Section 150.2 of the code specifically allows small additions that comply Prescriptively to be built with either 2×4 R-15 or 2×6 R-19 walls in cases where that wall is an extension of the existing wall. You are responsible for demonstrating that the wall is in fact an extension per this section, so we recommend discussing this with the CEC Hotline (800) 772-3300.
Assuming you do meet the exception, you can choose to ignore the warning that you get from EnergyPro about the wall not meeting the 0.051 required maximum U-Factor. Just be aware, this is a specific exception that only applies to those walls that meet the CEC definition of an extension.
The current CF1R-ADD form does not have the ability to document this, so when you issue the form, you will need to append a note indicating that the walls with the higher U-Factor are meeting the exception.
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