Surely this comes down to the purpose of the question. If wanting to know how thick to make cedar walls for them to become equivalent to poly, I agree measurement would be most appropriate. However, if weighing up poly against wood when considering lots of different factors such as price, longevity, environmental impact and benefits for the bees, a more vague (although still needing to be accurate) concept of 'this is roughly the same as/this is slightly better than/this is significantly better than' might be sufficient rather than needing an exact value.