• The National Roofing Contractors Association argues that long-term thermal resistance calculated in a laboratory for polyisocyanurate and polyurethane foams is not the same as the real-world value.
• Test R values can meet code, but they it does not take into account thermal resistance loss when metal fasteners are employed, according to a study.
• Designers also should adjust R values based on the climate where the insulation will be used, the organization recommends.

From the article

To minimize the loss of thermal resistance in design, the R-value of roof insulation should be computed based on the actual performance of the insulation material and the local climate, says the National Roofing Contractors Association. ...