We have an interesting question posed by a project team member. The project is in Alaska, where the temperature is more often 0 F than 75 F (with about 13000 HDD). The insulation material being considered is expanded polystyrene, which has different R-values at different mean insulation temperatures (R value increases as temperature decreases).The standard ASTM test rating for R value, apparently, is derived from a test at 75 F, and the insulation actually performs better at lower temperatures. Is there anything in 90.1 modeling protocol that speaks to this? Is it acceptable to use a value closer to the mean temperature in the climate zone? The mean annual temperature is 26.7 F, and the mean high temp is 37 F. The average DB temperature in July at 3 pm is 70. Can we take credit for improved performance of the insulation at lower mean temperatures? We haven’t found any reference to this in the sections of 90.1 dealing with envelope insulation… have you run into this before?