The contents of earthbags can modify their R-value, for example some light volcanic tuff can make it a better insulator. I am not aware of ways to increase the r-value of cob.
Here's the wiki link: http://en.wikipedia.org/wiki/R-value_%28insulation%29
Earth/dirt has an R value but cob + straw or wood or some other insulation material would have better R value than just dirt alone.
Dry dirt has better thermal resistance than wet dirt... wet dirt would transfer the heat faster.
Imagine you have a roof over your head and no walls to give resistance to heat loss... You build a fire in your wood stove under your roof and the heat just goes out in all directions without any resistance. You are cold. So now you wrap a thin sheet of plastic all the way around as walls and you have an R value of around 1. wuhuu you are not so cold... The more insulation material you add to your roof and walls - the easier it is to keep your heat in and the more R value you have (Keeping your insulation dry is important too.)
One inch of cob has a small R value and one foot of dry cob has a greater R value.
I agree with ronnie except for thin plastic sheeting having an R value of 1 or that an R value of 1 would make a difference.
Plastic sheeting doesnt really have any R value but it has long been used as an air barrier in very cold climates.
Stopping Air infiltration is MUCH more important than R value. I see some cob installs having this as an issue as well due to splits in the wood over time. Surely a cob expert can tell me Iam wrong on this or there is a technique to avoid it..
I think that it would be best for people to stick with DOE bare minimum R value levels. Found a cool link thats adustable based on heating/cooling source and ZIP code: http://www.ornl.gov/sci/roofs+walls/insulation/ins_16.html
Also important for understanding of Cob, it does not function because of its R-value ( very low) It functions on a thermal mass idea. It traps heat when the sun shines and gives it back when the sun goes down. Very good for moderating temperature and humidity, as cob will absorb moisture form the air. The walls breath! If your climate is very cold for the winter with little no solar gains available then a hybrid structure might serve. IE: a north and a bit of your east and west walls made of something with more insulation, then cob in the south. Good luck.
Awesome, thanks! And I actually just ordered that book! Yay! Can't WAIT to read it.
Check their section on "SLIC." They also get higher r-value by additives.
Here is a blog by a man who built a cob back east in snow and humid regions. The lessons he learned are invaluable. http://www.small-scale.net/yearofmud/
The Year of Mud blog