Insulation or R-value is a measure of apparent thermal conductivity, and thus describes the rate that heat energy is transferred through a material or assembly. Materials are assigned a number, the higher the number the greater the insulation quality.
For a straw bale wall there are various factors which affect the R-value. This includes tightness of each individual bale, how the bales are oriented and so on.
According to data from Ausbale (Australian Straw Bale Assocation) the R-value of R35 can be assumed.