[Cob] Insulative values of cob and strawclay

[Sky Orndoff]

Hello!

Neat recent posts!  This is replying to them about insulative values.  

Both: One thing not often mentioned in these discussions is airflow:  a sealed house is better thermally than a leaky house.  Cob can be either, but if you make a tight cob house, it'll outperform a batt-insulated house any day.

Mike: Here's my two cents about insulation values:  Straw and slip-straw are going to have fairly different insulative values, because of the effect of thermal bridging.  Inside a strawbale, there's the grass stalks, air trapped inside the grass stalks, and then there's air between the grass stalks.  That's all.  It is this air-solid-air-solid pathway that makes it resistant to heat transfer.  Fiber on its own (without the dead air) does not conduct heat very well, but it's not an insulant.  Almost all insulants work in this way: air-solid-air-solid etc. When you mix an insulative material with a solid that transfers heat effectively (clay, concrete, steel, etc) heat will transfer through the material, making the pathway air-solid-conductive solid-solid-air-conductive solid etc.  This provides much less dead airspace than the insulation would alone, which increases heat transfer and therefore decreases r-values.  I would optimistically guess an
 r-value of between 1.5 and 2.0 for claystraw.  Which is pretty decent--maybe a little more insulative than wood.  Have fun with your studframe claystraw project--I think it's a great idea!  You'll use a lot of straw and clay.

Janet: about your r-values:  The effective measure of r-19 in Arizona is probably "effective" due to daily temperature swings: thermal transfer happens slowly but steadily through cob, which means that if your daily temperature average is comfortable even if your predawn and early evening temperatures are not, then your cob building's inside will be comfy.  If it's 40 at night and 85 during the day, you'll average around 63 degrees inside, and that will be lovely.  That's a likely scenario during any season in Arizona.  In Montana, if I relied on this "effective R-19" value to get me through the six months of subfreezing weather, I would have to burn many many tons of firewood to stay warm.   So I won't take Arizona's effective R-value and apply it to my situation: here in Montana I need to insulate.  This isn't to dissuade you from your plan--it's to clarify my opinion on how to make cob cozy.

Everyone: Here in Montana we're nearing completion of our cobwood room.  It's a marriage of cob and cordwood techniques, which uses insulation in the middle of two linked-by-log cob walls.  It's a very small space, but designed a la cobhouse, with curved walls and a built-in rocket mass heater, which makes it feel both roomy and snug.  Check out the project at www.earthartbuild.wordpress.com

Thanks!

Sky

A big shoutout to Ron Becker and Tys Sniffen for their willingness to share their experiences with us in the design phases.  It was lovely seeing your incredible handiwork.  Thanks guys!