Do you typically spec your roof insulation for a minimum R-value or an average R-value?
Average R-value (for thermal), but minimum thickness (for span and durability).
Where the insulation is used to control condensation I would want to control the minimum value. I would give the Mechanical Engineer a call.
I give a minimum thickness and a minimum average thickness. I don’t want to engage in disputes over the interpretation of R-values.
I would also suggest that some AHJs are very specific about what they want – either “minimum” or “minimum average” value for the roof insulation and this has not been subject to our interpretation. in my experience, the values required by the local energy code are on the code sheet of the drawings; my specs require whoever supplies the insulation to meet the required R values – in whatever form they are required.
To repeat a favorite smart remark:
“Designing a building that meets the code merely means that if you required anything less, it would be illegal.”
I specify the Materials, and their appropriate LTTR, and the sequence in which they get installed, but leave actual thickness of intermdiate layer(s) to drawing notes and details.