HEY HEIDI – The Dear Abby of Concrete Masonry
MERV asks what the difference between code based R-values and “effective” R-values is
Q: For pre-insulated Concrete Masonry systems, what is the difference between code based R-values & effective R-values?
–Misleading Effective R Values
A: Dear MERV,
For concrete masonry, effective R-values typically mean that the R-value is adjusted to account for the thermal mass of a CMU wall system. It can also take into account other variables, such as thermal lag or annual averages. Code based R-values are calculated based on either test data or the method described in the ASHRAE Handbook – the series parallel (isothermal planes) method. Code based R-values are reported under strictly defined conditions, such as exterior and interior temperature, still air on the inside of the assembly and a defined wind mph on the outside. Code based R-values remain the same whether in Alaska, Florida and all of the climate zones in between. There a few reasons using effective R-values are problematic. Since effective R-value adjustments typically take in to consideration the benefits of thermal mass, and the benefits of thermal mass change due to the climate, effective R-values calculated in Arizona would be very different from effective R-values calculated in New England. Another problem with using effective R-values is code compliance. The energy code already takes into account the benefits of thermal mass for each climate zone, allowing mass walls to have less insulation. In the 2012 IECC for climate zone 5 (Massachusetts), the prescriptive insulation R-value requirement for a mass wall is 11.4. A wood framed structure requires an R of 20 and a metal structure requires an R-value of 26. Using effective R-values would essentially be double dipping. For more information, see NCMA TEK 6-2.