Hello!
For Climate Zone 5, IECC requires a minimum insulation value of R-13 + R-7.5 c.i. for a steel framed building. If the studs are 16" on center, the thermal bridging degrades the batt R-value of 13 down to an effective R-6 c.i. which bring the total insulation value to R-13.5 c.i. Currently, I am reviewing a building with 2.5" of R-5 extruded polystyrene total an insulation value of R-12.5 c.i. However, the building wall assembly exceeds the 'Assembly Minimum' of U-value 0.064. Since IECC 2009 is the effective energy code of Massachusetts since June 30, 2010, is the insulation assembly in agreement with the Minimum Envelope Requirements if the 'Assembly Maximum' is exceeded? Any input will be much appreciated!
Regards,
Janne Kairento
Hi Janne,
For energy modeling purposes, as long as your exterior/underground walls and slabs are built up from constructions and layers and exceed the minimum performance of the assembly U-value (read as lower assembly U-value), you should be in good shape. From a building inspection standpoint, I don?t know whether the 2.5? of R-5 extruded polystyrene is in compliance or not.
John T. Forester, P.E., LEED AP