IECC 2009 Tables 502.1.2 & 502.2(1)

[QuestionThat]

Am I correct in that the values for individual building envelope opaque components can be substituted as suits between tables 502.1.2 & 502.2(1)?
If so, am I also understanding correctly that I can elect by using table 502.1.2 not to use continuous insulation for metal framed stud walls?
If that's the case then why am I allowed to use a lower R value when using table 502.1.2?
For example for metal framed walls in Zone 3 Commercial...
Table 502.1.2- U=0.084 therefor R=11.90
Table 502.2(1)- R=13 + R=7.5 ci for total R of 20.5
Why are they allowing R11.9 with no ci for this particular category (wood framed is similar).

Anyone know why I would have marked up table 502.1.2 to be used with "UFactor alternative approach"

 

[RLGA]

The R-value method requires that only the insulation materials comply with the stated R-values. The actual performance of R-13 insulation in a metal-stud-framed wall is actually around R-5.98 for 3-5/8" studs spaced at 16" o.c. It is R-7.15 for studs spaced at 24" o.c. Therefore, the U-factor would be around 0.07 just for insulation alone. However, the U-factor method allows you to take into consideration the thermal performance of all materials in the wall assembly (e.g., gypsum board, sheathing, etc.), including interior and exterior air films.

You would use Table 502.1.2 when you know you cannot comply with the prescriptive R-value requirements for the insulation materials per Table 502.2(1).

 

[QuestionThat]

OK Thanks. So MUST take ASSEMBLY into consideration for table 502.1.2. Therefor steel studs have large impact on design decisions..... thus what appears to be allowing for a lower R value is somewhat deceiving.
Did some math yesterday and turns out that to comply with table 502.1.2 u factor of .084 would need to have very close to the same R value requirements (for the insulation) as table 502.2(1). 5/8" at 4.1R rigid and R13 Batt. Difference is that if go with table 502.1.2 it offers some flexibility (for the assembly). Example can use 2" rigid ci and meet requirements without using any batt insulation.