The ASTM C1289 Standard Specification for Faced Rigid Cellular Polyisocyanurate Thermal Insulation Board (ASTM C1289-13) was recently updated and features important improvements regarding the prediction of long-term thermal resistance (LTTR) R-values (R) for polyiso roof insulation boards.
As a result, LTTR values for polyisocyanurate (polyiso) roof insulation will change from a design value of 6.0 R per inch to 5.7 R per inch, which will impact the amount of polyiso needed to meet building-code requirements. Professionals involved in the design, procurement or installation of roof systems that include polyiso roof insulation should be aware of this change.