I developed the first table which is the estimated R-Values of blown cellulose that has been compressed. The following assumptions have been made:
- The manufacturers of cellulose list the R-Value of their product “after settlement” which is about 10 to 20 percent.
- The density after settlement is approximately 3.62 lbs/ft3
- The ASHRAE handbook was used to determine the change in the Thermal Conductivity versus the change in Density. Basically as the cellulose compresses the density goes up. As the density goes up the thermal conductivity goes up. The R-Value is the reciprocal of thermal conductivity or (1 divided by the thermal conductivity), which means the R-value goes down.
- The compressed R-Values were rounded to the nearest 0.5 so that’s why you see several numbers the same. For example R-22, R-24, & R-26 all show R-18 when compressed into a 2x6. However the true R-Values were 18.2, 18.0, & 17.8
You could use this as a guide for sections of an attic where foot traffic has compressed the insulation or plywood was placed over the insulation for storage.
The second chart is for compressed fiberglass batts produced by Owens Corning.
These charts are very generic so don’t stick you neck out too far when using them… 
