For anyone else that wants to know this answer… the correct answer and how to calculate if you ever need is as follows…
BTW @bulbatron9 never included the thermal conductivity he was told by the supplier, so we dont know if it was SI or Imp value… but it does not seem correct.
High density polyethylene is not commonly used for thermal insulation therefore you will not find published answers in r-value. However, engineering texts and plastics industry manuals list thermal conductivity is in the range from .46 to .52 W /( m-K ). This is a metric based SI value
You cannot convert a SI number into an imperial R-value by using the inverse as was stated by bulbatron9. It is an SI thermal conductivity value and not the imperial value…. we need to either invert to the RSI and convert that value to imperial R-value or convert the SI conductivity into imperial and then invert to determine the R-value.
To convert this SI unit to an imperial value we need to divide the SI thermal conductivity by the conversion factor of 1.7307, when we do this we find that the imperial thermal conductivity is .46/1.7307 = .26 to .52/1.7307 = .30 BTU / (hr-ft-°F).
Now we can calculate the inverse to determine the true r-value.
Thermal conductivity is the inverse of R value so now that we have the imperial conductivity we calculate the inverse 1/.26 = R 3.76 to 1/.30 = R 3.33