , You are correct. My apology's Jean Paul, just an old man rambling...
     

thomas rubino wrote:Well Gerry ;
Did you notice my post about admitting I Might be getting old ?
I have this horrible illness... called CRS disease , you may have heard of it.
So ahh yes  , You are correct. My apology's Jean Paul, just an old man rambling...
     

Thanks Thomas, Gerry!

My current plan for the bench is:

 * Rear brick "retaining" wall providing 4" airgap along the first run (right out of the manifold and the final duct run before the chimney)
 * Dry brick and fieldstone rubble directly on (quite dry) concrete slab
 * Cob and fieldstone on top of that

It sounds like I should have thermal cob (clay and sand) around all of the ductwork to form the backup ductwork when the metal ductwork fails.  This should be "filler" for most of the bench.  I should have structural cob (clay, sand, straw) below my metal ductwork (on top of my dry rubble layer) and on top (around 2" thick).

Does that sound right?  You asked about encasing the bench in another material.  I have the brickwork on one side.  The other side will be up against sheetrock.  If I am using cob for the other sides, it seems like that should be a couple inches of structural cob (clay, sand, straw) as well?

Thanks!

Yes; Jean Paul.  That should work great.
And as Gerry said , we await photos!

thomas rubino wrote:Well Gerry ;
Did you notice my post about admitting I Might be getting old ?
I have this horrible illness... called CRS disease , you may have heard of it.

Had to think on that one..... but if its what I think it stands for then I most definitely have my blonde moments too!

Just remember to put lots of rock in your mass Jean Paul. Saves mixing the sand and clay and holds the heat better too.

Well, time passes...  We had a couple good cob parties and made some progress but now progress is kind of slow and intermittent...

Here's some current pics